Science — AI-generated practice question

In a combination reaction, two or more substances combine to form a single new substance:

$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq)$$

Here, two reactants give one product.

In a decomposition reaction, a single substance breaks down into two or more simpler substances:

$$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)$$

Here, one reactant gives two products.

These reactions are opposite because combination builds one compound from many, while decomposition breaks one compound into many.

Source: Chapter 1, Section 1.2 – Types of Chemical Reactions

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• The examiner expects you to state the contrast clearly (1 mark), give a balanced equation for combination (1 mark), and a balanced equation for decomposition (1 mark).
• Any correct, balanced example equations earn full credit — the CaO/CaCO₃ pair is ideal because CaO + H₂O → Ca(OH)₂ is the reverse of the decomposition of Ca(OH)₂, making the "opposite" point vivid.
• Always include state symbols and the heat arrow (Δ) for thermal decomposition — examiners notice these details.