Heat, like work (processes that transfer energy), is measured in the joule, a unit named after James Prescott Joule. Although the joule is the international standard for measuring energy, heat flow is often measured in calories. A calorie defines the amount of energy needed to increase one gram of water by one degree Celsius. The energy required for one calorie is 4.186 joules, which is a relatively small amount of energy. Nutritionists also use the term “calorie” to describe the amount of energy a particular food can provide a person. A nutritional calorie is really a kilocalorie (1,000 calories), which is also written as the capitalized “Calorie.”

Another unit used to measure heat flow is the British Thermal Unit, or Btu. The Btu is the amount of energy needed to increase the temperature of one pound of water by one degree Fahrenheit. This unit, which is used only by countries such as the United States that still employ the English standard method of measurements, is equal to 252 calories.

A substance does not have to boil to change from liquid to gas state. The boiling point is where the pressure of the water vapor equals the atmospheric pressure. At all temperatures there is a great variation in the kinetic energy of the molecules that make up the substance. When the molecules with high energy reach the surface they have the possibility of escaping into the air. The process of changing from liquid to gas at a temperature below the boiling point is called evaporation.

When the molecules with higher energy leave the liquid the remaining molecules have a lower average energy, and thus a lower temperature. It is cooled by evaporation.