Final answer:
When a gas expands, it does work on its surroundings, resulting in positive work done by the gas.
Step-by-step explanation:
When a gas expands, it does work and its internal energy decreases. The work done by the gas is positive when work is done by the system, meaning that the gas is doing work on its surroundings. This occurs when the gas expands against a movable piston, pushing it outward. The force exerted by the gas over a distance results in positive work, increasing the energy of the system.