Complete the following using the correct word given in the brackets.
Hydrogen chloride gas is formed when sodium chloride is treated with concentrated _____acid. (hydrochloric/sulphuric) 

Explanation

To complete the sentence, we need to determine which acid, hydrochloric acid or sulfuric acid, can react with sodium chloride (NaCl) to produce hydrogen chloride (HCl) gas.

### Step-by-Step Solution:

1. **Identify the Reactants**: We have sodium chloride (NaCl) and two options for acids: hydrochloric acid (HCl) and sulfuric acid (H2SO4).

2. **Consider the Reaction with Sulfuric Acid**:
- When sodium chloride reacts with concentrated sulfuric acid, the reaction can be represented as:
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