When two bodies at different temperatures are brought in contact, heat is transformed from the body at a higher temperature to that at a lower temperature. So, the former starts to cool down whereas the latter starts to warm up. The flow of heat continues till both reach the same temperature. The state where both bodies are at the same temperature is called thermal equilibrium.
Now, we know that heat is a form of energy. According to the law of conservation of energy, energy can neither be created nor destroyed. So during such heat transfers, if the heat doesn’t transform into another form of energy then,
heat released by a body = heat absorbed by another body
This is the fundamental principle of calorimetry. This is also true for one number of bodies brought in contact with one another. In that case, we can write down the principle of calorimetry as:
heat released by the hot bodies = heat absorbed by the cold bodies
Basic Principle of Calorimetry:
With the help of a calorimeter, heat lost or heat gained by a body can be measured.
The conditions under which the basic principle of the calorimeter is valid are –
i. There should not be any heat exchange between the calorimeter and its surroundings.
ii. No exothermic or endothermic, physical, or chemical changes should take place between the calorimetric substance and the substance put into the calorimeter.
iii. The substance put inside the calorimeter should not be soluble in the calorimeter substance.
Note: Water is not a good calorimetric substance. Nowadays, online is considered the bet calorimetric substance.