Cooling and heating are opposite processes. Cooling is the removal of heat energy from an object or space and heating is the addition of heat energy to an object or space. We use these opposite processes a great deal in our daily lives. For example, in the kitchen we use the cooling provided by a refrigerator to keep food cold. We also use the heat from a stove to cook food.
Nearly 75 percent of the energy used by the average family household in the United States goes for cooling and heating purposes. Air conditioning and refrigeration are the major cooling requirements of a home, while water and space heating are the most important heating requirements.
In the experiments that follow you will learn more about cooling and heating. You will also learn alternative ways of cooling and heating, using such unusual materials as gases, salts, water, and trees.
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Materials
- Epsom salt
- Aluminum pie pan
- Water
- Oven
- Insulated mitt
- Two small, zipper-close, plastic bags
- Sink
- Measuring cups
Procedure
ASK AN ADULT TO HELP YOU WITH THIS EXPERIMENT. DO NOT USE THE STOVE BY YOURSELF.
Ask an adult to turn on the oven and to set the temperature of the oven to 450° F
(232°C). Pour one-half cup of Epsom salt into an aluminum pie pan and gently shake it to evenly spread the Epsom salt over the bottom of the pan. Place the pie pan in the oven. Heat the Epsom salt in the hot oven for thirty minutes.
Ask an adult to remove the pan of Epsom salt from the hot oven using an insulated mitt. Place the pan on the stovetop and allow it to cool for ten minutes. Make sure the oven is turned off.
Add one-quarter cup of the Epsom salt that was not heated in the oven to a small plastic zipper-close bag. Next add one-quarter cup of room temperature water to the bag, seal, and shake the bag. Feel the temperature of the outside of the bag.
Next add one-quarter cup of the cooled Epsom salt that was heated in the oven to the second zipper-close bag. Add one-quarter cup of room temperature water to the bag and seal the bag. Give the bag a couple of shakes and then feel the outside of the bag.
When you are finished with this experiment, pour the contents of both bags down a sink drain. Then flush the bags and the sink with water. Also rinse out the aluminum pie pan with water. DO NOT DRINK ANY OF THE LIQUID AND DO NOT EAT ANY OF THE EPSOM SALT. EPSOM SALT CAN MAKE YOU SICK IF YOU EAT IT.
Observations
Do you notice any difference in the appearance of the Epsom salt after it is heated in the oven? Does the bag containing the water and the Epsom salt that was not heated feel warm or cool after shaking? Does the bag containing the water and the Epsom salt that was heated feel warm or cool after shaking?
Discussion
Epsom salt is a hydrate of the salt called magnesium sulfate. A hydrate is a chemical substance containing water combined with another chemical substance (usually a salt). The water molecules in a hydrate are called waters of hydration. They can usually be removed from the hydrate by heating. The process of removing water from a hydrate is called dehydration.
In this experiment, when you heated the Epsom salt in the oven, you removed most of the water molecules (waters of hydration) in the salt.
A salt is made of positive and negative ions. Ions are charged atoms or groups of atoms. In Epsom salt, magnesium ions are positive and sulfate ions are negative. The waters of hydration surround these ions in Epsom salt.
You should find that the bag containing water and the dehydrated Epsom salt feels warm. The bag containing water and the hydrated Epsom salt feels cool. When dehydrated Epsom salt is mixed with water, heat is given off. When hydrated Epsom salt is mixed with water, heat is absorbed. When something gives off heat it feels warm, while something that absorbs heat feels cool.
Energy is required to remove individual ions from a salt crystal. However, energy is given off when the individual ions that break away from the crystal become surrounded by water molecules that are dissolving the salt. If more energy is required to remove individual ions from a salt crystal than is given off when the ions become surrounded by water, then the salt solution becomes cold. If more energy is given off when the ions become surrounded by water than is needed to remove individual ions from a salt crystal, then the salt solution becomes warm.
When hydrated Epsom salt dissolves in water, more energy is required to remove the magnesium and sulfate ions (and the waters of hydration) from the crystals than is given off when the magnesium and sulfate ions become surrounded by the dissolving water molecules.
This is why the bag containing the unheated Epsom salt became cold when you added water.
On the other hand, when dehydrated Epsom salt dissolves in water, more energy is given off by the ions becoming surrounded by water molecules than is needed to break the magnesium and sulfate ions from the crystals. This is why the bag containing the dehydrated Epsom salt became warm when you added water.
Most instant hot packs and cold packs that are available in drugstores work on the principle just discussed. When the cold or hot pack is needed, the bag is squeezed to cause the water and salt to mix. Depending on the salt used in the pack, energy is either absorbed (cold pack) or given off (hot pack). Ammonium nitrate is the most commonly used salt in cold packs. And calcium chloride is the most commonly used salt in hot packs.
Other Things to Try
Repeat this experiment using a thermometer to measure the temperature change when the Epsom salt is dissolved in water. Use the thermometer to measure the temperature change when dehydrated magnesium sulfate is dissolved in water.
Repeat this experiment using table salt. Do you observe a temperature change when the table salt dissolves in water?
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A baking sheet will be too big and it is difficult to pour from a baking sheet. It is best to use an aluminum pie dish.
Can we use a baking sheet with sides to heat the salt or do we need to only use an aluminum pie dish?
Oops – I made a mistake. Yes, hotter is better as it will dry out the crystals more thoroughly.
In the live class, I thought you said 250 degrees, yet it says 450 degrees in this video. Which is correct?
Student worksheets are right under the videos. Not every experiment has these yet – we are still creating them. There’s about 700 finished right now however!
where is the exercise sheets?