Which of the following pairs cannot be represented with a chemical formula?

Analysis of the Question

The question asks us to identify a pair of substances that cannot be represented by a single chemical formula. To answer this, we must distinguish between pure substances (elements and compounds) and mixtures.

Concept Explanation

• Chemical Formula: A chemical formula (like $H_2O$ or $NaCl$) represents the fixed ratio of atoms in a pure substance (an element or a compound).
• Pure Substances: These have a constant composition and can be represented by a formula. • Elements: e.g., Copper ($Cu$), Oxygen ($O_2$). • Compounds: e.g., Sodium chloride ($NaCl$), Caustic soda ($NaOH$), Washing soda ($Na_2CO_3 \cdot 10H_2O$).
• Mixtures: These consist of two or more substances physically combined in varying proportions. Because their composition is not fixed, they cannot be represented by a single chemical formula. • Air: A mixture of gases (nitrogen, oxygen, argon, carbon dioxide, etc.). • Bronze: An alloy (a solid mixture) of copper and tin.

Step-by-Step Solution

1. Analyze Option A: • Air: A mixture of gases. No fixed formula. • Bronze: An alloy (mixture) of copper and tin. No fixed formula. • Conclusion: Neither can be represented by a chemical formula.

2. Analyze Option B: • Bronze: Mixture (No formula). • Sodium chloride: Compound ($NaCl$). Has a formula.

3. Analyze Option C: • Copper: Element ($Cu$). Has a formula. • Sodium chloride: Compound ($NaCl$). Has a formula.

4. Analyze Option D: • Caustic soda: Compound ($NaOH$). Has a formula. • Washing soda: Compound ($Na_2CO_3 \cdot 10H_2O$). Has a formula.

Since both air and bronze are mixtures with variable compositions, they are the only pair that cannot be represented by a chemical formula.

Final Answer

The correct option is A.

Exam Tip: Always remember that alloys (like bronze, brass, and steel) and solutions (like air or salt water) are mixtures. If you see an alloy or a mixture in a "chemical formula" question, that is almost certainly the one that cannot be represented by a formula!