Science (086) — AI-generated practice question

Changing subscripts in a chemical formula changes the substance itself. For example, H₂O₄ is a completely different compound from H₂O. We can only add coefficients (e.g., 4 H₂O) to balance an equation, because the law of conservation of mass requires that the number of atoms of each element remains the same — not that we alter the identities of the reactants or products.

Source: Chapter 1, Section 1.1.2 (Balanced Chemical Equations)

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Examiners look for two key points (1 mark each):

1. Changing subscripts changes the identity/formula of the compound — it becomes a different substance entirely.
2. Only coefficients may be changed; this follows from the law of conservation of mass, which governs why we balance equations in the first place.

The textbook explicitly states: "we cannot alter the formulae of the compounds or elements involved in the reactions… we can put coefficient '4' as 4 H₂O and not H₂O₄." Quote or paraphrase this directly if possible.