While balancing a chemical equation, a student writes H₂O₄ instead of 4H₂O to increase the number of oxygen atoms. Why is this approach incorrect?
Generated by claude-sonnet-4-6 · 2026-06-26 00:53 · grounding rag
Model Answer
Writing H₂O₄ is incorrect because it changes the chemical formula of water, implying a compound that does not exist. While balancing an equation, we can only change the coefficients (numbers in front of the formula), not the subscripts. The correct way is to write 4H₂O, which means four molecules of water, keeping the formula of each molecule unchanged.
Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations
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Explanation
- Examiners expect two clear points: (1) H₂O₄ is a non-existent/wrong compound — changing subscripts alters the identity of the substance; (2) only coefficients may be changed during balancing.
- The textbook explicitly states: "we cannot alter the formulae of the compounds… we can put coefficient '4' as 4H₂O and not H₂O₄." Quote or paraphrase this for full marks.
- Avoid lengthy explanations — two crisp points are enough for 2 marks.