At constant pressure, what happens to air density when temperature increases?

• A. Air density increases.
• B. Air density decreases.
• C. Air density remains constant.
• D. Air density becomes zero.

Answer: (B)

When a gas is heated at constant pressure, it expands. The amount of air (mass) is the same, but the volume increases as temperature rises, so density (mass per unit volume) decreases. This follows from the ideal gas relationship: with pressure fixed and fixed mass, volume is proportional to temperature, so density is inversely proportional to temperature. In practical terms, hotter air is less dense, which is why engines lose some power on hot days.