In the laboratory preparation of hydrogen, pure zinc is not used because

Explanation

**Step-by-Step Solution:**

1. **Understanding the Reaction**: In the laboratory, hydrogen gas is typically prepared by reacting zinc with an acid, such as hydrochloric acid (HCl) or sulfuric acid (H2SO4). The general reaction can be represented as:
\[ \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \]

2. **Identifying the Problem with Pure Zinc**: Pure zinc is not used for this reaction because it is non-porous in nature. This non-porous characteristic means that the surface area available for the reaction is limited.
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