How the colour changes when the gases after thermal decomposition of ferrous sulphate come in contact with an acidified solution of potassium dichromate?

Options

A. Green to orange
B. red to colourless
C. orange to green
D. blue to green

Answer

To understand how the color changes when the gases produced from the thermal decomposition of ferrous sulfate react with an acidified solution of potassium dichromate, we can break down the process into several steps:

### Step 1: Thermal Decomposition of Ferrous Sulfate
- **Reaction**: When ferrous sulfate (FeSO4·7H2O) is heated, it undergoes thermal decomposition. Initially, it loses its water of crystallization to form anhydrous ferrous sulfate (FeSO4).
- **Further Decomposition**: With prolonged heating, the anhydrous ferrous sulfate decomposes into iron(III) oxide (Fe2O3), sulfur dioxide (SO2), and sulfur trioxide (SO3) gases.

### Step 2: Gases Produced
- **Gases**: The gases produced during the decomposition are primarily sulfur dioxide (SO2) and sulfur trioxide (SO3). Both of these gases are colorless and have a pungent smell.

### Step 3: Reaction with Acidified Potassium Dichromate
- **Potassium Dichromate**: The acidified solution of potassium dichromate (K2Cr2O7) is typically orange in color due to the presence of chromium in the +6 oxidation state (Cr6+).
- **Reaction**: When the colorless gases (SO2 and SO3) come into contact with the acidified potassium dichromate, a redox reaction occurs. The sulfur dioxide acts as a reducing agent, reducing Cr6+ to Cr3+.

### Step 4: Color Change
- **Formation of Chromium(III)**: The reduction of chromium from +6 to +3 results in the formation of chromium(III) sulfate (Cr2(SO4)3), which is green in color.
- **Observation**: Therefore, the initial orange color of the potassium dichromate solution changes to green as the reaction proceeds.

### Summary of Color Change
- **Initial Color**: Orange (due to Cr6+ in K2Cr2O7)
- **Final Color**: Green (due to Cr3+ in Cr2(SO4)3)