Water triple point?


I’m five, and I’d like a simple way of understanding water’s triple point.

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The triple point is the specific temperature where water can exist in all three common states of matter – solid, liquid, and gas. It is 32.02°F or about 0.01°C.

If you put water into a vacuum chamber and keep removing all gas, it starts boiling. Boiling takes energy, but there is no heat source. Thus the water gets colder and colder. And it keeps going, because nothing is really stopping water from evaporating/boiling away from its liquid form.

It only stops when the water gets so cold that it freezes! And the freezing point is almost unaltered by (sane amounts of) pressure, so close to 0°C. Now you get some ice forming in the liquid water, but it also is still somewhat boiling. Thus you found a condition where water has all common states at once: solid, liquid, gaseous. This is the _triple point_!

The same more or less works for most pure substances. Stuff gets very complicated if there are mixtures, though.

Water has 3 states of matter.

Liquid, frozen and vapour.

What state of matter it is in, depends on two things; Temperature and pressure.

Example: We are used to dealing with mostly the same pressures, and only changes in temperature. But if you were to go up a mountain, you would notice that water boils only at like 70-90 degrees C instead of the usual 100.

You can think of it like this: The water is made of small drops, high temperature makes drops go move around a lot, high pressure pushes the water together. Frozen: The water is tight, liquid: The water is somewhat loose, and vapour: the water is very far from each other.

So because the state of water depends on both pressure and vapour, the 3 states of matter meet at one point where a small difference in temperature or pressure results in a jumping to a different phase of matter. You can see it here: [https://64.media.tumblr.com/68160af16a7f26384d0ca5a9497d2e30/tumblr_mp7bjokR3g1qa0fruo1_1280.jpg](https://64.media.tumblr.com/68160af16a7f26384d0ca5a9497d2e30/tumblr_mp7bjokR3g1qa0fruo1_1280.jpg)

Water is a liquid. Ice is solid. Water vapor (which I’m just going to call “vapor” to not repeat the word “water” as much) is a gas.

If you heat up water, it turns into vapor. Cool it down, you get ice.

But it takes some energy to switch over. If water on earth is exactly 0C, some of it will freeze, but not all of it. Likewise, water doesn’t all boil when you get to 100C – you can have both water and vapor at that temperature.

But also, the temperatures that these happen change if you remove pressure. Remove too much pressure, and you can’t have water: ice turns directly into gas. This is what happens with dry ice (CO2) – it turns straight from solid to gas.

The “triple point” is the name for when you have the pressure just right so that the boiling point and freezing point are exactly the same – which means that ice is melting to water, then boiling to vapor, at the same temperature. All three – ice, water, and vapor exist at the same temperature and pressure. Lower pressure at all, and water can’t exist. Raise pressure, and there’s a temperature that water is stable at (not freezing or boiling). But at just the right pressure, there’s a temperature that all three can exist.

Water can exist in three forms, as a solid (ice), liquid (water) and a gas (steam). You probably see and drink liquid water every day. During the winter, depending where you live, you probably encounter solid water ice frequently as well. When you heat water up, like on a stove, you can see the mist from water gas, steam.

Water changes between these three forms depending on the temperature. Cold, it is ice. Medium, it is liquid, and hot, it is steam. Water also can change forms when pressure changes. If you take water up onto a high mountain where the pressure is low, it turns into steam at a lower temperature. If you were to go in a pressure chamber, it would change into steam at a higher temperature.

At some temperatures, water can exist in two forms at once. For instance, when it is freezing out, you can have both puddles of water and snow at the same time, without the puddles freezing or the snow melting. Similarly, when you boil water, the water doesn’t boil all at once, but it exists as steam and hot liquid water at the same time. You could consider these to be “double points” because water exists in two forms. There is also a set of “triple points” for water, where it can exist as a solid, liquid, and gas, all at the same time. This isn’t a condition that occurs naturally on Earth, because it requires very low pressure, closer to what you might find on Mars.