Final answer:
When heat is applied to a piston chamber with gas, the gas particles will speed up, leading to an increase in kinetic energy, while the volume increases to maintain constant pressure.
Step-by-step explanation:
When heat is applied to a system where a gas is contained in a piston chamber, and the piston moves to keep the pressure constant, the behavior of the gas will change in accordance with the principles of thermodynamics. Specifically, the correct answer to this scenario is that the gas particles will speed up. This increase in speed translates to an increase in the kinetic energy of the gas molecules, as heating the system adds internal energy. Although the pressure remains constant due to the movement of the piston, the volume will increase to maintain the pressure as the kinetic energy of the gas particles increases. This situation is reflected in Gay-Lussac's Law, which states that the pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature.