At fixed pressure, what happens to the density of a gas as temperature increases?

• A. Density increases
• B. Density decreases
• C. Density remains the same
• D. Density varies unpredictably

Answer: (B)

At fixed pressure, a gas expands when its temperature rises, so the volume increases while the amount of gas stays the same. Since density is mass per volume, increasing volume means the density decreases. This relationship follows PV = nRT, so with P constant and n fixed, V grows in proportion to T. Equivalently, ρ = PM/RT for a gas with molar mass M, showing ρ decreases as T increases. In short, raising temperature at constant pressure makes a gas less dense.