Air density is determined by the number of air molecules per unit volume.

• A. False
• B. Depends on humidity
• C. True
• D. Irrelevant to pressure

Answer: (C)

Density is about how much stuff is packed into a given volume. For air, that means mass per volume, which depends on two things: how many molecules occupy that space and how heavy those molecules are on average. Pushing air into a smaller volume or cooling it increases the number of molecules per unit volume, raising density. Humidity changes the mix by adding water vapor, which lowers the average molecular weight of the gas, so density can shift with humidity even if the number of molecules per volume isn’t dramatically different. In short, density ties to both how many molecules are present in a volume and their masses, so saying air density is determined by the number of molecules per unit volume is true in practical terms, with humidity affecting it through composition.