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ICSE Class 9 Chemistry Question 2 of 18

Chemical Changes & Reactions — Question 7

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Question 7

Define the following types of chemical changes or reactions with a suitable example of each.

(a) Direct combination reaction or synthesis

(b) Decomposition reaction

(c) Displacement reaction or substitution reaction

(d) Double decomposition reaction

Answer

(a) A chemical reaction in which two or more elements or compounds react to form one new compound is called a Direct combination reaction or synthesis.

For example,

Hydrogen burns in air to give water:

  2H2 [g] + O2 [g] ⟶ 2H2O [l]

(b) A chemical reaction in which a chemical compound decomposes into two or more simpler substances (elements and/or compounds) is called a Decomposition reaction.

For example,

Calcium carbonate on heating decomposes to Calcium Oxide and Carbon dioxide:

  CaCO3 →Δ\xrightarrow{\enspace\Delta\enspace} CaO + CO2

(c) A chemical reaction in which an element or radical has replaced another element in a compound is known as Displacement reaction or Substitution reaction.

For example,

Magnesium displaces Copper from Copper [II] sulphate solution:

Mg + CuSO4 ⟶ MgSO4 + Cu

(d) A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is known as Double decomposition reaction.

For example,

Silver nitrate + Potassium chloride ⟶ Silver chloride + Potassium nitrate

AgNO3 + KCl ⟶ AgCl + KNO3

Chapter Overview: Chemical Changes and Reactions

This chapter distinguishes between physical changes (reversible, no new substance formed) and chemical changes (usually irreversible, new substances formed with different properties). Chemical reactions are identified by signs such as change in colour, evolution of gas, formation of precipitate, change in temperature, or change in smell. The chapter covers types of reactions: direct combination (synthesis), decomposition (analysis), single displacement, double displacement, and neutralisation. Energy changes in reactions are discussed through exothermic reactions (release heat, e.g., combustion) and endothermic reactions (absorb heat, e.g., photosynthesis). Students learn about factors affecting rate of reaction including temperature, concentration, surface area, and catalysts. Catalysts are substances that alter reaction rates without being consumed. The concepts of oxidation (gain of oxygen / loss of hydrogen / loss of electrons) and reduction (loss of oxygen / gain of hydrogen / gain of electrons) are introduced, along with redox reactions where both occur simultaneously.

Key Definitions

Term Definition
Chemical ChangeChange that produces new substances with different chemical properties
Physical ChangeChange in physical properties only; no new substance formed; usually reversible
Exothermic ReactionReaction that releases heat energy to surroundings (e.g., combustion, neutralisation)
Endothermic ReactionReaction that absorbs heat energy from surroundings (e.g., photosynthesis, thermal decomposition)
OxidationGain of oxygen, loss of hydrogen, or loss of electrons
ReductionLoss of oxygen, gain of hydrogen, or gain of electrons
Redox ReactionReaction where oxidation and reduction occur simultaneously
CatalystSubstance that alters the rate of reaction without being consumed

Must-Know Concepts

  • Signs of chemical change: colour change, gas evolution, precipitate formation, temperature change, odour change
  • Combination: 2Mg + O2 → 2MgO; Decomposition: 2KClO3 → 2KCl + 3O2 (MnO2 catalyst)
  • Displacement: Zn + H2SO4 → ZnSO4 + H2↑ (more reactive metal displaces less reactive one)
  • Redox example: CuO + H2 → Cu + H2O (CuO reduced, H2 oxidised)
  • Rate increases with: higher temperature, higher concentration, larger surface area, presence of catalyst
  • Rusting: 4Fe + 3O2 + xH2O → 2Fe2O3·xH2O (needs both oxygen and moisture)

Physical vs Chemical Changes

Feature Physical Change Chemical Change
New SubstanceNot formedFormed
ReversibilityUsually reversibleUsually irreversible
Energy ChangeSmallSignificant
ExamplesMelting ice, dissolving sugarBurning wood, rusting iron

Important Diagrams to Practice

  • Energy diagrams for exothermic and endothermic reactions (reactant/product energy levels)
  • Classification chart of types of chemical reactions with examples
  • Experiment setup for demonstrating conditions required for rusting

Common Mistakes

  • Classifying dissolving salt in water as a chemical change (it is physical)
  • Saying all chemical changes are irreversible (some are reversible, like crystallisation)
  • Confusing oxidation with reduction in redox reactions
  • Not identifying both the oxidising and reducing agents in a redox reaction

Scoring Tips

  • When asked to identify reaction type, write the balanced equation first then classify
  • For redox reactions, clearly label which substance is oxidised and which is reduced
  • Remember: the oxidising agent itself gets reduced, the reducing agent gets oxidised
  • Give at least two examples for each type of reaction for better answers

Frequently Asked Questions

Is dissolving sugar in water a chemical or physical change?

It is a physical change. Sugar dissolves but retains its chemical identity (C12H22O11). You can recover the sugar by evaporating the water. No new substance is formed.

Can a reaction be both exothermic and a decomposition?

Most decomposition reactions are endothermic (need heat to break bonds). However, some are exothermic, like the decomposition of hydrogen peroxide: 2H2O2 → 2H2O + O2, which releases heat.

Why is rusting a chemical change?

Rusting forms a new substance - hydrated iron(III) oxide (Fe2O3·xH2O) - which has completely different properties from iron. It is irreversible, involves a change in composition, and releases energy slowly over time.