Hydrolysis is a chemical reaction that happens when a molecule splits into two parts when water is added. One part gains a hydrogen (H+) and the other gets the hydroxyl (OH–) group. The reaction in the experiment forms starch from glucose, and when we add water, it breaks down the amino acid components just like the enzymes do in your stomach when they digest food.
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Displacement: There are several different types of displacement reactions, including single, double, and acid-base.
An example of a single substitution reaction (A + BC AC + B) occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen: Zn + 2 HCl ZnCl2 + H2
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A double displacement (metathesis) reaction has two compounds exchanging bonds to form new compounds (AB + CD –> AD + CB). Antacids like calcium hydroxide (CaOH) combine with stomach acid (HCl) to form calcium chloride salt (CaCl2) and water (H2O).
CaOH + HCl CaCl2 + H2O
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This reaction happens when simple compounds come together to form a more complicated compound.
The iron (Fe) in a nail combines with oxygen (O2) to form rust, also called iron oxide (Fe2O3).
2Fe + O2 Fe2O3
We’re about to do a synthesis reaction with sulfur. Sulfur is element #6 on the periodic table. Sulfur is used in fertilizer, black powder, matches, and insecticides. In pioneer times sulfur was put into patent medicines and used as a laxative.
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To further the evil reputation of sulfur, or brimstone, when sulfur is burned in a coal fired power plant, sulfur dioxide is produced. The sulfur is spewed into the air, where it is reacts with moisture in the air to form sulfuric acid. The clouds get full and need to let go of this sulfuric acid. Down comes the acid rain to wreak havoc on the masonry and plant life below.
In our experiment, sulfur and oxygen are heated and sulfur dioxide is produced. This is a synthesis reaction because the sulfur and the oxygen react and form a new substance, sulfur dioxide. We see the flame of sulfur dioxide burn in air. Small flame, little smoke. When the flame is left lit and placed in the oxygen, the flame flares up and lots of white smoke is generated. It appears that sulfur’s flame burns brighter and stronger in pure oxygen.
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A combustion reaction gives off energy, usually in the form of heat and light. The reaction itself includes oxygen combining with another compound to form water, carbon dioxide, and other products.
A campfire is an example of wood and oxygen combining to create ash, smoke, and other gases. Here’s the reaction for the burning of methane (CH4) which gives carbon dioxide (CO2) and water (H2O):
CH4 + 2 O2 CO2 + 2 H2O
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We will be creating hydrogen gas by making a heterogeneous mixture of zinc powder and calcium hydroxide and heat it. The hydrogen bubbles into test tube in a water bath. When we mix our test tube of hydrogen with the air the room, the hydrogen burns…it actually explodes. Our amounts are small, but you will witness a cool, small, explosion.
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If you guessed that electrochemistry has to do with electricity and chemistry, you’re right! But you might wonder how they work together. Back in 1800, William Nicholson and Johann Ritter were the first ones to split water into hydrogen and oxygen using electrolysis. (Soon afterwards, Ritter went on to figure out electroplating.) They added energy in the form of an electric current into a cup of water and captured the bubbles forming into two separate cups, one for hydrogen and other for oxygen.
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The chemical reaction inside electrochemical cells is also a redox reaction. Batteries (also known as galvanic or voltaic cells) use a spontaneous chemical reaction inside to create energy. The acid inside the battery reacts with the metal electrodes (the plus and minus ends of the battery) to provide electricity (energy).
Most metals oxidize – the corrosion itself is the oxidative deterioration. You can protect metals from corrosion (but not completely) by inhibiting the oxidant (when you paint the surface or even allow a thin layer of oxide to form then seal it to protect it. You can also make a coating layer that isn’t affected by water or oxygen and use that to coat the metal surface (like coating iron with sodium chromate).
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Chemical equilibrium is the condition that happens when the concentration of the reactants and products don’t have any net change over time. This doesn’t mean that the reaction stops, just that the producing and consuming of the molecules is in balance.
Most chemical reactions are reversible, just like phases changes. Do you remember the hot icicle experiment? Do you remember how to get it back to the starting point? You have to add energy to the solid sodium acetate to turn it back into a liquid, so it can turn back into a solid again. Then let that experiment sit for a bit (overnight or about 12 hours) and in the morning, you’ll have crystals growing on your pipe cleaner. Now if you want to reverse this reaction, all you have to do is add energy to the system and the crystals will dissolve back into the solution. You can heat it up in the microwave or in a pot of water on the stove, and the crystals will disappear.
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When your reaction is in equilibrium, you can do things to disturb it, like increasing the temperature or adding more of something. The system will respond and shift to account for these changes.
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Plasma makes up a very large percentage of the matter in the universe. Not much of it is on Earth and the plasma that is here is very short lived or stuck in a tube. Plasma is basically what happens when you add enough energy to a gas that the atoms move and vibrate around so energetically that they smack into each other and rip electrons off each other, so you have positively charged atoms (called ions) that lost their electrons, and also the electrons themselves which are negatively charged, all zinging around in the gas.
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This gives the gas electrical properties that gases don’t usually have, so it’s classified as a different state of matter, or known as “ionized gas” – it’s gas that is electrically charged. The stuff in florescent light bulbs is plasma. Plasma TV’s have plasma inside of them. Lightning and sparks are actually plasma!
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Solids
What makes the solids, liquids, gases etc. different is basically the energy (motion) of the atoms. From BEC, where they are so low energy that they are literally blending into one another, to plasma, where they are so high energy they can emit light. Solids are the lowest energy form of matter that exist in nature (BEC only happens under laboratory conditions).
In solids, the atoms and molecules are bonded (stuck) together in such a way that they can’t move easily. They hold their shape. That’s why you can sit in a chair. The solid molecules hold their shape and so they hold you up. The typical characteristics that solids tend to have are they keep their shape unless they are broken and that they do not flow.
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Elasticity is what allows you to bounce a basketball and shoot a rubber band across the room. All solids have some elasticity. A rubber band has a lot of elasticity, a diamond on the other hand has very little elasticity. Elasticity is basically the ability of solids to be stretched, twisted or squashed and come back to its original shape. You can stretch a rubber band quite a bit and when you stop stretching it comes back to the way it was. A basketball actually squashes a bit when it hits the sidewalk and when it unsquashes it bounces back up. If you stretch, twist or squash something beyond its elastic limit it will break or deform.
Imagine taking a rubber band, for example, and stretching it so much that it breaks. You’ve stretched it beyond its elastic limit and it broke. Another example, would be taking a wire pipe cleaner. If you bend it just a bit, it will bend back to its original shape. If you go to far, it stays in the new shape. You have bent it beyond its elastic limit.
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A liquid has a definite volume (meaning that you can’t compress or squish it into a smaller space), but takes the shape of its container. Think of a water-filled balloon. When you smoosh one end, the other pops out. Liquids are generally incompressible, which is what hydraulic power on heavy duty machinery (like excavators and backhoes) is all about.
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Here are the most important things about gases to remember:
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How do they make liquid helium, liquid oxygen, liquid nitrogen… atoms that are normally in the gases state?
The basic idea is that they compress the gas (remember the room full of ping pong balls? Now squish the room so it’s only half the size. Do the balls bounce faster or slower? Faster! So the temp increases.) When they compress the gas, it heats it up, so they cool it, then squish it even more to higher pressure and cool to near room temperature. They keep repeating this until it becomes a very high pressure, then finally they release the pressure (which is like suddenly expanding the squished room to the size of a football field), which makes the temperature drop way fast and the gas becomes extremely cold, condensing into a liquid.
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By knowing the value of the bond energy, we can predict if a chemical reaction will be exothermic or endothermic. If the bonds in the products are stronger than the bonds in the reactants, then the products are more stable and the reaction will give off heat (exothermic).
Exothermic chemical reactions release energy as heat, light, electrical or sound (or all four). Usually when someone says it’s an exothermic reaction, they usually just mean energy is being released as heat.
