Why is alcohol considered a cleaner fuel than petrol?

The correct option is It produces less smoke.

To understand why alcohol is considered a cleaner fuel than petrol, we can compare their chemical structures and how they undergo combustion:

1. Chemical Composition and Oxygen Content:
Alcohols, such as ethanol (C₂H₅OH), are oxygenated hydrocarbons. This means that oxygen is already present within the molecular structure of the fuel. In contrast, petrol consists of a complex mixture of heavier, longer-chain hydrocarbons (such as octane, C₈H₁₈) that contain only carbon and hydrogen, with no oxygen in their structure.

2. Completeness of Combustion:
When a fuel burns, it undergoes a combustion reaction with oxygen. Complete combustion converts all carbon atoms in the fuel into carbon dioxide (CO₂) and all hydrogen atoms into water (H₂O). Because alcohol molecules already contain oxygen, they require less atmospheric oxygen to burn completely. Petrol, having a higher concentration of carbon and lacking internal oxygen, is much more likely to undergo incomplete combustion under the same conditions.

3. Reduction of Pollutants (Smoke and Soot):
Incomplete combustion leads to the formation of carbon monoxide (CO) and unburnt carbon particles, which are released into the atmosphere as soot and smoke. Because alcohol undergoes cleaner, more complete combustion, it releases far fewer unburnt carbon particles and harmful emissions, producing significantly less smoke than petrol.