How does increasing water vapor in the air affect air density?

How does increasing water vapor in the air affect air density?

• A. Decreases density
• B. Increases density
• C. No effect
• D. Increases pressure

Answer: (A)

Increasing water vapor lowers air density. Air density is mass per volume, and water vapor has a lower molecular weight (18 g/mol) than the main components of dry air (mostly N2 and O2, with average molar mass about 28–29 g/mol). When humidity rises at the same temperature and pressure, the average molar mass of the air decreases, so the mass per unit volume decreases as well. So the air becomes less dense. The other ideas would require changes in pressure or temperature, or a heavier gas, which isn’t what's being changed here.

Increasing water vapor lowers air density. Air density is mass per volume, and water vapor has a lower molecular weight (18 g/mol) than the main components of dry air (mostly N2 and O2, with average molar mass about 28–29 g/mol). When humidity rises at the same temperature and pressure, the average molar mass of the air decreases, so the mass per unit volume decreases as well. So the air becomes less dense. The other ideas would require changes in pressure or temperature, or a heavier gas, which isn’t what's being changed here.