A heterogeneous mixture where the particles of a material are spread though a liquid or gas not are too large to stay mixed without being stirred or shaken - particles will settle.

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?