Here's why:
* Kinetic Energy and Temperature: The average kinetic energy of gas molecules is directly proportional to the absolute temperature. This means that at the same temperature, all gas molecules have the same average kinetic energy.
* Kinetic Energy and Molecular Mass: Kinetic energy is also related to the mass and velocity of a molecule: KE = 1/2 * mv², where KE is kinetic energy, m is mass, and v is velocity.
* The Trade-Off: Since lighter molecules have less mass, to maintain the same average kinetic energy as heavier molecules, they must have a higher average velocity.
Example: Imagine two gas molecules: one is oxygen (O2), and the other is helium (He). Helium is much lighter than oxygen. If you heat both gases to the same temperature, the average kinetic energy of both will be the same. To achieve this, the helium molecules will move much faster than the oxygen molecules.
Important Note: This is a statistical average. At any given moment, some individual lighter molecules might be moving slower than some individual heavier molecules. However, overall, the lighter molecules will have a higher average speed.
