Cambridge Science – States of Matter Practice Test

Temperature must be expressed in Kelvin in gas law equations because these laws describe a direct, proportional relationship between temperature and the average kinetic energy of gas molecules. Kelvin is an absolute scale with zero at absolute zero, the point where molecular motion would stop. This means that as temperature rises, kinetic energy (and thus pressure for a fixed amount of gas in a fixed volume) increases in a clean proportional way.

Using Celsius or Fahrenheit introduces an offset: zero on those scales does not correspond to zero energy. If you plug Celsius temperatures into the equations, the relationship would include a constant term, breaking the pure proportionality and leading to nonphysical predictions (for example, predicting nonzero pressure even as the thermal energy would be vanishing). Expressing temperature in Kelvin preserves the correct linear relationship and ensures the math matches the underlying physics.