Besides, does mass increase when heated?
As Loring Chien said, for all practical purposes, the mass does not change as you add heat. If you are thinking that material expands when it gets hot and therefore its mass increases, then that is incorrect. It's just that the molecules are slightly farther apart.
Subsequently, question is, why does the volume of a gas increase as it is heated? The volume of the gas increases as the temperature increases. As temperature increases, the molecules of the gas have more kinetic energy. They strike the surface of the container with more force. If the container can expand, then the volume increases until the pressure returns to its original value.
Hereof, what will happen when gas is heated?
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. At high temperatures, the gas turns into a plasma.
Why does heat increase mass?
Heating does not increase mass. Heating increases internal motion of molecules, so the energy imparted by heating is embodied as kinetic energy. An object does not become heavier because it is in motion. However, kinetic energy can be converted to mass.
