Which equation demonstrates Charles' law, where volume is directly proportional to temperature at constant pressure?

• A. V1/T1 = V2/T2
• B. PV = nRT
• C. P1V1 = P2V2
• D. V = kT

Answer: (A)

Charles' law says that volume changes directly with temperature when pressure is held constant. For two states of the gas, that relationship is written as V1/T1 = V2/T2, which shows the ratio V/T stays the same as T changes (with P and the amount of gas fixed). Using Kelvin for temperature is important because it ensures the proportionality is meaningful across the entire range, including near zero. This two-state form is the clearest way to demonstrate that direct proportionality at constant pressure. While V = kT also expresses V growing with T, the V1/T1 = V2/T2 form explicitly ties the change to two states under the fixed-pressure condition, which is exactly what Charles’ law describes. The other equations describe related ideas (the general ideal gas law or Boyle’s law for different conditions) but do not illustrate the constant-pressure, two-state proportionality as directly.