At 100^o C, water boils at atmospheric pressure. This means that the liquid is able to turn into vapor and the air gets pushed away. If we attempt to raise the temperature above 100^oC, the liquid water turns into water vapor as fast as energy is added: the water is boiling and the temperature does not change at all. At high elevations the air pressure is less, and water boils at a lower temperature. The graph shows how the boiling temperature varies with pressure: for example, if the pressure is 70% of normal atmospheric pressure, the boiling temperature is reduced to 90^oC. In the pink region above the curve, liquid water can exist (we are below the boiling temperature), while in the white region, we have only water vapor.

This graph is closely related to the one on the previous page. "Vapor pressure" is proportional to the amount of water in the air, with a scale that varies a little with temperature.

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What have we learned about changes of phase?