The yellow colour of chromates changes to orange on acidification due to formation of:

Options

A. `Cr^(3+)`
B. `Cr_(2)O_(3)`
C. `Cr_(2)O_(7)^2-`
D. `CrO_(4)^(-)`

Answer

To solve the question regarding the color change of chromates upon acidification, we will follow these steps:

### Step-by-Step Solution:

1. **Identify the Chromate Ion**:
The chromate ion is represented as \( \text{CrO}_4^{2-} \). In solution, this ion exhibits a yellow color.

**Hint**: Remember that chromate ions are yellow in color due to their specific electronic structure.

2. **Understand Acidification**:
Acidification refers to the addition of hydrogen ions (\( \text{H}^+ \)) to the solution. This typically occurs when an acid is added.

**Hint**: Recall that acids release \( \text{H}^+ \) ions in solution, which can affect the equilibrium of certain chemical species.

3. **Reaction of Chromate with Hydrogen Ions**:
When \( \text{H}^+ \) ions are added to the chromate solution, a chemical reaction occurs:
\[ \text{CrO}_4^{2-} + 2 \text{H}^+ \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \]
This reaction results in the formation of the dichromate ion, \( \text{Cr}_2\text{O}_7^{2-} \).

**Hint**: Look for how the addition of \( \text{H}^+ \) ions shifts the equilibrium between chromate and dichromate.

4. **Color Change**:
The dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \) is orange in color. Therefore, the yellow color of chromate changes to orange upon acidification due to the formation of dichromate ions.

**Hint**: Remember that different oxidation states and coordination environments can lead to different colors in transition metal compounds.

5. **Conclusion**:
The yellow color of chromate changes to orange on acidification due to the formation of the dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \).

**Final Answer**: The correct option is the formation of dichromate ion.