Q1. [2] medium initial-understanding
While balancing a chemical equation, why is it not allowed to change the formula of a compound — for example, writing H₂O₄ instead of placing the coefficient 4 in front of H₂O?
Generated by claude-sonnet-4-6 · 2026-06-26 00:53 · grounding rag
Model Answer
Changing the formula of a compound (e.g., writing H₂O₄ instead of H₂O) would represent an entirely different substance that does not exist in the reaction. Each chemical formula is fixed and represents a specific compound with definite properties. To balance an equation, only coefficients (numbers placed in front of formulas) may be changed, so that the number of atoms of each element remains equal on both sides, satisfying the Law of Conservation of Mass.
Source: Chapter 1, Section 1.1.2 — Balanced Chemical Equations
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Explanation
- Examiners expect two clear points: (1) altering the formula changes the identity/composition of the substance, and (2) only coefficients are allowed because the law of conservation of mass must be satisfied without changing what substances are present.
- Quoting or referencing the textbook example (H₂O → 4H₂O, not H₂O₄) shows you understand the rule directly from the source.
- Avoid writing a long paragraph; two crisp points earn full marks here.