Two colourless solutions `X` and `Y` were mixed together. On mixing, a yellow precipitate `Z` was formed. Which of the following statements is correct regarding `X, Y` and `Z` ?

Options

A. `X` and `Y` were lead nitrate and potassium iodide solutions respectively. The yellow precipitate `Z` was lead iodide.
B. `X` and `Y` were potassium chloride solution and water respectively. The yellow precipitate `Z` was of chloride ions.
C. `X` and `Y` were sodium hydroxide solution and hydrochloric acid respectively and the yellow precipitate `Z` was sodium chloride.
D. `X` and `Y` were potassium hydroxide solution and nitric acid respectively and the yellow precipitate `Z` was potassium nitrate.

Explanation

To solve the question regarding the two colorless solutions `X` and `Y` that form a yellow precipitate `Z`, we need to analyze the possible reactions that can occur between various combinations of colorless solutions.

### Step-by-Step Solution:

1. **Identify the Solutions**:
- We have two colorless solutions, `X` and `Y`. We need to determine what these solutions could be.

2. **Consider Possible Reactions**:
- A common reaction that produces a yellow precipitate is the reaction between lead(II) nitrate and potassium iodide.
- The chemical formulas for these compounds are:
- Lead(II) nitrate: \( \text{Pb(NO}_3\text{)}_2 \)
- Potassium iodide: \( \text{KI} \)

3. **Write the Reaction**:
- When lead(II) nitrate reacts with potassium iodide, the following reaction occurs:
\[ \text{Pb(NO}_3\text{)}_2 + 2\text{KI} \rightarrow \text{PbI}_2 \downarrow + 2\text{KNO}_3 \]
- Here, \( \text{PbI}_2 \) (lead iodide) is the yellow precipitate formed, and \( \text{KNO}_3 \) (potassium nitrate) remains in solution.

4. **Identify the Precipitate**:
- The yellow precipitate \( Z \) formed in this reaction is \( \text{PbI}_2 \).

5. **Evaluate the Statements**:
- Now, we need to evaluate the correctness of the statements regarding `X`, `Y`, and `Z`:
- **Statement 1**: `X` and `Y` are lead nitrate and potassium iodide respectively, and the yellow precipitate `Z` is lead iodide. This statement is correct.
- **Statement 2**: If `X` and `Y` were potassium chloride and water, no yellow precipitate would form. This statement is incorrect.
- **Statement 3**: If `X` and `Y` were sodium hydroxide and hydrochloric acid, the product would be sodium chloride and water, which are both colorless, and no yellow precipitate would form. This statement is incorrect.
- **Statement 4**: If `X` and `Y` were potassium hydroxide and nitric acid, the product would be potassium nitrate and water, which are also colorless, and no yellow precipitate would form. This statement is incorrect.

6. **Conclusion**:
- The only correct statement is the first one, which states that `X` and `Y` are lead nitrate and potassium iodide respectively, and the yellow precipitate `Z` is lead iodide.

### Final Answer:
The correct statement regarding `X`, `Y`, and `Z` is:
- `X` and `Y` are lead nitrate and potassium iodide respectively, and the yellow precipitate `Z` is lead iodide.