Match List I with List II and select the answer using the code given below the Lists: List I (Reaction) A. CaO (s) + H2O (l) → Ca(OH)2 (aq) B. CaCO3 (s) → CaO (s) + CO2 (g) C. Pb (s) + CuCl2 (aq) → PbCl2 (aq) + Cu (s) D. Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq) List II (Type) 1. Single displacement 2. Double displacement 3. Combination 4. Decomposition Code:
- (a)A-2, B-1, C-4, D-3
- (b)A-2, B-4, C-1, D-3
- (c)A-3, B-4, C-1, D-2
- (d)A-3, B-1, C-4, D-2
Correct — C, A-3, B-4, C-1, D-2. In reaction A two substances (CaO and water) join to form one product, so it is a combination reaction (3). In B a single compound, calcium carbonate, breaks into two on heating — a decomposition reaction (4). In C lead displaces copper from copper chloride, a single displacement (1). In D the ions of two compounds swap partners to precipitate barium sulphate, a double displacement (2). This gives A-3, B-4, C-1, D-2.
- (a)A-2, B-1, C-4, D-3 — It mislabels the combination of CaO and water as a double displacement and calls the lead-copper displacement a decomposition — none of these four pairings matches the actual reaction types.
- (b)A-2, B-4, C-1, D-3 — B and C are right (decomposition and single displacement), but it wrongly calls A a double displacement and D a combination; A is a combination and D is a double displacement.
- (d)A-3, B-1, C-4, D-2 — A and D are right, but it swaps B and C — calling the breakdown of CaCO3 a single displacement and the lead-copper reaction a decomposition, when it is the reverse.
Chemical reactions are commonly sorted into four basic types. In a combination reaction two or more reactants form a single product; in a decomposition reaction one compound splits into two or more substances. In a single displacement a more reactive element pushes out a less reactive one from its compound, while in a double displacement two compounds exchange ions, often forming a precipitate.
Read each equation by its shape. Two-in and one-out is combination; one-in and many-out is decomposition; a lone element replacing another is single displacement; and two compounds trading partners is double displacement. Matching the pattern to the name, rather than memorising the chemicals, is the fastest route to the code.
- A combination reaction joins two or more reactants into one product; CaO + H2O forming Ca(OH)2 is an example.
- A decomposition reaction breaks one compound into simpler substances, as when CaCO3 is heated to CaO and CO2.
- A single displacement reaction has a more reactive element replacing a less reactive one, as lead displaces copper from CuCl2.
- A double displacement reaction exchanges ions between two compounds, often giving a precipitate such as BaSO4.
Matching each equation's shape to its type gives A-3, B-4, C-1, D-2 — option (c).
- Single displacement involves one free element; double displacement is two compounds exchanging ions — do not mix them up.
- Decomposition (one reactant) is the reverse pattern of combination (one product) — check how many species are on each side.
Reaction types are asked as 'classify this reaction' or as a match-the-columns item — count the reactants and products and look for a free element or an ion swap.
No directly related past PYQ was found.
- practice — not a real PYQ
The reaction 2H2O → 2H2 + O2 is an example of
- (a)combination
- (b)decomposition
- (c)single displacement
- (d)double displacement
Answer(b) decomposition — one compound breaks down into simpler substances.
- practice — not a real PYQ
The reaction AgNO3 + NaCl → AgCl + NaNO3 is an example of
- (a)combination
- (b)decomposition
- (c)single displacement
- (d)double displacement
Answer(d) double displacement — the two compounds exchange ions and AgCl precipitates.