Reaction of magnesium in acid. This is an example of a metal-acid reaction. Here, the acid is hydrochloric acid. The reaction proceeds vigorously, due to the high reactivity of the magnesium, producing large amounts of heat. The products of the reaction are magnesium chloride and hydrogen gas (the bubbles seen here). Because calcium is more reactive than magnesium, that reaction is even more vigorous than this reaction (see A500/757).