Question #97168

Let us do a couple experiments.

1. Heat some water on a stove until it starts to boil. Then turn the stove off and observe that it quickly stops boiling. The water is hot enough to keep boiling, so why does it stop?

2. After taking a shower step out and note that if you do not dry off immediately you feel cold. Why?

In both cases boiling or evaporation of water is not just a matter of temperature. It takes additional energy, too. In Experiment 1 you had to keep the stove on to supply this energy as additional heat. In Experiment 2 the energy to evaporate water came from your body heat making you feel cold.

This extra energy required to change a liquid to a gas, or change a solid or liquid to a gas is called latent heat. It's stored or "latent" without contributing to the temperature, because the energy went into forming the higher energy phase instead.

In the case of water the latent heat is exceptionally high, making the experiments above easy to see. Converting solid ice to liquid water under normal conditions requires a latent heat of #80"cal/g"#. For converting liquid water to steam or vapor it's a whopping #540"cal/g"#.