In Charles' Law, how are volume and temperature related when pressure is held constant?

In Charles' Law, how are volume and temperature related when pressure is held constant?

• A. Temperature is directly proportional to pressure at constant volume.
• B. Volume is directly proportional to temperature at constant pressure.
• C. Volume inversely proportional to temperature at constant pressure.
• D. Pressure is inversely proportional to volume at constant temperature.

Answer: (B)

At constant pressure, volume changes directly with absolute temperature: as temperature increases, the gas expands, and as it decreases, the gas contracts, while pressure stays fixed. This relationship is written as V ∝ T, or V = kT when P is constant. The reason is that higher temperature means higher average molecular kinetic energy, causing more collisions with the container walls and, if the container allows it, more space for the gas to occupy. Using Kelvin is essential because temperature must be on an absolute scale for this direct proportionality to hold. Other descriptions reflect different gas laws: increasing temperature at constant volume affects pressure (Gay-Lussac’s law), not volume; volume inversely proportional to temperature at constant pressure would contradict Charles’ law; and Boyle’s law describes pressure related to volume at constant temperature.

At constant pressure, volume changes directly with absolute temperature: as temperature increases, the gas expands, and as it decreases, the gas contracts, while pressure stays fixed. This relationship is written as V ∝ T, or V = kT when P is constant. The reason is that higher temperature means higher average molecular kinetic energy, causing more collisions with the container walls and, if the container allows it, more space for the gas to occupy. Using Kelvin is essential because temperature must be on an absolute scale for this direct proportionality to hold.

Other descriptions reflect different gas laws: increasing temperature at constant volume affects pressure (Gay-Lussac’s law), not volume; volume inversely proportional to temperature at constant pressure would contradict Charles’ law; and Boyle’s law describes pressure related to volume at constant temperature.