The reaction between hydrochloric acid (HCl) and magnesium (Mg) to produce hydrogen gas (H2) is a chemical change.
When magnesium metal is added to hydrochloric acid, it reacts to form magnesium chloride (MgCl2) and hydrogen gas. The overall reaction can be represented by the following equation:
Mg + 2 HCl → MgCl2 + H2
This is a chemical change because the substances involved are transformed into new products. In this case, magnesium metal reacts with hydrochloric acid to form magnesium chloride solution and hydrogen gas. The properties of the original materials (magnesium and hydrochloric acid) are different from those of the products (magnesium chloride and hydrogen). Additionally, the reaction represents a change in chemical composition, which is a hallmark of chemical changes.
In contrast, a physical change would involve a change in state or appearance without altering the chemical identity of the substance, such as melting ice or boiling water. Since this reaction results in the formation of new substances, it is classified as a chemical change.