At constant pressure, the volume of a gas is directly proportional to its temperature. Which Law is this?

• A. Amontons' Law
• B. Boyle's Law
• C. Dalton's Law
• D. Charles' Law

Answer: (D)

Direct proportionality between volume and temperature at fixed pressure is Charles’s Law. From the ideal gas equation PV = nRT, keeping pressure and the amount of gas constant gives V = (nR/P) T, so volume increases linearly with temperature. The proportionality constant (nR/P) depends on how much gas there is and the pressure applied, so the V–T graph is a straight line through the origin when temperature is measured in Kelvin. The Kelvin scale is important because it uses absolute temperature, ensuring zero Kelvin would correspond to zero volume in the idealized case. Physically, raising the temperature increases the average kinetic energy of molecules, causing them to push outward against the container more, so the volume expands to keep the pressure constant. The other laws describe different conditions (like constant temperature with respect to pressure, or constant volume with respect to temperature) and don’t capture the V–T relationship at constant pressure.