Are all gases at the same temperature moving at the same speed?

• A. True
• B. False; lighter gas molecules move faster than heavier ones
• C. False; heavier gas molecules move faster
• D. False; speed is independent of molecular mass

Answer: (B)

At a given temperature, gas molecules don’t all have the same speed. The average kinetic energy is the same for all molecules, but speed depends on mass. Since kinetic energy is (1/2) m v^2, if two molecules share the same temperature, the lighter one must reach a higher speed to have the same kinetic energy as a heavier one. That means lighter gas molecules move faster on average, while heavier molecules move slower. There’s a range of speeds for every gas, described by the Maxwell–Boltzmann distribution, but the overall trend is that lighter molecules are faster at the same temperature.