Reaction between which of the following two reactants will produce hydrogen gas?

Metals react with dilute acids to produce a salt and hydrogen gas [1]. Magnesium is a highly reactive metal that readily displaces hydrogen from hydrochloric acid, resulting in magnesium chloride and hydrogen gas evolution. In contrast, copper is below hydrogen in the reactivity series and does not react with dilute hydrochloric acid. While metals generally produce hydrogen with acids, nitric acid (HNO3) is a strong oxidizing agent; it typically oxidizes the hydrogen produced into water and is itself reduced to nitrogen oxides (like NO, NO2, or N2O) rather than evolving hydrogen gas. Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water, and carbon dioxide gas, not hydrogen [3]. Therefore, only the reaction between magnesium and hydrochloric acid reliably produces hydrogen gas in this context.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.3 > p. 20
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > QUESTIONS > p. 22