The average kinetic energy of a substance increases when:

• A. Its temperature increases
• B. Its temperature decreases
• C. It changes from liquid to solid
• D. It changes from liquid to gas

Answer: (A)

When you heat a substance, you add energy that makes the particles move faster. The average kinetic energy is the part of the energy due to motion, so it goes up as temperature rises. In gases, this relationship is direct: the average translational kinetic energy is proportional to temperature. In liquids and solids, higher temperature means more vigorous molecular vibrations and motions, so the average kinetic energy increases too.

If the temperature drops, particle motion slows and the average kinetic energy decreases. During a phase change—like melting or boiling—the temperature stays roughly constant while energy goes into changing the arrangement of particles, not their speed, so the average kinetic energy doesn't rise until after the change is complete and the temperature climbs again.