Physical states are described as not always necessary in a chemical equation. However, in some reactions specifying them becomes essential. Give ONE situation where omitting the physical state would make the equation misleading or incomplete, and explain why.
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Model Answer
When water (H₂O) can exist as liquid or steam, omitting its physical state makes the equation misleading. For example, in the reaction of iron with water:
$$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$$
Here, the symbol (g) is essential to indicate that water is used as steam, not liquid water. Without it, the reader cannot determine the actual reaction conditions, making the equation incomplete and potentially misleading about how the reaction proceeds.
Source: Chapter 1, Section 1.1.2 (Step VII — Writing Symbols of Physical States)
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Explanation
- Examiners want: (1) a specific, relevant example, (2) the physical state symbol shown, and (3) a clear reason why omission is misleading.
- The textbook explicitly uses the Fe/H₂O example to illustrate exactly this point — always use it here rather than inventing another.
- Key phrase to include: "water is used in the form of steam" — this is the textbook's own wording and signals to examiners you know the source.
- Don't just say "physical states are important in general"; the question asks for ONE specific situation with an explanation.