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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The one I’m demonstrating with water in a syringe is the absolute easiest to do. The second easiest is carbon dioxide (CO₂): Triple Point: -56.6°C (216.6 K), 5.2 atm (517 kPa). Dry ice (solid CO₂) can be used to observe sublimation and approach the triple point if you control the pressure. A sealed pressure chamber with temperature control (e.g., a strong, transparent pressure vessel) could bring CO₂ to its triple point. Achieving 5.2 atm is challenging but possible with pressure-rated containers (note: a soda bottle can hold around 3 atm, so do NOT put it in a soda bottle because it will burst). This is best done by a chemist or other professional.
Is there a compound that its critical or triple point could be at something close enough to standard conditions that we would be able to do it at home.