Which of the following statements is NOT a postulate of the kinetic molecular theory?

• A. Gases consist of tiny particles (atoms or molecules).
• B. The volume of the particles is negligible compared with the container.
• C. The particles do not attract or repel each other.
• D. The average kinetic energy of the gas particles is directly proportional to the Celsius temperature.

Answer: (D)

The essential idea here is how the motion of gas particles relates to temperature. In kinetic molecular theory, the average kinetic energy of gas particles is proportional to the absolute temperature, meaning it scales with Kelvin, not with Celsius. Because the Kelvin scale starts at absolute zero, the energy goes to zero there. If you express temperature in Celsius, you introduce a fixed offset (0 °C is 273.15 K), which would distort the direct proportionality between kinetic energy and temperature. So saying the average kinetic energy is directly proportional to Celsius temperature is not correct, which is why this statement isn’t a postulate.

The other statements align with the theory: gases consist of tiny particles; the particles’ own volumes are negligible compared with the container; and there are no attractive or repulsive forces between particles in the idealized model.