Reaction of calcium and an acid in a test tube. This is an example of a metal-acid reaction. Here, the metal is calcium (Ca) and the acid is dilute hydrochloric acid (HCl). The reaction proceeds vigorously, due to the high reactivity of the calcium, producing large amounts of heat. The products of the reaction are calcium chloride (CaCl2) and hydrogen gas (H2, the bubbles seen here).