Science (086) — AI-generated practice question

Combination Reaction — two or more reactants combine to form a single product:

$$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$$

Decomposition Reaction — a single reactant breaks down into two or more products:

$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\text{Heat}} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$

Fundamental Difference: In a combination reaction, multiple reactants converge to form one product (many → one). In a decomposition reaction, one substance splits into multiple products (one → many). This shows that the direction of chemical change is opposite in the two reactions — combination builds complexity while decomposition breaks it down.

Source: Chapter 1 — Types of Chemical Reactions, What You Have Learnt

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• The examiner expects one equation each, both balanced — award 1 mark each.
• The third mark is for clearly stating the directional difference (many→one vs one→many).
• Avoid writing long descriptions; use the equations to do the work. State symbols earn you extra credit but are not strictly required for 3 marks here.
• Key phrase from the textbook: "Decomposition reactions are opposite to combination reactions" — use this language.