The color change is caused by holes and other defects that are created in the zinc oxide lattice as the compound is heated. The temperature changes . It takes energy to break chemical bonds. And energy is released when new chemical bonds form. When the reaction involves more bond-breaking than bond making, the energy required is often absorbed from the surroundings, making them cooler.
Light is emitted .
Sometimes energy is released by bond-forming reactions in the form of light. This occurs in most combustion reactions. A change in volume occurs. Density is a characteristic of a compound, and if new compounds are produced as othe compounds are consumed in the reaction, the change in density can cause the reacting mixture to expand or contract as the reaction proceeds. Of course, volume increases and decreases will also accompany temperature and pressure changes, so these must be held constant if volume is being monitored to see if a reaction is occurring.