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