When a cup of hot coffee is placed in a room, its temperature reduces as it is attaining thermal equilibrium with the atmosphere. The energy that is transferred from the coffee due to the temperature gradient is called heat, and the energy transfer that takes place in the process is called heat flow. The SI unit of heat is the joule. Another standard unit of heat is the calorie, where one calorie is equal to 4.186 joules. Further, the quantity of heat required to raise the temperature of mass m of a certain material from T1 to T2 is proportional to mass and change in temperature. The proportionality constant c is called specific heat. Thus specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by one-degree celsius. If the molar mass, which is the mass per mole of a substance, is used in the formula for specific heat, it is called the molar heat capacity and is denoted by capital C.