The correct option is Magnesium and manganese.
Explanation
Typically, when metals react with acids, they displace hydrogen to form a salt and hydrogen gas. However, nitric acid ($HNO_3$) behaves differently because it is a strong oxidizing agent. In most reactions between metals and nitric acid, the hydrogen produced is immediately oxidized to water ($H_2O$), and the nitric acid is reduced to various nitrogen oxides (such as $N_2O$, $NO$, or $NO_2$). Consequently, hydrogen gas is rarely evolved.Analysis of the Exception:
There is a specific exception to this rule involving very dilute nitric acid (approximately 1% concentration). Under these specific conditions, the oxidizing power of the acid is sufficiently low to allow the evolution of hydrogen gas with two specific metals:
- Magnesium ($Mg$): $Mg + 2HNO_3 \rightarrow Mg(NO_3)_2 + H_2$
- Manganese ($Mn$): $Mn + 2HNO_3 \rightarrow Mn(NO_3)_2 + H_2$
Analysis of Incorrect Options:
- Copper and Silver: These metals are less reactive than hydrogen (placed below hydrogen in the reactivity series). They do not displace hydrogen from dilute acids. Instead, they react with nitric acid to form metal nitrates, water, and nitrogen oxides.
- Zinc and Lead: While more reactive than hydrogen, their reaction with nitric acid is dominated by the acid's oxidizing nature. Zinc typically produces nitrous oxide ($N_2O$) or nitrogen gas, while lead produces nitrogen monoxide ($NO$), along with water and the respective nitrates.
- Aluminium: Aluminium reacts with concentrated nitric acid to form a passive oxide layer that prevents further reaction (passivity). It does not evolve hydrogen with dilute nitric acid due to the oxidizing effects mentioned above.
Key Takeaway:
Magnesium and Manganese are the only metals that evolve hydrogen gas when reacting with very dilute nitric acid. For other metals, nitric acid acts as an oxidizing agent, producing water instead of hydrogen gas.