Science (086) — AI-generated practice question

(i) Decomposition of calcium carbonate (thermal decomposition):

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

(ii) Calcium oxide (CaO) / Quick lime is directly used in the construction industry. It reacts vigorously with water to form slaked lime [Ca(OH)₂], which is used for whitewashing walls and in making cement and mortar.

(iii) This reaction is a chemical change because new substances with different properties are formed (CaO and CO₂ from CaCO₃), bonds are broken and new bonds are formed, and the reaction is irreversible under normal conditions. The composition of matter changes permanently, which is the hallmark of a chemical change.

Source: Chapter 1, Section 1.2 (Decomposition Reactions); Section 1.2.1 (Combination Reaction – slaked lime use)

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• (i) Always include the heat symbol (Δ) above the arrow and state symbols — examiners award a mark for a correctly balanced equation with conditions.
• (ii) The textbook specifically mentions CaO (quick lime) reacting with water → slaked lime → whitewashing. Stick to this use.
• (iii) Key indicators of chemical change: new substances formed, different properties, irreversibility, bond breaking/making. Mentioning "no new elements" is not enough — you must explain why it is still chemical (new compounds with new properties are produced).