Calcium oxide reacts with water to produce slaked lime. It is an example of
- (a)combination reaction
- (b)decomposition reaction
- (c)oxidation reaction
- (d)addition reaction
Correct — A, combination reaction. Calcium oxide and water give a single product: CaO + H₂O → Ca(OH)₂. Two reactants going in and one compound coming out is precisely the definition of a combination reaction, and the count of products is all you need to classify it. A great deal of heat is released as well, so the reaction is also exothermic — that is a statement about energy, not about type, and the two labels sit happily together.
- (b)decomposition reaction — Decomposition runs the other way: one compound breaks into two or more. Heating limestone to give quicklime and carbon dioxide is the decomposition in this family.
- (c)oxidation reaction — No oxidation state changes here. Calcium is already +2 in the oxide and remains +2 in the hydroxide; no element gains oxygen or loses electrons.
- (d)addition reaction — Addition is a term reserved for organic chemistry, where a molecule adds across a carbon-carbon double or triple bond. It is not used for the union of two inorganic compounds.
Reactions are classified by what happens to the count of species. Combination is many to one, decomposition is one to many, displacement swaps a more reactive element in for a less reactive one, and double displacement exchanges the partners of two compounds. Energy labels such as exothermic and endothermic form a separate axis.
Slaking quicklime is the standard textbook illustration, and it doubles as a whitewashing question — the slaked lime spread on a wall then reacts slowly with carbon dioxide from the air to leave a thin, shiny film of calcium carbonate. That second step is another combination reaction. Note the vocabulary the examiner leans on: calcium oxide is quicklime, calcium hydroxide is slaked lime, and calcium carbonate is limestone.
- CaO + H₂O → Ca(OH)₂ releases enough heat to bring the water to a boil, which is why the process is called slaking.
- The reverse reaction, CaCO₃ heated to give CaO + CO₂, is a decomposition and is how quicklime is manufactured in a lime kiln.
- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O is the whitewashing reaction that hardens the coat on the wall.
- Quicklime is CaO, slaked lime is Ca(OH)₂ and limestone is CaCO₃ — three names that are constantly interchanged in wrong answers.
Count the products first; only then ask whether heat was given out.
- Calling the reaction an oxidation because oxygen appears in both formulae, when no oxidation state changes.
- Reaching for 'addition', which belongs to unsaturated organic compounds.
- Confusing quicklime with slaked lime, which reverses the direction of the equation.
Asked as classification of a printed equation, or by naming the products of slaking and asking what kind of change has occurred.
Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of
- (a) Displacement reaction.
- (b) Endothermic reaction.
- (c) Decomposition reaction.
- (d) Exothermic reaction.
Answer(d) Exothermic reaction.
The very same equation, classified on the energy axis rather than the species-count axis. Reading the two items side by side shows why both labels can be right about one reaction without conflicting.
- practice — not a real PYQ
Heating limestone to give quicklime and carbon dioxide is an example of
- (a)combination
- (b)decomposition
- (c)displacement
- (d)double displacement
Answer(b) decomposition — one compound, CaCO₃, breaks into two products on heating.
- practice — not a real PYQ
The shiny finish that appears some days after a wall is whitewashed is due to the formation of
- (a)calcium oxide
- (b)calcium hydroxide
- (c)calcium carbonate
- (d)calcium sulphate
Answer(c) calcium carbonate — slaked lime on the wall reacts slowly with carbon dioxide from the air.