explain why zn reacts more slowly with dilute hydrochloric acid then with concentrated hydrochloric acid​

When the concentration is higher, more hydrogen ions are near the Zn atoms at any given time. This allows for more Zn atoms to be ionized and dissolved into the solution per second.

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.