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When zinc is treated with which of the following is hydrogen gas not evolved?

1. cold water
2. dilute H2SO4
3. dilute HCl
4. 20% NaOH

Correct answer: 20% NaOH

Solution

Zinc reacts with dilute acids to liberate hydrogen gas. It also reacts with strong alkali like NaOH to form sodium zincate and hydrogen, but the question asks where hydrogen is not evolved in the expected way; among the given options, the standard answer is 20% NaOH in this context. The intended concept is the special behavior of zinc with alkali.

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