Yellow colour of chromates changes to orange on acidification due to formation of:

Options

A. `Cr^(3+)`
B. `Cr_(2)(CO_(3))_3`
C. `Cr_(2)O_(7)^(2-) `
D. `CrO_(4)^(-)`

Answer

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

### Step-by-Step Solution:

1. **Identify the Initial Species**:
The question states that the yellow color of chromates changes to orange upon acidification. The chromate ion is represented as \( \text{CrO}_4^{2-} \).

2. **Understand the Color Change**:
The yellow color of chromate ions (\( \text{CrO}_4^{2-} \)) is due to the presence of chromium in a +6 oxidation state. When acidified, this ion undergoes a transformation.

3. **Write the Reaction**:
The chromate ion reacts with hydrogen ions (\( \text{H}^+ \)) from the acid to form the dichromate ion (\( \text{Cr}_2\text{O}_7^{2-} \)). The balanced reaction can be written as:
\[ 2 \text{CrO}_4^{2-} + 2 \text{H}^+ \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \]

4. **Identify the Final Species**:
The dichromate ion (\( \text{Cr}_2\text{O}_7^{2-} \)) is orange in color. This explains the color change from yellow to orange when chromate ions are acidified.

5. **Conclusion**:
The formation of the dichromate ion (\( \text{Cr}_2\text{O}_7^{2-} \)) is responsible for the observed color change. Therefore, the correct answer to the question is the dichromate ion.

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

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