Final answer:
In the reaction of calcium with hydrochloric acid to form calcium chloride, calcium is oxidized as its oxidation number increases from 0 to +2 and hydrogen is reduced with its oxidation number decreasing from +1 to 0, confirming it as a redox reaction.
Step-by-step explanation:
To show that the reaction where calcium reacts with hydrochloric acid to form calcium chloride is a redox reaction, we need to analyze the oxidation numbers of each element before and after the reaction. In the reactants, calcium (Ca) is in its elemental form and has an oxidation number of 0. Hydrogen (H) in hydrochloric acid (HCl) has an oxidation number of +1, and chlorine (Cl) has an oxidation number of -1. In the products, calcium in calcium chloride (CaCl₂) has an oxidation number of +2, and chlorine remains with an oxidation number of -1. Hydrogen in hydrogen gas (H₂) has an oxidation number of 0.
Therefore, calcium undergoes oxidation as its oxidation number increases from 0 to +2. Hydrogen from the hydrochloric acid undergoes reduction as its oxidation number decreases from +1 in HCl to 0 in H₂ gas. Since there's both oxidation and reduction taking place, this reaction is indeed a redox reaction.