The mechanism of hot matter rising and cool matter falling is caused by a change in what factor?

Convection

Explanation:

Convection is the transfer of heat by the movement of molecules within solid, liquids, and gases. It includes advection and diffusion that is the transfer of energy or mass particle along a gradient. Convection takes place in a mantel that is a slow-motion of the movement of convection current that carries the heat form into the surfaces. The material that is dense is thermally contracted and sinks under its weight in the process.

2cool and cold temperature unless its an ever green tree 

Answergranulation;