What will occur when solid calcium sulfate is added
towater?

Explanation:

Calcium sulfate is slightly soluble in water. The small amount of ions given by calcium sulfate on dissolving in water does not interact with water acting as a spectator ions. Due to presence of covalent bonds in calcium sulfate it doesn't dissolves in water which polar solvent.

Hardening of water occurs when solid calcium sulfate is added  to water.This hardening of water is due to presence of calcium ions.

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.