When objects at different temperatures are placed in contact with each other but isolated from everything else, they attain thermal equilibrium. A container that prevents heat transfer in or out is called a calorimeter, and the use of a calorimeter to make measurements is called calorimetry. Generally, these measurements involve heat or specific heat capacity. The term "calorimetry problem" is used for any problem where the specified objects are thermally isolated from their surroundings. An important objective in solving calorimetry problems is that during a heat transfer between objects isolated from their surroundings, the heat gained by the colder object must equal the heat lost by the hotter object due to the conservation of energy.
For example, consider a cup of water poured into a hot aluminum pan placed off the stove. Here, the pan is placed on an insulated pad, and heat transfer to the air is neglected in the short time needed to reach equilibrium. As a result, this is a calorimetry problem, even though no isolating container is specified. Originally, the pan and water were not in thermal equilibrium. The pan is at a higher temperature than the water. Heat transfer restores thermal equilibrium once the water and the pan are in contact; it stops once thermal equilibrium between the pan and the water is achieved. The heat lost by the pan equals the heat gained by the water, which is the basic principle of calorimetry.