Some release heat gradually (for example, a disposable hand-warmer), while others are more explosive (like burning magnesium). The energy comes from breaking the bonds within the chemical reaction.
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Other chemical reactions will just sit there and do nothing, unless you add energy to it first. These types of reactions need to absorb energy in order to react, so you’ll notice a temperature drop when the reaction takes place (a disposable ice pack, for example, is a chemical reaction that takes place using the energy from the water, so it makes the water colder when it uses this energy).
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A molecule is the smallest unit of a compound that still has the compound’s properties attached to it. Molecules are made up of two or more atoms held together by covalent bonds.
In the space where electrons from different atoms interact with each other, chemical bonds form. The electrons in the outermost shell are the ones that form the bonds with other atoms.
When the atoms share the electron(s), a covalent bond is formed. Electrons aren’t perfect, though, and usually an electron is more attracted to one atom than another, which forms a polar covalent bond between atoms (like in water, H2O).
While it may seem a bit random right now, with a little bit of study, you’ll find you can soon understand how molecules are formed and the shapes they choose once you figure out the types of bonds that can form.
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Let’s take an example:
Why does ice float? In the water molecule (H2O) is held together by polar covalent bonds. Water molecules are also attracted to each other by weak (hydrogen) bonds between the atoms. As water cools below 4°C, the hydrogen bonds forms a hexagonal crystal lattice (known as ‘ice’). The solid form of water is a larger structure than the liquid form, as the crystal structure has a hole in the center. In other words, ice takes up about 9% more space than liquid water, so a liter of ice weighs less than a liter water. By peeking into the molecules closely, you can explain why ice is one of the very few solids that is lighter than its liquid form.
Water is also a polar molecule, which happens because one end of the molecule has slightly more charge than the other end.
When two different kinds of atoms, like oxygen and hydrogen form a bond, one attracts the shared pair of electrons more strongly than the other. (When a bond forms between two of the same kind of atom do we assume the attraction is equal, so atoms like O2 and N2 are not polar).
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There are different kinds of bonds that can form in a molecule. When two atoms approach each other close enough for their electron clods to interact, the electrons of one repels the electrons in the other, and the same thing happens within the nucleus of the atoms. At the same time, each atom’s negatively charged electron is attracted to the other atom’s positively charged nucleus. If the atoms still come closer, the attractive forces offset the repulsive and the energy of the atom decreases and bonds are formed – the atom sticks together. When the energy decrease is small, the bonds are van der Waals. When the energy decrease is larger, we have chemical bonds, either ionic or covalent.
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Ionic bonds generally involve metals from the left side of the periodic table that interact with nonmetals from the far right side. Look at your periodic table now – do you see the atoms near the rare gases? Those usually form ions. Covalent bonds form when elements lie closer to one anther in the periodic table. Most chemical bonds are somewhere between purely ionic and purely covalent.
Ionic compounds aren’t really real molecules. When ionic compounds are solids, they are really a structure of charged particles. When one atom accepts or donates an electron to another atom, an ionic bond is formed, like in table salt (NaCl). Do you see how there’s only 1 electron in sodium in the outermost shell? And notice how chlorine has seven, not 8 in the outermost shell? Chlorine wants to feel full (8), and sodium has only 1 out of the 8, so it donates it to chlorine. Not all atoms hold onto their valence electrons with equal strength.
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You’re going to try to determine what is happening during the flame test when you see different colors. Think about what particles are found in the chemicals you’re using, and why the different chemicals emit different colors of light? Where else have you seen colorful light emissions?
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Did you aim your razor slit at a light source such as a fluorescent light, neon sign, sunset, light bulb, computer screen, television, night light, candle, fireplace… ? Make sure that the diffraction grating does right up to your eye. Move the spectrometer around until you can get the rainbow to be on the scale inside the tube.
Once you’ve got the hang of it, you might be wondering, wow – cool… but what am I looking at exactly? Ok – so those lines you saw inside the tube – those are spectral lines. Can you see how there are brighter lines? Which frequencies are those? Well we need a ruler to measure those. Can you see how if we lined up a ruler as could tell what the frequencies are?
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What other light sources work? Use your spectrometer to look at computer screens, laptops, night lights, neon lights, candles, campfires, fluorescent lights, incandescent lights, LEDs, stoplights, street lights, and any other light sources you can find. When you walk down town at night and look at various “neon” signs.
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Energy can take one of two forms: matter and light (called electromagnetic radiation). Light is energy that can travel through space. When you feel the warmth of the sun on your arm, that’s energy from the sun that traveled through space as infrared radiation (heat). When you see a tree or a bird, that’s light from the sun that traveled as visible light (red, orange… the whole rainbow) reflecting and bouncing off objects to get to your eye. Light can travel through objects sometimes… like the glass in a window.
Light can take the form of either a wave or a particle, depending on what you’re doing with it. It’s like a reversible coat – fleece on the inside, windbreaker on the outside. It can adapt to whatever environment you put it in.
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When Einstein aimed a red light at the metal sheet, nothing happened. Even when he cranked the intensity (brightness) of the red light, still nothing happened. So it was the energy of the light (wavelength, or color), not the number of photons (brightness or intensity) that made the electrons eject from the plate. This is called the ‘photoelectric effect’. A UV light makes ever more electrons jump off the plate!
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One of the dreams of early chemists was to figure out how to transform lead into gold. Lead has 82 protons in its core whereas gold contains only 79. So conceivably all you’d need to do is remove three protons and presto! So how do you do that? Since protons can’t be stripped off with a chemical reaction, you need to smack it hard with something to knock off just the right amount. Lead, however, if a very stable element, so it’s going to require a lot of energy to remove three protons. How about a linear accelerator?
In a linear accelerator, a charged particle moves through a series of tubes that are charged by electrical and/or magnetic fields. The accelerated particle smacks the target, knocking free protons or neutrons and making a new element (or isotope). Glenn Seaborg (I actually met him!), 1951 Nobel Laureate in Chemistry, actually succeeded in transmuting a tiny quantity of lead into gold in 1980. He actually discovered (or helped discover) 10 elements on the periodic table, 100 new isotopes, and while he was still living (which usually doesn’t happen), they named an element after him (Seaborgium – 106).
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The strong force. Well, actually the residual strong force. This force is the glue that sticks the nucleus of an atom together, and is one of the strongest force we’ve found (on its own scale – it’s not felt at all beyond 1 femtometer 10-15m – outside the nucleus). This force binds the protons and neutrons together and is carried by tiny particles called pions. When you split apart these bonds, the energy has to go somewhere… which is why fission is such a powerful process.
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Naturally radioactive elements emit energy without absorbing it first. Fluorescence for example – the atom absorbs a photon before emitting another photon. You have to “charge it up” or mix chemicals together before light comes out. With radioactive materials, they emit energy on their own, sometimes in the form of light, but sometimes they emit other particles. Let me explain.
Chemical reactions usually deal with only electron or atom exchanges. Nuclear reactions deal with changes inside the nucleus of an atom.
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Which one of these things you see on the screen now is radioactive? Most kids think that anything that glows must be radioactive, but it turns out that there’s a lot of things that glow that aren’t radioactive at all. Many minerals (called phosphors) glow after being exposed to sunlight which contains UV light. In 1897, Henri Becquerel was studying phosphorescence when he accidentally discovered radioactivity. Naturally radioactive elements emit energy without absorbing it first. Let me explain…
Cold light refers to the light from a glow stick, called luminescence. A chemical reaction (chemiluminescence) starts between two liquids, and the energy is released in the form of light. On the atomic scale, the energy from the reaction bumps the electron to a higher shell, and when it relaxes back down it emits a photon of light. Glow sticks generate light with very little heat, just like the glow you see from fireflies, jellyfish, and a few species of fungi. Chemiluminescence means light that comes from a chemical reaction.
