How did sodium and hydrogen chloride change into two different substances?

Sodium metal reacts with hydrochloric acid to produce salt and hydrogen gas. This means that your reactants will be sodium metal and hydrochloric acid, since these are the substances that are being transformed to form salt and hydrogen gas.

Explanation:

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.