Which of the following statements regarding copper salts is not true?

Options

A. Copper(I) disproportionates to copper and copper (II) ion in aqueous solution
B. Copper(I) can be stabilised by the formation of insoluble complex compounds such as `CuCl_(2)^(-) and Cu(CN)_(2)^(-)`
C. Copper(II) oxide is red powder
D. The water of crystallization of copper sulphate is five

Answer

To determine which statement regarding copper salts is not true, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Copper(I) disproportionates to copper and copper(II) ion in aqueous solution.**
- **Analysis:** Copper(I) ions (Cu⁺) are less stable and can disproportionate into copper metal (Cu) and copper(II) ions (Cu²⁺). This is true because Cu²⁺ is more stable due to its higher hydration energy, which compensates for its second ionization energy.
- **Conclusion:** This statement is **true**.

2. **Statement 2: Copper(I) can be stabilized by the formation of insoluble complex compounds CuCl₂⁻ and Cu(CN)₂²⁻.**
- **Analysis:** Copper(I) ions can indeed form stable complexes with chloride ions and cyanide ions, such as CuCl₂⁻ and Cu(CN)₂²⁻. This stabilization occurs because these complexes are insoluble and help maintain the stability of Cu⁺.
- **Conclusion:** This statement is **true**.

3. **Statement 3: Copper(II) oxide is a red powder.**
- **Analysis:** This statement is incorrect. Copper(I) oxide (Cu₂O) is a red powder, while copper(II) oxide (CuO) is actually a black powder.
- **Conclusion:** This statement is **not true**.

4. **Statement 4: The water of crystallization of copper sulfate is 5.**
- **Analysis:** Copper sulfate (CuSO₄) commonly exists as pentahydrate (CuSO₄·5H₂O), which means it contains five molecules of water of crystallization.
- **Conclusion:** This statement is **true**.

### Final Conclusion:
The statement that is not true regarding copper salts is **Statement 3: Copper(II) oxide is a red powder**.