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Fluorescence is what you see on those dark amusement-park rides that have UV lights all around to make objects glow. The object (like a rock) will absorb the UV light and remit a completely different color. The light strikes the electron and bumps it up a level, and when the electron relaxed back down, emits a photon, a light particle. Can you find the image with the glowing rocks? There’s two of them – one with the lights on and one with the lights off. Right – on the left side. The reason stuff glows is that fluorescent objects absorb the UV light and then spit it back almost instantaneously. Some of that energy gets lost during that process, and that changes the wavelength of the light, which makes this light visible and causes the material to appear to ‘glow’.
Sometimes things glow even after you turn off the UV light source. Phosphorescence light is the ‘glow-in-the-dark’ kind you have to ‘charge up’ with a light source. This delayed afterglow happens because the electron gets stuck in a higher energy state. Lots of toys and stick-on stars are coated with phosphorescent paints. Those are like the stars and planets you see in the middle of the slide. Atoms continue to emit light even after the electrons return to their normal energy states. While it looks like seconds to minutes that the glow lasts, some samples have been found to phosphoresce for years using highly sensitive photographic methods. Only a few minerals phosphoresce, such as calcite from Terlingua, Texas.
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Okay, so now I want you to imagine a room full of ping pong balls that can bounce all by themselves. They go zipping all over the place all on their own. Now take those ping pong balls and add energy to them so now they bounce twice as fast. Got it?
Now what happens if we take away energy from them? Do they bounce slower? Yup!
Okay, now get them back to their original bouncing speed. Now take the room and make it smaller, like half it’s size, but keep the ping pong ball speed the same. Do they hit the walls more or less frequently? More! Are they speeding up or slowing down? Speeding up!
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Now take the room and expand it ten times it’s normal size. Do the balls hit the walls more or less now? Less! Do they still have the same speed? No, they should be slowing down, too.
So those ping pong balls are molecules, and when you add energy, you’re increasing the temperature so they fly around faster. When you increase the temperature, the molecules zip around faster and faster.
Dalton’s Law of partial pressures is related to the Ideal Gas law. Dalton’s Law states that in a mixture of non-reacting gases, like air, for example, the total pressure exerted is the sum of the partial pressures of each of the individual pressures. For air, you would simply sum up all the partial pressures of each of the individual gases of oxygen, nitrogen, argon, carbon dioxide, and water vapor to get the total air pressure.
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Find a low pressure (like the pressure you feel right now – it’s called 1 atm). Put your finder on the 1 mark on the vertical side (next to the “P”, which stands for Pressure) and follow the dashed line straight across. As you move across, so you notice how at low temperatures you’re in the ice region, but when you hit zero, you turn to water, and for temperatures below 100 deg C you’re only in the liquid water phase?
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The triple point is where a molecule can be in all three states of matter at the exact same time, all in equilibrium. Imagine having a glass of liquid water happily together with both ice cubes and steam bubbles inside, forever! The ice would never melt, the liquid water would remain the same temperature, and the steam would bubble up but not melt the ice. In order to do this, you have to get the pressure and temperature just right, and it’s different for every molecule.
The triple point of mercury happens at -38oF and 0.000000029 psi. For carbon dioxide, it’s 75psi and -70oF. So this isn’t something you can do with a modified bike pump and a refrigerator.
However, the triple point of water is 32oF and 0.089psi. The only place we’ve found this happening naturally (without any lab equipment) is on the surface of Mars.
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When something changes state, goes from like a liquid to a solid, all of the substance must change to the next state. For example, at 100° C all the water must change from a liquid to a gas. The temperature stays constant until it’s completely changed state. It’s kind of weird when you think about it.
If you were able to take the temperature of water as it changed from a solid (ice) to a liquid you would notice that the temperature stays at 32° F until that piece of ice was completely melted. The temperature would not increase at all.
Even if that ice was in an oven, the temperature would stay the same. Once all the solid ice had disappeared, then you would see the temperature of the puddle of water increase.
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Do you remember when I said that heat and temperature are two different things? Heat is energy – it is thermal energy. It can be transferred from one object to another.
Here’s what you do:
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Water is very good at absorbing heat without increasing in temperature which is why it is used in car radiators and nuclear power plants. Whenever someone wants to keep something from getting too hot, they will often use water to absorb the heat.
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They can have a thermal energy but they can’t have heat. Heat is really the transfer of thermal energy. Or, in other words, the movement of thermal energy from one object to another.
If you put an ice cube in a glass of lemonade, the ice cube melts. Which way does heat flow?
The thermal energy from your lemonade moves to the ice cube.
The movement of thermal energy is called heat. The ice cube receives heat from your lemonade. Your lemonade gives heat to the ice cube.
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Thermal energy is how much the molecules are moving inside an object. The faster molecules move, the more thermal energy it has.
Objects whose molecules are moving very quickly are said to have high thermal energy or high temperature. Like a cloud of steam, for example. The higher the temperature, the faster the molecules are moving.
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Remember that temperature is just a speedometer for molecules. The speed of the molecules in ice cream is way slower than it is in a hot shower.
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Energy is the capacity to do work or to transfer heat. You do work when you walk up a flight of stairs. You can feel the heat from the sun when you step in the sunlight. Both are energy.
Heat is associated with changing the temperature of an object. The temperature changes because energy is being transferred to it. Another word for heat is thermal energy.
Thermochemistry is the science of heat or thermal energy transfer and how to use it with chemical reactions.
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Your silver turns black because of the presence of sulfur in food. Here’s how the cleaning works: The tarnished spoon has silver sulfide on it, and when you put it in the solution, the silver sulfide combines with the baking soda and salt in the water solution to break apart into sulfur (which gets deposited on the foil) and silver (which goes back onto the spoon). Using the heat from your stove, you’ve just relocated the tarnish from the spoon to the foil. Just rinse clean and wipe dry!
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The reaction between silver sulfide and aluminum takes place when the two are in contact while they are immersed in a baking soda solution. The reaction is faster when the solution is warm. The solution carries the sulfur from the silver to the aluminum. The aluminum sulfide may adhere to the aluminum foil, or it may form tiny, pale yellow flakes in the bottom of the pan. The silver and aluminum must be in contact with each other, because a small electric current flows between them during the reaction. This type of reaction, which involves an electric current, is called an electrochemical reaction. Reactions of this type are used in batteries to produce electricity.
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The oxidation number of an element is the charge the atom has
I. For an atom in its elemental form the oxidation number is zero.
II. For any monatomic ion the oxidation number equals the charge of the ion.
III. For nonmetals the oxidation number is usually negative.
a) Oxygen is usually -2 in all compounds.
b) Fluorine is -1 in all compounds.
c) Hydrogen is +1 when bonded to nonmetals and -1 when bonded to metals (metal hydrides).
IV. The sum of the oxidation numbers for all atoms is zero for neutral
compounds or equals the charge for polyatomic ions.
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Sterling silver is an alloy (a solid solution) of silver and copper. In order to find the percent of silver, you have to break it apart from the copper, which will make it an ion floating around in a liquid. Then you will need to bond it to something that will make it turn back into a solid so you can measure it.
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In order to mix up chemicals in the right amounts (so we get the right amount out of the reaction), we have to figure out how much of a chemical to put in in the first place. Sometimes chemists have this problem: they need for example 2.0 L of 1.5 M solution of Na2CO3 (sodium carbonate). They find a bottle of Na2CO3 on the shelf, some distilled water, and a 2.00L flask. How much Na2CO3 do they put in the flask with the water?
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Gas forming reactions are also exchange reactions. The best example I can think of for this type of reaction is what happens when you put a piece of chalk in a cup of vinegar. The chalk, which is mostly CaCO3 (calcium carbonate) and vinegar (acetic acid) forms calcium chloride and carbonic acid, which isn’t stable and quickly turns into water and carbon dioxide. A faster version of this experiment is what happens when you take an effervescent tablet, like alka seltzer, and stick it in water, because the tablet is actually a solid form of baking soda and vinegar put together. What happens when you mix baking soda and vinegar together?
