When calcium reacts with hydrochloric acid, it undergoes a chemical reaction to form calcium chloride and hydrogen gas. The chemical equation for this reaction is:

Ca + 2HCl → CaCl2 + H2

In this reaction, calcium (Ca) reacts with two molecules of hydrochloric acid (HCl) to produce calcium chloride (CaCl2) and hydrogen gas (H2). Calcium chloride is a white, crystalline solid that is soluble in water, while hydrogen gas is a colorless, odorless, and flammable gas.