Chemical Changes and Reactions
ICSE Class 9 · Chemistry · Chapter 2
What this chapter covers
- Chemical changes and how to recognise them
- Direct combination
- Decomposition
- Displacement
- Double decomposition
- Three double displacement reactions to know well
- Energy changes in a chemical change
- Evolution and absorption of heat, light and electricity
Key points
- A chemical change forms new substances and is usually accompanied by an energy change; signs include colour change, gas, precipitate, temperature change and new smell.
- Direct combination: two or more substances form one product, A + B → AB.
- Decomposition: one compound breaks into simpler substances by heat (thermal), electricity (electrolytic) or light (photochemical); thermal dissociation is reversible.
- Displacement: a more reactive element replaces a less reactive one; the activity series decides whether it happens.
- Double decomposition: two compounds exchange radicals; it goes when a precipitate, a gas or water forms.
- Na2SO4 + BaCl2 gives white BaSO4; AgNO3 + NaCl gives white curdy AgCl; Pb(NO3)2 + 2KI gives yellow PbI2.
- Exothermic reactions give out heat; endothermic reactions absorb heat; the label depends on the overall energy change, not on how the reaction starts.
- Reactions can also give out or absorb light (burning magnesium; photosynthesis) and electricity (cells; electrolysis).
Questions answered
Define a chemical change. Give three signs that show a chemical reaction has occurred.
A chemical change is a permanent change in which new substances with new properties are formed. Signs include a change of colour, evolution of a gas and formation of a precipitate.
Define direct combination and give one example in which two compounds combine.
Direct combination is a reaction in which two or more substances combine to form one new substance; CaO + H2O → Ca(OH)2.
Name the three types of decomposition according to the energy supplied, with one equation each.
Thermal: CaCO3 → CaO + CO2. Electrolytic: 2H2O → 2H2 + O2. Photochemical: 2AgCl → 2Ag + Cl2.
What is thermal dissociation? How is it different from thermal decomposition?
Thermal dissociation is a reversible decomposition by heat; the products recombine on cooling, as with ammonium chloride. In ordinary thermal decomposition the products do not recombine.
An iron nail is placed in copper sulphate solution. State two observations and write the equation.
A reddish-brown coating of copper forms on the nail, and the blue solution turns pale green. Fe + CuSO4 → FeSO4 + Cu.
Why does copper not react with dilute hydrochloric acid?
Copper is below hydrogen in the activity series, so it cannot displace hydrogen from the acid.
Write the balanced equation and the colour of the precipitate when barium chloride is added to sodium sulphate solution.
Na2SO4 + BaCl2 → BaSO4 + 2NaCl; the precipitate is white.
Silver nitrate solution is added to sodium chloride solution. Name the precipitate and state how it behaves with dilute nitric acid and with ammonium hydroxide.
Silver chloride, a white curdy precipitate; it does not dissolve in dilute nitric acid but dissolves in ammonium hydroxide.
Past board exam questions
ICSE 2019 sample paper · 4 marks
Classify each of the following reactions as combination, decomposition, displacement or double displacement: (i) 2Al + Fe₂O₃ → Al₂O₃ + 2Fe (ii) 2KClO₃ → 2KCl + 3O₂ (iii) BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl (iv) CaO + H₂O → Ca(OH)₂
Show answer
(i) Displacement
(ii) Decomposition
(iii) Double displacement
(iv) Combination