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Lots of bubbles! Baking soda and vinegar is a gas forming exchange reaction.
There’s actually two reactions going – the first one is a double displacement where the vinegar reactions with the backing soda to make sodium acetate and carbonic acid, but the carbonic acid is unstable and breaks into carbon dioxide and water. The bubbles you see from this reaction are the carbon dioxide bubbles escaping., Since CO2 is heavier than air, it sits on the surface or overflows off the side of the container. If you add soap to this reaction, you’ll see the bubbles more clearly. If you warm up the vinegar first, the reaction will happen faster. The white sludge at the bottom os sodium acetate that’s left ver. Adults use this in making rubber tires, for curing headaches… that sort of thing.
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Strong acids and strong bases (which we’ll talk about in a minute) all have one thing in common: they break apart (completely dissociate) into ions when placed in water. This means that once you dunk the acid molecule in water, it splits apart and does not exist as a whole molecule in water. Strong acids form H+ and a negative ion
The seven strong acids are: hydrochloric acid (HCl), nitric acid (HNO3) used in fireworks and explosives, sulfuric acid (H2SO4) which is the acid in your car battery, hydrobromic acid (HBr), hydroiodic acid (HI), and perchloric acid (HClO4). The record-holder for the world’s strongest acid are the carborane (CAR-bor-ane) superacids (over a million times stronger than concentrated sulfuric acid).
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Carborane acids are not highly corrosive even though are super-strong. Here’s the difference between acid strength and corrosiveness: the carborane acid is quick to donate protons, making it a super-strong acid. However, it not as reactive (negatively charged) as hydrofluoric (HF) acid, which is so corrosive that it will dissolve glass, many metals, and most plastics.
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Precipitate reactions are like watching a snow globe, but the snow appears out of nowhere.
For example, you can combine two liquid solutions that are totally clear and when you put them together, they each break apart into ions and then recombine in a way that looks like white snow in your test tube. Basically precipitate reactions make it possible to see the ions in a solution because they form a salt that’s not soluble – it doesn‘t dissolve in the solution. You can also get different colors of the precipitate snow, depending on which reactants you start out with. If you were to use potassium bromide (KBr) with silver nitrate, you’d find a yellowish snowstorm of silver bromide (AgBr).
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When a substance is mixed with water it’s called an aqueous solution. Solubility is a property that a solid, liquid, or gases has when mixed with a solvent. If it can dissolve into the solvent, then it’s soluble. Dissolving marbles in water is a physical change. The marbles don’t break apart in the water to form new molecules with the water.
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A reagent is chemical compound that creates a reaction in another substance; the product of that chemical reaction is an indicator of the presence, absence, or concentration of another substance.
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Let’s do a real example problem of how you’d do a calculation for figuring out how much oxygen you would need for the complete combustion of 454 grams of propane.
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A decomposition reaction breaks a complicated molecule into simpler ones usually by heating, but not always. In fact, if you leave a bottle of hydrogen peroxide on the counter, it decomposes into water (H2O) and oxygen (O2) without any heating at all. 2H2O 2O2 + 2H2
A very common type of decomposition is shown by the chemistry of metal carbonates. Calcium, one of the most abundant elements on earth, usually is locked up in limestone, called calcium carbonate. CaCO3. When heated to about 1000 degrees C, it decomposes to make lime (a solid metal oxide) and CO2 gas. Chemical engineers make more then 348 million tonnes of lime to make steel, cement and other chemicals.
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If you have one element, like sulfur, which is S, and it’s a negative ion, just add “ide” to the end, like sulfide. Or if you have a carbon ion, it’s carbide. Nitrogen would be nitride, chlorine would be chloride.
If there’s more than one atom, especially if one of them is oxygen, then they have special names. The one with more oxygen atoms is the “ate” and the one with less is the “ite”. Sulfate has 4 oxygen atoms, and sulfite only has 3. Nitrate has three oxygen, and nitrite has only 2.
If there’s more than two ions, the one with the largest number of atoms gets the “per” and “ate”, like perchlorate. And the smallest one gets the “hypo” and “ite”, like hypochlorite.
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A lot of chemical reactions happen in a solution (it allows the chemicals to interact much more easily with each other when it is), so chemists define how much of the solute is in the solution by the term MOLARITY.
Molarity is a really convenient unit of concentration and it works like this. If I have 10 moles of solute in 10 liters of water, what’s the molarity? 10/10 = 1! So it’s a 1M solution. What if I have 20 moles in 10 liters? Then it’s a 2M solution. See how easy that is?
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Mole means “heap” or “pile” and is a unit for measuring the amount of a pure substance. It’s a chemist’s dozen. It’s a lot bigger than 12 though. It’s 6.022 x 10^23. So if you had a mole of eggs, you’d have… that huge number at the bottom of the slide. The most confusing part is this…
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Elements are arranged so that the ones with similar chemical and physical properties are stacked in vertical groups, and there are 8 groups (see the numbers at the top?) with either an A or B after the number? I know they’re written in Roman… just remember that IV means four, and VI means six. Sometimes you’ll see them numbered 1-18 starting with hydrogen on the left.
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The rows are called periods. Now point to the metals… what colors are those? There are lots of them!
Atoms are made of protons, neutrons, and electrons. The protons and the neutrons make up the nucleus (the center) of the atom. The electron lives outside the nucleus in an electron cloud and are way too small to see. Protons and neutrons are made up of smaller little particles, which are made of smaller little particles and so on. Atoms can have anywhere from only one proton and one electron (a hydrogen atom) to over 300 protons, neutrons and electrons in one atom. It is the number of protons that determines the kind of atom an atom is, or in other words, the kind of element that atom is. How many protons does Zinc have?
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Matter that is made of only one kind of atom is an element, like helium. Helium likes to hang out in groups of two helium atoms.
An atom is the smallest particle of an element that still has its chemical properties. If you have a gold atom and you split it into smaller parts (which you can do), it won’t still act like it did chemically as it did when it was a whole atom.
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When doing your experiments, you’ll often repeat an experiment again and again for various reasons. One reason is to make sure the experiment you’re doing is repeatable – it’s not just a one-time thing. You might also be checking to be sure you’ve done it right, or written down the amounts of chemicals correctly, or need to observe something you didn’t previously.
Precision measures how well your answers agree with each other from experiment to the next.
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Read the temperature from the thermometer… what do you get? This thermometer is reading in Celsius.
We’ll cover thermometers and the four temperature scales in a bit when we get to thermochemistry, but I just wanted to make sure we’re all on the same page when it comes to reading a thermometer, especially now that so many are digital and some kids may have not yet had the experience of reading a temperature scale.
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If you’re going to do a chemistry experiment, you’re going to use chemicals. How much of each one you use is going to change the results you get, so it’s important to find a way to accurately measure out the same amount of chemical each time.
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We already talked about how matter is anything that takes up space, like air, kittens, your left armpit… Mass can exist in different states. What are they?
Solid, liquid and gas. You also know about two more additional states: what are they? Plasma and BEC! Can matter exist in more than one state at a time? Sure – ever had a glass of water? That has liquid water and solid water molecules (ice) at the same time!
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Glow sticks generate light with very little heat, just like the glow you see from fireflies, jellyfish, and a few species of fungi. Chemiluminescence means light that comes from a chemical reaction. When this happens in animals and plants, it’s called bioluminescence.
In a glow stick, when you bend it to activate it, you’re breaking a little glass tube inside which contains hydrogen peroxide (H2O2). The tube itself is filled with another chemical (phenyl oxalate ester and a fluorescent dye) that is kept separate from the H2O2, because as soon as they touch, they begin to react. The dye in the light stick is what gives the light its color.
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Materials:
You’ll need a really dark room to see this reaction take place, as the amount of light it gives off is low, but it’s still there! Allow your eyes to adjust to the darkness for about 10 minutes, and you’ll definitely see a blue glow in the liquid.
