Classifying substances: one element, one compound, one mixture

Steps:

• Define categories: elements are pure substances of one atom type (e.g., O₂); compounds are two or more elements chemically bonded (fixed ratio, e.g., C₆H₁₂O₆); mixtures are physically combined substances (variable ratios, e.g., air).
• Examine each row for one of each type.
• Classify: elements (copper, zinc, nitrogen, oxygen); compounds (methane CH₄, CO₂, water H₂O, glucose C₆H₁₂O₆); mixtures (brass alloy, petrol hydrocarbons, air gases).
• Select row with exactly one element, one compound, one mixture.

Why D is correct:

• Oxygen is an element (O₂, diatomic gas); glucose is a compound (C₆H₁₂O₆, fixed molecular formula); air is a mixture (78% N₂, 21% O₂, variable composition).

Why the others are wrong:

• A: Two elements (copper, zinc) and one mixture (brass), missing compound.
• B: Two compounds (methane, CO₂) and one mixture (petrol), missing element.
• C: One element (nitrogen) and two compounds (CO₂, water vapour), missing mixture.

Final answer: D