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. 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.
Because of these numbers, we can get water to boil here on Earth while it stays at room temperature by changing the pressure using everyday materials. (If you have a vacuum pump, you can have the water boil at the freezing point of 32oF.)
Here’s what you need to do:
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Materials:
- plastic syringe (no needle)
- room temperature water
Bonus Idea: Do this experiment first with water, then with carbonated water.
Why does that work? How did you get the pressure to decrease? Easy – when you pulled on the plunger and increased the volume inside the syringe. Since your finger covered the hole, no additional air was allowed in when you did this (which is why it was probably a little tough to do), so the number of molecules inside the syringe stayed the same, but the space they had to wiggle around got a lot bigger, meaning that the pressure decreased.
The air inside the syringe isn’t just plain old air… it has water vapor inside, too. And that’s not all – the water from your sink isn’t just plain old water, it has air bubbles mixed in with it. When you brought down the pressure (by pulling the plunger), you are forcing the air bubbles to come out of the water, which makes it boil. When you shove the plunger back in and increase the pressure, you’ll find that the air bubbles mix back into the water and disappears.
Did you try the soda water yet? Soda has carbon dioxide already mixed in for you, which is under pressure. You can release this pressure by opening the bottle (you’ll hear a PSSST!), which is the carbon dioxide bubbles coming out of the soda. Go ahead and try that now before reading further…
When you place the soda water into the syringe and decrease the pressure, the carbon dioxide comes out quickly Try tapping the syringe to make all the tiny bubbles combine into one larger bubble. When you increase the pressure (push the plunger back in), some of the bubbles will redissolve back into the soda.
If you’ve ever had a glass of hot water suddenly erupt in an explosion of bubbles, you’ve experienced superheated water (water that’s above it’s normal boiling point) that hasn’t been able to form bubbles yet. By adding a tea bag or simply just jiggling it around is usually enough to cause the bubbles to start, which often splatters HOT HOT water everywhere. (This isn’t something you want to try without adult help.)
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