The light comes from the copper sulfate reacting with the luminol, and will continue until one of the reactants is used up.
For advanced students, you can do this experiment with Cold Light.
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Everyone old enough to remember the Rubik’s Cube craze of the 1980s in the USA also remembers how it was near impossible to solve the thing! Originally created by a professor of architecture Erno Rubik, it was sold to a toy company in 1980 as the “Magic Cube”.
To date, over 350 million cubes have been sold worldwide, making it the world’s top selling puzzle game, and most people think of it as the best-selling toy of all time as well.
The original goal of creating this object was to help teach his students how to create something that rotated independently in layers without falling apart. Rubik didn’t realize he had created a puzzle until he scrambled it, and it took him over a month to solve it the first time!
There are eight corners and twelve edges, and when you do the math to figure out the number of possible combinations the puzzle has, it’s about 43 quintillion, or:
43,252,003,274,489,856,000
So what do you do with this thing? How DO you solve it?
It has to do with identifying the different layers, and solving one layer at a time. Here’s how you can do it:
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Download the official solver’s guide here. Or you can build a LEGO machine like JP Brown did to solve it for you!
There’s also a World Cube Association where folks keep track of cube competitions and records. The fastest cube solve was set by Mats Valk in 2013 – he can solve it in under 6 seconds. Some of the more creative competitions include solving the cube while blindfolded (record is 23.8 seconds), with only one hand (record is 12.6 seconds), only using the feet (record is 27.93 seconds), and underwater using a single breath.
Kaleidocycles are a three-dimensional paper sculpture you can turn around and round! Flexagons were first created by Arthur Stone at Princeton University in 1939, which were later published in 1959 to the general public in Scientific American.
These are simple to make and fun to play with. When I first showed them to my own kids, they immediately made one for each kid in their class, and also stumped the teacher that day when they asked how it worked. [am4show have=’p8;p9;p11;p38;p154;’ guest_error=’Guest error message’ user_error=’User error message’ ]
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Materials:
Click to download the car template or the animal template!
Now that you’ve made a kaliedocycle, let’s take it step further and make a flexagon. Flexagons looks like hexagons, but you can turn them inside out over and over. A hexaflexagon (this is the one we’re going to make in the next video) is made up of 19 triangles folded into six faces from a single strip of paper. You’ll need a strip of paper and the instructions below. (You can draw images on it when you’re done.)
Materials:
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The video below is made by Vi Hart, a smart and spunky mathemusician who has made amazing videos about the history of hexaflexagons that are fast-paced and fun.
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By controlling how and when a circuit is triggered, you can easily turn a simple circuit into a burglar alarm – something that alerts you when something happens. By sensing light, movement, weight, liquids, even electric fields, you can trigger LEDs to light and buzzers to sound. Your room will never be the same.
Switches control the flow of electricity through a circuit. There are different kinds of switches. NC (normally closed) switches keep the current flowing until you engage the switch. The SPST and DPDT switches are NO (normally open) switches.
The pressure sensor we’re building is small, and it requires a fair amount of pressure to activate. Pressure is force (like weight) over a given area (like a footprint). If you weighed 200 pounds, and your footprint averaged 10” long and 2” wide, you’d exert about 5 psi (pounds per square inch) per foot.
However, if you walked around on stilts indeed of feet, and the ‘footprint’ of each stilt averaged 1” on each side, you’d now exert 100 psi per foot. Why such a difference?
The secret is in the area of the footprint. In our example, your foot is about 20 square inches, but the area of each stilt was only 1 square inch. Since you haven’t changed your weight, you’re still pushing down with 200 pounds, only in the second case, you’re pressing the same weight into a much smaller spot… and hence the pressure applied to the smaller area shoots up by a factor of 20.
So how do we use pressure in this experiment? When you squeeze the foam, the light bulb lights up! It’s ideal for under a doormat or carpet rug where lots of weight will trigger it.
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Here’s what you need:
Download Student Worksheet & Exercises
Troubleshooting: There are a few problem areas to watch out for when building this sensor. First, make sure the hole in your foam is big enough to stick a finger (or thumb) easily through. The foam keeps the foil apart until stepped on, then it squishes together to allow the foil to make contact through the hole.
The second potential problem is if the switch doesn’t turn the buzzer off. If this happens, it means you’re bypassing the switch entirely and keeping the circuit in the constant ON position. Check the two foil squares – are they touching around the outside edges? Lastly, make sure your foam is the kind that pops back into shape when released. (Thin sponges can work in a pinch.)
What’s happening? You’ve made a switch, only this one is triggered by squeezing it. If you’re using the special black foam without the hole, it works because the foam conducts more electricity when squished together, and less when it’s at the normal shape.
First, the special black foam is conducting some (but not enough) electricity when you squeeze it. It’s just the nature of the black foam included with the materials kit. Second, when you squeeze it, you’re getting the two foil squares to touch through the hole, and this is what really does it for your LED. When you release it, the foil spreads apart again because they are on opposite sides of the foam square.
Bonus Idea: Stick just the sensor under a rug and run longer wires from the sensor to your room. When someone comes down the hallway, they’ll trigger the sensor and alert you before they get there!
Exercises
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This experiment is for advanced students. All chemical reactions are equilibrium reactions. This experiment is really cool because you’re going to watch how a chemical reaction resists a pH change.
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Materials:
We had two solutions that were both around 7. When we added an acid to one of them, the pH should have decreased. But why when we added the acid to the baking soda-carbonated soda solution, did it not change at all? That’s because it’s a buffer solution, which resists changes in pH.
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This experiment is for advanced students. Hydrolysis is a chemical reaction that involves breaking a molecular bond using water. In chemistry, there are three different types of hydrolysis: sat hydrolysis, acid hydrolysis, and base hydrolysis. In nature, living organisms survive by making their energy from processing food. The energy converted from food is stored in ATP molecules. To release the energy stored in food, a phosphate group breaks off an ATP molecule (and becomes ADP) using hydrolysis and releases energy from the bonds.
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Materials:
Put simply, hydrolysis is a chemical reaction that happens when a molecule splits into two parts when water is added. One part gains a hydrogen (H+) and the other gets the hydroxyl (OH–) group. The reaction in the experiment forms starch from glucose, and when we add water, it breaks down the amino acid components just like the enzymes do in your stomach when they digest food.
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This experiment is for advanced students. We’re going to look at the strength of redox reactions using copper, zinc, and acids.
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Materials:
We’re going to get zinc to react with different molecules in solution. You’re looking for a reaction that either changes the color of the nail, the solution, or forms tiny bubbles on the surface of the nail.
For the calcium carbonate, you’ll find tiny bubbles up and down the nail. The calcium ions are reduced and zinc ions are oxidized. For the copper sulfate, the nail changed color dramatically!
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Imagine you have a thin rope attached to a thick rope, and you jerk the thin rope so it creates a pulse that travels down the rope. When it hits the boundary between the two ropes, the wave just doesn’t stop and go away. Some of the energy from the wave is reflected back toward the source along the thin rope, and some of the energy is transmitted to the thicker (more dense) rope.
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Since light is a wave, when it goes from a less dense to a more dense medium, some of the energy gets reflected back while some of it gets transmitted through. Aim a flashlight at a window and you’ll find when the light goes from air to glass, it will both reflect back and transmit through the window.
When the light hits the glass, it not only reflects and transmits, it also changes speed and wavelength as it crosses the boundary AND it also changes directions. When it bends to change direction, it’s called refraction.
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If you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,282 miles per second), called optical density, and the result is bent light beams and broken pencils.
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When light rays strikes a surface, part of the beam passes through the surface and the rest reflects back, like a ball bouncing on the ground. Where it bounces depends on how you throw the ball.
