Describe exothermic reaction please

Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light.

Explanation: In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.