The metal that will librate H$_2$ gas from dilute HNO$_3$ is

Analyze the question: Which metal releases hydrogen gas (H₂) when it reacts with dilute nitric acid?

Concept: Most metals react with dilute acids like HCl or H₂SO₄ to give a salt and hydrogen gas. Nitric acid is different. HNO₃ is a strong oxidising agent, so the hydrogen produced is usually oxidised to water, and nitrogen oxides (NO, NO₂, N₂O) form instead. So HNO₃ normally does not liberate H₂.

The exception is very dilute, cold HNO₃ reacting with the most reactive metals, especially magnesium (and manganese). In this case, H₂ is actually given off:

Mg + 2HNO₃ (very dilute, cold) → Mg(NO₃)₂ + H₂↑

Going through the options:

• A. Cu: It is below hydrogen in the reactivity series, so it cannot displace H₂ from any dilute acid. With HNO₃ it gives NO or NO₂.
• B. Mg: The classic textbook case. Mg liberates H₂ from very dilute HNO₃.
• C. Ca: It is very reactive, but it reacts violently with water first, and with HNO₃ the nitrate is reduced, so it is not the standard answer.
• D. Zn: With dilute HNO₃, Zn gives nitrogen oxides (such as N₂O) or ammonium nitrate, not H₂.

Honest note: Strictly speaking, Ca is so reactive that it could also produce some H₂, so the question is a bit loose. However, the established JAMB/WASSCE fact is that magnesium is the metal that liberates hydrogen from dilute nitric acid.

Answer: B. Mg

Exam tip: Remember "Mg and Mn" as the only metals that give H₂ with very dilute HNO₃. For any other metal with HNO₃, expect nitrogen oxides, not hydrogen.