Have you ever looked into a pool of clear, still water and seen your own face? The surface of the water acts like a mirror and you can see your reflection. (In fact, before mirrors were invented, this was the only way people had to look at themselves.) If you were swimming below the surface, you’d still see your own face – the mirror effect works both ways.
Have you ever broken a pencil by sticking it into a glass of water? The pencil isn’t really broken, but it sure looks like it! What’s going on?
When a beam of light hits a different substance (like the water), the wavelength changes because the speed of the light changes. If you’re thinking that the speed of light is always constant, you’re right… in a vacuum like outer space between two reference frames.
But here on Earth, we can change the speed of light just by shining a light beam through different materials, like water, ice, blue sunglasses, smoke, fog, even our own atmosphere. How much the light speed slows down depends on what the material is made of. Mineral oil and window glass will slow light down more than water, but not as much as diamonds do.
How broken the pencil appears also depends on where you look. In some cases, you’ll see a perfectly intact pencil. Other times, you’ll guess neither piece is touching. This is why not everyone can see a rainbow after it rains. The sun must be at a low angle in the sky, and also behind you for a rainbow to appear. Most times, you aren’t at the right spot to see the entire arc touch the ground at both ends, either.
Lenses work to bend light the way you want them to. The simplest lenses are actually prisms. Prisms unmix light into its different wavelengths. When light hits the prism, most of it passes through (a bit does reflect back) and changes speed. Since the sunlight is made up of many different wavelengths (colors), each color gets bent by different amounts, and you see a rainbow out the other side.
Here are a few neat activities that experiment with bending light, doubling your money, and breaking objects. Here’s what you do:
Materials:
Download Student Worksheet & Exercises
Here’s what you do:
1. Toss one coin into a water glass (pickle jars work great) and fill with an inch of water. Hold the glass up and find where you need to look to see TWO coins. Are the coins both the same size? Which one is the original coin? (Answer at the bottom of this page.)
2. Look through the top of the glass – how many coins are there now? What about when you look from the side?
3. Toss in a second coin – now how many are there?
4. Remove the coins turn out the lights. Shine a flashlight beam through the glass onto a nearby wall. (Hint – if this doesn’t work, try using a square clear container.) Stick a piece of paper on the wall where your light beam is and outline the beam with a pencil.
5. Shine the light at an angle up through the water so that it bounces off the surface of the water from underneath. Trace your new outline and compare… are they both the same shape?
6. Add a teaspoon of milk and stir gently. (No milk? Try sprinkling in a bit of white flour.) Now shine your flashlight through the container as you did in steps 4 and 5 and notice how the beam looks.
7. Use a round container instead of square… what’s the difference?
Answers:
1. The smaller coin is the reflection.
2. One coin when glanced from above, two from the side.
3. Four.
4. Beam is a circle.
5. Beam is an oval.
6. I can see the beam through the water!!
7. The round container distorts the beam, and the square container keeps the light beam straight. Both are fun!
The coin water trick is a neat way for kids to see how refraction works. In optics, refraction happens when light waves travel from one medium with a certain refractive index (air, for example) to another medium which has a different refractive index (like water). At the boundary between the two (where air meets water), the wave changes direction.
The wavelength increases or decreases but the frequency remains constant. When you sine light through a prism, the wavelength changes and you see a rainbow as the prism un-mixes white light into its different colors.The light wave changed direction when it traveled from air to glass, and then back to air again as it leaves the backside of the prism.
Did you try the pencil experiment? Did you notice how if you look at the pencil (placed at a slant) partially in the water, it appears to bend at the water’s surface? The light waves bend as they travel from water to air. To further complicate things, the way the eye received information about the position of the pencil actually makes the pencil to appear higher and the water shallower than they really are! Can you imagine how important this is for trying to spear a fish? The fish might appear to be in a different place, so you need to account for this when you take aim!
Click here for the Disappearing Beaker experiment!
Exercises
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You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil. When light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refraction does not occur.
However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil? (Hint: move the pencil back and forth slowly.)
The refractive index provides a measure of the relative speed of light in that particular medium which allows us to figure out speeds in other mediums as well as predict which way light will bend.
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We’re going to bend light to make objects disappear. You’ll need two glass containers (one that fits inside the other), and the smaller one MUST be Pyrex. It’s okay if your Pyrex glass has markings on the side. Use cooking oil such as canola oil, olive oil, or others to see which makes yours truly disappear. You can also try mineral oil or Karo syrup, although these tend to be more sensitive to temperature and aren’t as evenly matched with the Pyrex as the first choices mentioned above.
Here’s what you need:
Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph
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When a beam of light hits a different substance (like glass), the speed of light changes. The color of the light (called the wavelength) can also change. In some cases, the change of wavelength turns into a change in the direction of the beam.
For example, if you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,000 miles per second), called optical density, and the result is bent light beams and broken pencils.
You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil.
This is a very fine point about refraction: when light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refractions do not occur.
However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil? (Hint: move the pencil back and forth slowly.)
Depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts. Glass is optically denser than water, which is denser than air.
Here’s a chart:
Vacuum 1.0000
Air 1.0003
Ice 1.3100
Water 1.3333
Pyrex 1.4740
Cooking Oil 1.4740
Diamond 2.4170
This means if you place a Pyrex container inside a beaker of vegetable oil, it will disappear. This also works for some mineral oils and Karo syrup. Note however that the optical densities of liquids vary with temperature and concentration, and manufacturers are not perfectly consistent when they whip up a batch of this stuff, so some adjustments are needed.
Not only can you change the shape of objects by bending light (broken or whole), but you can also change the size. Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes.
Questions to Ask
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But why does light bend? You can imagine a toy car going from a wood floor to carpeting. One wheel hits the carpet first and slows down before the other, causing the toy to turn. The direction of the wave changes in addition to the speed. The slower speed must also shorten its wavelength since the frequency of the wave doesn’t change.
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The bottom line is that bending is caused by the change in speed of light when it crosses a boundary. This is true everywhere, even in the vacuum of space if it’s going from space to our atmosphere.
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Do you remember the eye balloon that you made earlier? The white portion of the balloon represents your sclera, which you may have already guessed is also the white part of your eye. It is actually a coating made of protein that covers the various muscle in your eye and holds everything together.
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Of course, the lens you inserted represents the actual lens in your eye. The muscles surrounding the lens are called ciliary muscles and they are represented by the rubber neck of your balloon. The ciliary muscles help to control the amount of light entering your eyes.
The retina is in the back of your eye, which is represented by the inside back of your balloon. The retina supports your rods and cones. They collect information about light and color and send it to your brain.
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How does light know which way to bend? It depends on whether the wave is speeding up or slowing down when it moves across the boundary, which depends on the optical density of the mediums.
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Wowza… you’ve made it through the ENTIRE course in Advanced Physics! Wa-hoo!!
Time for one more video… ready?
If you’ve ever tried to skewer something under the water from above it, you know that you can’t aim directly at the object, because of the way light bends when it goes from a slower to a faster medium. Can you guess the one condition where light doesn’t bend as it crosses a boundary?
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How much incident light bends as it crosses a boundary can be calculated and measured if we know about the mediums, including information about the index of refraction. Snell’s Law is a mathematical relationship between the refractive and incident angles of light and the optical density of the different mediums.
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If you’re scratching your head during math class, wondering what you’ll ever use this stuff for, here’s a cool experiment that shows you how scientists use math to figure out the optical density of objects, called the “index of refraction”.
How much light bends as it goes through one medium to another depends on the index of refraction (refractive index) of the substances. There are lots of examples of devices that use the index of refraction, including fiber optics. Fiber optic cables are made out of a transparent material that has a higher index of refraction than the material around it (like air), so the waves stay trapped inside the cable and travel along it, bouncing internally along its length. Eyeglasses use lenses that bend and distort the light to make images appear closer than they really are.
