Exothermic red solid surface gradually turns black

Quick answer：

This phenomenon may occur in several chemical reactions or physical processes. For example, it could be due to a red solid being heated and reacting with atmospheric oxygen, causing it to turn black as a result of carbonization or oxidation. It could also be a phase change where the red solid is a compound that decomposes or reacts at high temperatures, producing black residue. The specific cause would depend on the identity of the red solid and the conditions under which it is being heated.