Pure compounds have fixed chemical composition

Steps:

• Define a pure compound as a substance with two or more elements in a fixed ratio, chemically bonded.
• Examine each option: dry air is a gas mixture; ethanol is C₂H₅OH; steel is an iron-carbon alloy; petrol is hydrocarbon blend.
• Identify ethanol as the only option with a single, fixed chemical formula.
• Confirm by excluding mixtures and elements.

Why B is correct:

• Ethanol (C₂H₅OH) is a compound of carbon, hydrogen, and oxygen in a definite 2:6:1 ratio, per the law of definite proportions.

Why the others are wrong:

• A: Dry air is a mixture of nitrogen, oxygen, and other gases, not chemically combined.
• C: Steel is an alloy, a homogeneous mixture of metals like iron and carbon.
• D: Petrol is a mixture of various hydrocarbons with varying compositions.

Final answer: B