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Materials:
Download Student Worksheet & Exercises
Experiment:
Questions to Ask:
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Incoming light refracts as it crosses two boundaries of a prism. Notice how prisms have non-parallel sides for a reason…
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Now let’s take a look at how to use Snell’s Law to figure out the optical density of a medium by measuring how much the light bends when it goes from one medium to another.
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The Law of Reflection states that when light reflects off the surface, the angle of incidence is equal to the angle of reflection. Snell’s Law states that when light crosses into a new medium, the relationship between the angle of incidence (θi) and angle of refraction (θr) are related by the equation:
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where n is the index of refraction.
There’s actually a connection between light reflection and refraction, since they usually happen at the same time.
Total internal reflection happens at large incident angles and when light travels from a more optically dense medium to a lesser dense medium. Total refers to no loss in intensity (plane mirrors have a loss of about 4%).
For total internal reflection to occur, two things have to happen: light must be going from more optically dense to less dense mediums, and the angle of incidence is greater than the critical angle.
Total internal reflection happens when light travels from water to air, not from air to water. It also happens when light bends away from the normal at large angles of incidence. For water-to-air, it’s greater than 48.6o. Each set of mediums have their own critical angle.
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Let’s take a look at a glass prism and total internal reflection critical angles to determine the optical density of the glass.
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Let’s look at total internal reflection and diamonds.
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Fiber Optics are one application of total internal reflection. Optical fibers are flexible, transparent fibers made from plastic or glass about the size of a human hair that can serve as a "light pipe" to transmit light from one location to another. The bundle of fibers is used in medical applications where doctors can see inside the body by attaching a small camera to one side of the cable. To make the project below, you can order this Fiber Optics kit.
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Materials:
Lightwave communication over optical fiber networks are used today everywhere in fiber optic communications. These transmit over long distances at higher bandwidths than metal cables and don't have problems with electromagnetic interference or losses typical with copper wiring.
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Dispersion is when visible light is separated into the colors that make up the light. We’ve already seen how optical density is a measure of how much a medium slows down light that travels through it. The index of refraction depends on the frequency of the light.
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The index of refraction is 1.51 for red light and 1.53 for violet, which means that as light goes through glass, it slows down the violet light just a little it more than it does the red. Because of this, prisms can unmix light by dispersion because the prism has two (or more) boundaries where this effect adds to separate white light into its colors.
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In this experiment, water is our prism. A prism un-mixes light back into its original colors of red, green, and blue. You can make prisms out of glass, plastic, water, oil, or anything else you can think of that allows light to zip through.
What’s a prism? Think of a beam of light. It zooms fast on a straight path, until it hits something (like a water drop). As the light goes through the water drop, it changes speed (refraction). The speed change depends on the angle that the light hits the water, and what the drop is made of. (If it was a drop of mineral oil, the light would slow down a bit more.) Okay, so when white light passes through a prism (or water drop), changes speed, and turns colors. So why do we see a rainbow, not just one color coming out the other side?
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The secret is because the light is made up of different wavelengths, and each gets bent by different amounts when they hit a new material. So one wave changes speed to red, another to yellow, another to green, etc. when the beam hits the prism. And water drops are tiny prisms.
The light passing through a water drop gets refracted twice, not once. The first time is when it enters the water drop, the second when it bounces off the other side of the drop and reflects back through the water drop and out again (some of the light does make it out the other side of the drop, but most of it bounces back). When the light emerges from the water drop, it changes speed again, and presto! You have a rainbow.
Natural rainbows (the ones that you see after it rains) happen when water drops (tiny prisms) in the air are hit by sunlight from behind you at just the right angle (which is relatively a low angle, near the ground). The best rainbows can be seen when half of the sky is darkened with rainclouds and you’re in a clear patch with sun behind you. And guess what? You can even see a nighttime rainbow (called a moonbow), although they’re pretty rare, usually near full moon.
Here’s what you do:
Materials:
Download Student Worksheet & Exercises
Set a clear tray of water in sunlight. Lean a mirror against an inside edge and adjust so that a rainbow appears on the wall. You can also use a light bulb shining through a slit in a flat cardboard piece as a light source.
Troubleshooting: This is one of the easiest experiments to do, and the most beautiful. The trouble is, you don’t know where the water shadow will show up, so make sure you point the mirror to the sky and play with the angle of the mirror until you find the wavering rainbow. Because the shadow is constantly moving, you can snap a few pictures when you’ve got it so you can look over the finer details later. If this project still eludes you, take a large sheet and use it instead of the tiny index card.
Exercises
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Ever notice how water has to be involved before you get a rainbow? Rainbows never happen on dry, clear days.
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I remember how surprised I was when I saw a rainbow appear on a cloudless day while I was misting a soapy car with the garden hose. I was so amazed that the arc was larger than I realized that I climbed up a ladder before I realized that I could make the rainbow form in a cull circle!
Moonbows (also known as lunar rainbows) form from light reflected off the moon form in the atmosphere. Since they are formed from reflected sunlight, they tend to be very faint. If you want to find one, look in the opposite part of the sky from the moon. It will look like a white instead of the usual rainbow colors, but that’s because the eye has a hard time seeing colors in the dark. If you take a long-exposure photograph, the colors will appear. Aristotle himself recorded observing moonbows on dark nights when the weather conditions were just right!
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Spectrometers are used in chemistry and astronomy to measure light. In astronomy, we can find out about distant stars without ever traveling to them, because we can split the incoming light from the stars into their colors (or energies) and “read” what they are made up of (what gases they are burning) and thus determine their what they are made of. In this experiment, you’ll make a simple cardboard spectrometer that will be able to detect all kinds of interesting things!
SPECIAL NOTE: This instrument is NOT for looking at the sun. Do NOT look directly at the sun. But you can point the tube at a sheet of paper that has the sun’s reflected light on it.
Usually you need a specialized piece of material called a diffraction grating to make this instrument work, but instead of buying a fancy one, why not use one from around your house? Diffraction gratings are found in insect (including butterfly) wings, bird feathers, and plant leaves. While I don’t recommend using living things for this experiment, I do suggest using an old CD.
CDs are like a mirror with circular tracks that are very close together. The light is spread into a spectrum when it hits the tracks, and each color bends a little more than the last. To see the rainbow spectrum, you’ve got to adjust the CD and the position of your eye so the angles line up correctly (actually, the angles are perpendicular).
You’re looking for a spectrum (the rainbow image at left) – this is what you’ll see right on the CD itself. Depending on what you look at (neon signs, chandeliers, incandescent bulbs, fluorescent bulbs, Christmas lights…), you’ll see different colors of the rainbow. For more about how diffraction gratings work, click here.
Materials:
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Download Student Worksheet & Exercises
Find an old CD and a cardboard tube at least 10 inches long. Cut a clean slit less than 1 mm wide in an index card or spare piece of cardboard and tape it to one end of the tube. Align your tube with the slit horizontally, and on the top of the tube at the far end cut a viewing slot about one inch long and ½” inch wide. Cut a second slot into the tube at a 45 degree angle from the vertical away from the viewing slot. Insert the CD into this slot so that it reflects light coming through the slit into your eye (viewing slot).
Aim the 1 mm slit at a light source such as a fluorescent light, neon sign, sunset, light bulb, computer screen, television, night light, candle, fireplace… any light source you can find. Look through the open hole at the light reflected off the compact disk (look for a rainbow in most cases) inside the cardboard tube.
Troubleshooting: This is a quick and easy way to bypass the need for an expensive diffraction grating. Use your spectrometer to look at computer screens, laptops, night lights, neon lights, candles, campfires, fluorescent lights, incandescent lights, LEDs, stoplights, street lights, and any other light sources you can find, even the moon through a telescope.
Exercises
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Mirages happen on sunny days when the roads are heated by the sun to a point where it also heats the air above the road. Since hot air is less (optically) dense that cool air, the light refracts as it travels through it.
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Like sound, light travels in waves. These waves of light enter your eyes through the pupil, which is the small black dot right in the center of your colored iris. Your lens bends and focuses the light that enters your eye. In this experiment, we will study this process of bending light and we will look at the difference between concave and convex lenses.
