Temperature and State Changes

At sea level, water freezes at 32 °F (0 °C) and boils at 212 °F (100 °C). These are the temperatures at which water changes state.

When a liquid boils (changes to a gas), it absorbs heat. When a gas condenses (changes back to a liquid), it gives off heat.

Water requires one BTU of heat per pound to rise one degree Fahrenheit. If you place one pound of water at 32 °F in a container over a flame, its temperature rises 1 °F for each BTU of heat the water absorbs from the flame. Once the water has reached a temperature of 212 °F, it has absorbed 180 BTUs of heat.

As the flame continues to heat the water, it boils, changing from a liquid to a gas, and it continues to boil until all of it has changed to a gas.







If this gas is collected in a container and checked with a thermometer; it would also have a temperature of 212 °F The temperature has not risen further, but the flame has applied an additional 970 BTUs of heat. The heat is absorbed by the liquid as it boils. It is "hidden" in the water vapor.







If the vapor contacted cool air, the heat would flow into the cooler air as the vapor condensed back into water. This hidden heat is called the "latent (hidden) heat of vaporization."







Water has a latent heat of vaporization of 970 BTUs. This means one pound of water at 212 °F will absorb 970 BTUs of heat when it boils and becomes a vapor. In the same way, the vapor will give off 970 BTUs of heat when it condenses back to water.

Latent Heat of Vaporization







In other words, water acts like a "heat sponge": it soaks up a small amount of heat (180 BTUs per pound) when its temperature rises from 32 °F to 212 °F; and it soaks up a large amount of heat (970 BTUs per pound) when it changes from a liquid to a gas.