Question

when zinc metal is mixed with hydrochloric acid, vigorous production of what substance is an indication that a chemical reaction is occurring?

Explanation:

When zinc reacts with hydrochloric acid, a single - displacement reaction occurs. Zinc replaces hydrogen in hydrochloric acid. The products are zinc chloride and hydrogen gas. The vigorous production of gas is a sign of a chemical reaction. In this case, the gas produced is hydrogen.

Answer:

Hydrogen gas