Sometimes chlorine gas is passed through water for its purification. What will be the pH of such a sample of water?

Options

A. Equal to 7
B. More than 7
C. Less than 7
D. Very near to 8

Answer

To determine the pH of water when chlorine gas is passed through it for purification, we can follow these steps:

### Step 1: Understand the Reaction
When chlorine gas (Cl₂) is passed through water, it reacts to form hydrochloric acid (HCl) and hypochlorous acid (HOCl). The chemical reactions can be represented as:
\[ \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HOCl} \]

### Step 2: Identify the Nature of the Products
- Hydrochloric acid (HCl) is a strong acid.
- Hypochlorous acid (HOCl) is a weak acid.

### Step 3: Determine the Effect on pH
Since HCl is a strong acid, it dissociates completely in water, contributing to an increase in the concentration of hydrogen ions (H⁺) in the solution. The presence of H⁺ ions in the solution will lower the pH.

### Step 4: Conclude the pH Value
According to the pH scale:
- A pH of 7 is neutral.
- A pH less than 7 indicates an acidic solution.
- A pH greater than 7 indicates a basic solution.

Since the solution contains a strong acid (HCl), the pH of the water sample will be less than 7.

### Final Answer
The pH of the water sample after passing chlorine gas through it will be less than 7.

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