Which of the following statement is incorrect?

Options

A. Basic copper carbonate is `CuCO_3.Cu(OH)_2`
B. On strong heating potassium dichromate decomposes with evolution of oxygen
C. CuS is white in colour
D. `KMnO_4` exists as dark purple black prismatic crystals

Explanation

To solve the question of identifying the incorrect statement among the given options, we will analyze each statement one by one.

### Step 1: Analyze Option 1
**Statement:** Basic copper carbonate is CuCO3.CuOH2.

- Basic copper carbonate is indeed represented as CuCO3.Cu(OH)2.
- This compound contains two copper atoms, both in the +2 oxidation state.
- Therefore, this statement is **correct**.

**Hint:** Check the chemical formula and oxidation states to verify the accuracy of the compound's representation.

### Step 2: Analyze Option 2
**Statement:** On strong heating, potassium dichromate decomposes with evolution of oxygen.

- The decomposition of potassium dichromate (K2Cr2O7) upon strong heating does produce potassium chromate (K2CrO4), chromium trioxide (CrO3), and oxygen gas (O2).
- The balanced reaction shows that oxygen is indeed evolved during this decomposition.
- Therefore, this statement is **correct**.

**Hint:** Consider the products of thermal decomposition reactions to confirm if oxygen is released.

### Step 3: Analyze Option 3
**Statement:** CuS is white in color.

- Copper(I) sulfide (CuS) is actually known to be black in color, not white.
- Therefore, this statement is **incorrect**.

**Hint:** Recall the common colors of metal sulfides to determine the accuracy of the statement.

### Step 4: Analyze Option 4
**Statement:** KMnO4 exists as dark purple black prismatic crystals.

- Potassium permanganate (KMnO4) is indeed known to exist as dark purple-black prismatic crystals.
- When dissolved in water, it gives a deep purple solution.
- Therefore, this statement is **correct**.

**Hint:** Familiarize yourself with the physical properties of common compounds to assess their appearance accurately.

### Conclusion
After analyzing all the statements, we find that the incorrect statement is:

**Option 3: CuS is white in color.**

This is the answer to the question.