The residue of coal decomposition that is used in the manufacture of steel is called: 1. coal gas 2. coke 3. coal tar

Alidded glass container is filled with a colored gas. after a period of time, it is observed that the gas is uniformly spread throughout the box and that the movement has slowed considerably. next, a warm iron plate is carefully placed under the box. why is there resumed movement of the gas in the container?