A calorimeter uses the principle that thermal energy flows from hotter ma

The principle of calorimeter is that thermal energy flows from a hotter substance to a colder substance until both of them reach the same final temperature.

Calorimeter is an instrument that is used to measure the changes in energy or enthalpy that takes place during the course of a reaction. A calorimeter has a reaction vessel enclosed in an outer vessel filled with water. The temperature of the water is measured before and after the reaction. This temperature difference is a measure of how much heat is transferred. It is assumed that the heat energy liberated in a reaction is absorbed by water which results in the raise of temperature of the water. Heat given out in the reaction will be equal to the heat absorbed by water, assuming no heat is lost to the surroundings.