Calorimetry is a method of measuring the heat transfer within a chemical reaction or other physical processes, such as a change between different states of matter.
The term "calorimetry" comes from the Latin calor ("heat") and Greek metron ("measure"), so it means "measuring heat." Devices used to perform calorimetry measurements are called calorimeters.
How Calorimetry Works
Since heat is a form of energy, it follows the rules of conservation of energy. If a system is contained in thermal isolation (in other words, heat cannot enter or leave the system), then any heat energy that is lost in one part of the system has to be gained in another part of the system.
If you have a good, thermally-isolating thermos, for example, that contains hot coffee, the coffee will remain hot while sealed in the thermos. If, however, you put ice into the hot coffee and re-seal it, when you later open it, you will find that the coffee lost heat and the ice gained heat… and melted as a result, thus watering down your coffee!
Now let's assume that instead of hot coffee in a thermos, you had water inside a calorimeter. The calorimeter is well insulated, and a thermometer is built into the calorimeter to precisely measure the temperature of the water inside. If we were to then put ice into the water, it would melt-just like in the coffee example. But this time, the calorimeter is continually measuring the temperature of the water. Heat is leaving the water and going into the ice, causing it to melt, so if you watched the temperature on the calorimeter, you'd see the temperature of the water dropping. Eventually, all of the ice would be melted and the water would reach a new state of thermal equilibrium, in which the temperature is no longer changing.
From the change in temperature in the water, you can then calculate the amount of heat energy that it took to cause the melting of the ice. And that, my friends, is calorimetry.