To most people, calories are usually associated with a very large piece of chocolate cake. But in that case, they are called nutritionist’s calories, or the unit of energy-producing potential equal to the amount of heat that is contained in food and released upon oxidation by the body. The body needs the calories in the foods we eat to use as energy. This is why nutrition and weight-control texts often contain such entries as “a 140 pound person walking for one hour at a moderate pace burns off 222 calories.”

In chemistry, a calorie also refers to a unit of energy. But in terms of chemical experiments, a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius from a standard initial temperature at a pressure of 1 atmosphere (sea level). The unit measurement for energy is a joule, in which 1 calorie equals 4.184 joules; 1 joule is translated (most often in metric) as the energy needed to lift 2,000 grams a distance of 10 centimeters.