Question

The pair of metals which displace Hydrogen from the caustic soda solution is:
(A) Na, Al
(B) Mg, Zn
(C) Al, Zn
(D) Na, Mg

Answer 1

Hint: The elements having oxidation state can only displace each other depending on their reactivity of the metal. The reactivity of Mg is less than that of Hydrogen. Elements react chemically in order to attain stable configurations, more the instability, more is the reactivity.

Complete solution step by step:
Those metals which have high reactivity from hydrogen can replace the hydrogen from caustic soda. Metals which lie above hydrogen in the activity series are most reactive in nature from the hydrogen. So that’s why they have much capability to replace hydrogen from caustic soda. Sodium, Aluminium, and Zinc have higher reactivity than hydrogen.
The metals that have lower reactivity than hydrogen cannot replace the hydrogen from caustic soda. Metals which lie below the hydrogen in the activity series are less reactive in nature from the hydrogen. So that’s why they have low capability to replace hydrogen from caustic soda. Reactivity of Magnesium is less than that of hydrogen.
Thus, the options containing Mg are incorrect. Hence, (B) and (D) are incorrect.
Caustic soda is basically NaOH, Na displaces hydrogen and forms sodium oxide. Aluminium and zinc also have the tendency to replace hydrogen from caustic soda. Therefore,

The correct choices are (A) and(C).

Note: Concepts of metals should be clear. Knowledge of their activity series should be there to solve such problems. Metals and their natures should be read carefully. There are two types of displacement reactions. Single displacement reaction, and Double displacement reaction. In double displacement reaction two ionic compounds are interchanged to give two new compounds as a result.