Using the common sign convention for work, which statement is correct about a gas expanding at constant pressure?

• A. Work is always zero if pressure is constant.
• B. Increasing the system's volume at constant pressure produces negative work.
• C. Increasing the system's volume at constant pressure produces positive work.
• D. Work is independent of volume change.

Answer: (B)

When a gas expands against a constant external pressure, it pushes from within and does work on the surroundings. In the common chemistry sign convention, this work is taken as negative, because energy leaves the system as it performs work. The amount of work is W = -P_ext ΔV, so with constant pressure, if the volume increases (ΔV > 0), the result is negative. That’s why increasing the system’s volume at constant pressure produces negative work. The magnitude depends on how much the volume changes, not on whether the pressure is constant alone. If there’s no volume change, there is no work; but with expansion, you do nonzero negative work.