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Here’s what you need
Download Student Worksheet & Exercises
Here’s what you do
What’s going on?
You can see that a convex lens bends outward and a concave lens bends inward. What does this do to light?
In a convex lens, the domed surface means that if light waves come in through the flat bottom surface, they will be spread out, or refracted, as they exit the curved portion of the lens. But since a concave lens dips inward it creates the opposite effect. When light waves exit the concave surface, they are brought together. This makes images appear smaller.
The lens does all the focusing work but it is actually the shape of the eye that determines what you see. If you have a tall, oblong eye, you are far-sighted. And conversely, if your eyes are short and fat, you are near-sighted. In either case, the lenses are functioning properly but the actual shape of the eye needs a slight adjustment.
Exercises
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When I was in grad school, I needed to use an optical bench to see invisible things. I was trying to ‘see’ the exhaust from a new kind of F15 engine, because the aircraft acting the way it shouldn’t – when the pilot turned the controls 20o left, the plane only went 10o. My team had traced the problem to an issue with the shock waves, and it was my job to figure out what the trouble was. (Anytime shock waves appear, there’s an energy loss.)
Since shock waves are invisible to the human eye, I had to find a way to make them visible so we could get a better look at what was going on. It was like trying to see the smoke generated by a candle – you know it’s there, but you just can’t see it. I wound up using a special type of photography called Schlieren.
An optical table gives you a solid surface to work on and nails down your parts so they don’t move. This is an image taken with Schlieren photography. This technique picks up the changes in air density (which is a measure of pressure and volume).
The air above a candle heats up and expands (increases volume), floating upwards as you see here. The Schlieren technique shines a super-bright xenon arc lamp beam of light through the candle area, bounces it off two parabolic mirrors and passes it through a razor-edge slit and a neutral density filter before reaching the camera lens. With so many parts, I needed space to bolt things down EXACTLY where I wanted them. The razor slit, for example, just couldn’t be anywhere along the beam – it had to be right at the exact point where the beam was focused down to a point.
I’m going to show you how to make a quick and easy optical lab bench to work with your lenses. Scientists use optical benches when they design microscopes, telescopes, and other optical equipment. You’ll need a bright light source like a flashlight or a sunny window, although this bench is so light and portable that you can move it to garage and use a car headlight if you really want to get creative. Once your bench is set up, you can easily switch out filters, lenses, and slits to find the best combination for your optical designs. Technically, our setup is called an optical rail, and the neat thing about it is that it comes with a handy measuring device so you can see where the focal points are for your lenses. Let’s get started:
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Materials:
Your lenses are curved pieces of glass or plastic designed to bend (refract) light. A simple lens is just one piece, and a compound lens is like the lens of a camera – there’s lots of them in there. The first lenses were developed by nature – dewdrops on plant leaves are natural lenses. The light changes speed and bends when it hits the surface of the drop, and things under the drop appear larger. (Read more about refraction here.) The earliest written records of lenses are found in the Greek archives and described as being glass globes filled with water.
Concave lenses are shaped like a ‘cave’ and curve inward like a spoon. Light that shines through a concave lens bends to a point (converging beam). Ever notice how when you peep through the hole in a door (especially in a hotel), you can see the entire person standing on the doorstep? There’s a concave lens in there making the person appear smaller.
You’ll also find these types of lenses in ‘shoplifting mirrors’. Store owners post these mirrors around help them see a larger area than a flat mirror shows, although the images tend to be a lot smaller.
If you have a pair of near-sighted glasses, chances are that the lenses are concave. Near-sighted folks need help seeing things that are far away, and the concave lenses increase the focal point to the right spot on their retina.
Concave lenses work to make things look smaller, so there not as widely used as convex lenses. You’ll find concave lenses inside camera lenses and binoculars to help clear weird optical problems that happen around the edges of a convex lens (called aberration).
Here’s a video on lenses, both convex and concave:
Convex lenses bulge outwards, bending the light out in a spray (diverging beam). A hand-held magnifying glass is a single concave lens with a handle. These lenses have been used as ‘burning glasses’ for hundreds of years – by placing a small piece of paper at its focal point and using the sun as a light source, you can focus the light energy so intensely that you reach the flash point of the paper (the paper auto-ignites around 450oF).
When you stack a large convex lens above a solar panel, the magnification effect makes it so you can get away with using a smaller photovoltaic cell to get the same amount of energy from the sun. You’ll find convex lenses in telescopes, microscopes, binoculars, eyeglasses, and more.
What if you coat one side of the lens with a reflecting silver coating? You get a mirror!
Stick wooden skewers into a piece of foam to simulate how the light rays reflect off the surface of the mirror. Note that when the mirror (foam) is straight, the light rays are straight (which is what you see when you look in the bathroom mirror). The light bounces off the straight mirror and zips right back at you, remaining parallel.
Now arch the foam. Notice how the light ways (skewers) come to a point (focal point).
After the focal point, the rays invert, so the top skewer is now at the bottom and the bottom is now at the top. This is your flipped (inverted) image. This is what you’d see when you look into a concave mirror, like the inside of a metal spoon. You can see your face, but it’s upside-down.
A slit allows light from only one source to enter. If you have light from other sources, your light beam is more scattered and your images and lines become blurry. Thin slits can be easily made by placing the edges of two razor blades very close together and securing into place. We’re going to use an anti-slit using a piece of hair, but you can substitute a thin needle.
Here’s a video on using filters and slits with your laser:
There are hundreds of different types of filters, used in photography, astronomy, and sunglasses. A filter can change the amount and type of light allowed through it. For example, if you put on red-tinted glasses, suddenly everything takes on a reddish hue. The red filter blocks the rest of the incoming wavelengths (colors) and only allows the red colors to get to your eyeball. There are color filters for every wavelength, even IR and UV.
UV filters reduce the haziness in our atmosphere, and are used on most high-end camera lenses, while IR filters are heat-absorbing filters used with hot light sources (like near incandescent bulbs or in overhead projectors).
A neutral density (ND) filter is a grayish-colored filter that reduces the intensity of all colors equally. Photographers use these filters to get motion blur effects with slow shutter speeds, like a softened waterfall.
It’s time to put all these pieces together and make cool optical stuff – are you ready?
Download Student Worksheet & Exercises
Click here for more experiments on building your own microscope and telescope.
Corner reflectors are U-turns for light beams. A corner mirror made from three mirrors will reflect the beam straight back where it came from, no matter what angle you hit it at. Astronauts placed these types of mirrors on the moon so scientists could easily bounce laser beams off the moon and have them return to the same place on Earth. They used these reflected laser beams to measure the speed of light.
You’ll find corner mirrors in “cat’s eye” reflectors on the road. Car headlights illuminate the reflectors and send the beam straight back the same way – right at the driver.
Exercises
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Convex lenses bulge outwards, bending the light out in a spray (diverging beam). A hand-held magnifying glass is a single concave lens with a handle. These lenses have been used as ‘burning glasses’ for hundreds of years – by placing a small piece of paper at its focal point and using the sun as a light source, you can focus the light energy so intensely that you reach the flash point of the paper (the paper auto-ignites around 450oF).
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When you stack a large convex lens above a solar panel, the magnification effect makes it so you can get away with using a smaller photovoltaic cell to get the same amount of energy from the sun. You’ll find convex lenses in telescopes, microscopes, binoculars, eyeglasses, and more.
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Thin lenses are either diverging or converging lenses that aren’t very thick in the middle. We can simplify our ray tracing diagrams and our math equations by assuming a lens is thin.
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Converging lenses take incoming light and focus it down to a point before diverging out again. You can have single or double convex lenses, depending on the shape of the lens.
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But what does the image look like? Remember the line of sight principle where in order to see an object, you have to be able to sight along a line at that object? This idea is how images are formed and what they will look like.
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