First law of thermodynamics: First law of thermodynamics states that, Heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa.

First law of thermodynamics also states that energy can neither be created nor destroyed, through it can be transformed from one form to another. According to this law, the energy due to heat supplied must be balanced by external workdone plus the gain in internal energy due to rise in temperature.

Addition of heat at constant pressure to a gas results in Raising its temperature and doing external work.
When you heat a gas, both its vapor pressure and the volume it occupies increase. The individual gas particles become more energetic and the temperature of the gas increases.