A compound of calcium 'X' is used in the purification of water. It reacts with water to form another compound 'Y' with the evolution of gas 'Z'. When carbon dioxide gas is passed through 'Y', it turns milky. Gas 'Z' helps in the purification of water.
Which of the following statements regarding the given piece of information is correct?

Options

A. X is bleaching powder, Y is calcium hydroxide, and Z is water vapour.
B. X is calcium hydroxide, Y is bleaching powder, and Z is chlorine gas.
C. X is bleaching powder, Y is calcium hydroxide, and Z is chlorine gas.
D. X is calcium hydroxide, Y is bleaching powder, and Z is water vapour.

Answer

To solve the question step by step, we need to analyze the information provided and identify the compounds and gases involved in the water purification process.

### Step 1: Identify Compound 'X'
The question states that a compound of calcium 'X' is used in the purification of water. Based on common knowledge, one such compound is **bleaching powder**, which has the chemical formula **Ca(OCl)₂** (calcium hypochlorite).

### Step 2: Reaction of 'X' with Water
When bleaching powder (Ca(OCl)₂) reacts with water, it produces another compound 'Y'. The reaction can be represented as follows:
\[ \text{Ca(OCl)}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{Cl}_2 \]
Here, 'Y' is **calcium hydroxide (Ca(OH)₂)**, and 'Z' is the gas **chlorine (Cl₂)** that is evolved during this reaction.

### Step 3: Identify Gas 'Z'
From the reaction above, we see that gas 'Z' is **chlorine (Cl₂)**. Chlorine is known for its disinfectant properties and is commonly used in water purification to kill pathogens.

### Step 4: Effect of Carbon Dioxide on 'Y'
The question states that when carbon dioxide gas is passed through 'Y' (calcium hydroxide), it turns milky. This is due to the formation of **calcium carbonate (CaCO₃)**, which is a white precipitate. The reaction can be represented as:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
The formation of calcium carbonate causes the solution to turn milky.

### Step 5: Conclusion
Based on the analysis:
- **Compound 'X' is calcium hypochlorite (Ca(OCl)₂)**.
- **Compound 'Y' is calcium hydroxide (Ca(OH)₂)**.
- **Gas 'Z' is chlorine (Cl₂)**.
- The statement that when carbon dioxide is passed through 'Y', it turns milky is correct as it forms calcium carbonate.

### Correct Statement
The correct statement regarding the given piece of information is that **gas 'Z' (chlorine) helps in the purification of water**.