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 can be prepared by reacting zinc with dilute sulfuric acid. The chemical reaction can be represented as:
\[ \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \uparrow \]

2. **Role of Pure Zinc**: Pure zinc is a reactive metal that readily reacts with acids to produce hydrogen gas. However, when pure zinc is used, it can lead to the formation of a passive layer.
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