Consider two identical cylinders A and B, filled with an ideal gas. Cylinder A is sealed, while B has a movable piston. Let some amount of heat be applied to both cylinders to increase the gas temperature equally. The volume of gas in the enclosed cylinder remains unchanged, so no work is done. In cylinder B, the gas is free to expand, resulting in some work being done under constant external pressure. From the First law of thermodynamics, the heat applied increases the system's internal energy in cylinder A. In cylinder B, a portion of heat is used to do work. As the temperature is the same in both the cylinders, their internal energy change is also the same. Since more heat should be added to cylinder B than A to raise their temperature equally, two distinct molar heat capacities are defined. CV and Cp are the amounts of heat needed to raise the temperature of one mole of gas by 1 unit under constant volume and pressure, respectively.