According to the kinetic theory, how do particle speeds relate to mass at a given temperature?

• A. Heavier particles move faster
• B. All particles move at the same speed
• C. Temperature has no effect on speed
• D. Heavier particles move slower at a given temperature

Answer: (D)

At a given temperature, the average kinetic energy of particles is determined by that temperature. Since kinetic energy is (1/2) m v^2, keeping the same energy while having a larger mass means the speed must be smaller. In other words, v is proportional to 1/√m, so heavier particles move slower on average at the same temperature. Lighter particles, with smaller mass, reach higher speeds to carry the same amount of energy. This is why, in a mixture at the same temperature, light particles tend to move faster than heavy ones.