Assertion : Through the use of catalytic converters, unburnt hydrocarbons are changed into carbon monoxide which in turn is changed into nitrogen oxides and water.
Reason : Motor vehicles equipped with catalytic converters should use leaded petrol to protect the catalyst from degradation.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that catalytic converters change unburnt hydrocarbons into carbon monoxide, which is then converted into nitrogen oxides and water. This needs to be evaluated for its accuracy.

2. **Function of Catalytic Converters**: Catalytic converters are devices in vehicles that reduce harmful emissions. They contain catalysts (like platinum, palladium, and rhodium) that facilitate chemical reactions to convert toxic pollutants into less harmful substances.

3. **Chemical Reactions in Catalytic Converters**:
- Unburnt hydrocarbons (HC) are primarily converted into carbon dioxide (CO2) and water (H2O).
- Carbon monoxide (CO) is converted into carbon dioxide (CO2).
- Nitric oxide (NO) is converted into nitrogen gas (N2).
- Therefore, the assertion that unburnt hydrocarbons are changed into carbon monoxide is incorrect.

4. **Understanding the Reason**: The reason states that motor vehicles equipped with catalytic converters should use leaded petrol to protect the catalyst from degradation. This also needs to be evaluated.

5. **Impact of Leaded Petrol**: Leaded petrol contains lead, which is harmful to the catalysts used in catalytic converters. Lead can deactivate these catalysts, making them less effective in reducing emissions. Thus, vehicles with catalytic converters should actually use unleaded petrol to avoid catalyst degradation.

6. **Conclusion**: Both the assertion and the reason are incorrect. The assertion misrepresents the chemical processes occurring in catalytic converters, and the reason incorrectly states the type of petrol that should be used.

**Final Answer**: Both the assertion and the reason are incorrect.

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