(2) Difference between heat quantity and enthalpy

Heat quantity is a thermal energy that a substance dissipates to its surrounding (or brings in from its surrounding).
Enthalpy is an energy (heat + pressure energy) that a substance possesses.
When the enthalpy of a substance changes, heat and power of the same amount with the change is dissipated to or brought in from the surrounding.
(This is the 1st law of thermodynamics, and is called the energy conservation law.)
For example, decrease in temperature of 1 kg water by 1�Ž is resulted from removal of heat of 4.186 kJ.
(4.186 is called specific heat of water whose unit is kJ/kg/K. It is 1 kcal/kg�Ž in the previous unit.)