Chemical Changes and Reactions — Question 8
Back to all questions(a) Double decomposition — This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called double displacement reaction.
AB + CD ⟶ AD + CB
Eg., AgNO3 + NaCl ⟶ AgCl ↓ + NaNO3
(b) Thermal dissociation — A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.
Heat some solid ammonium chloride in a test tube. Two colourless gases, ammonia and hydrogen chloride, are produced. As these gases move up to the upper part of the test tube which is cooler, they combine to form ammonium chloride, which appears as a white sublimate on the upper cooler side of the test tube.
NH4Cl ⇌ NH3 ↑ + HCl ↑
(c) Reversible reaction — A reaction that can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.
(d) Displacement — A chemical change in which a more active element displaces a less active element from its salt solution is called a displacement reaction.
Mg + H2SO4 ⟶ MgSO4 + H2 ↑
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 Change | Change that produces new substances with different chemical properties |
| Physical Change | Change in physical properties only; no new substance formed; usually reversible |
| Exothermic Reaction | Reaction that releases heat energy to surroundings (e.g., combustion, neutralisation) |
| Endothermic Reaction | Reaction that absorbs heat energy from surroundings (e.g., photosynthesis, thermal decomposition) |
| Oxidation | Gain of oxygen, loss of hydrogen, or loss of electrons |
| Reduction | Loss of oxygen, gain of hydrogen, or gain of electrons |
| Redox Reaction | Reaction where oxidation and reduction occur simultaneously |
| Catalyst | Substance 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 Substance | Not formed | Formed |
| Reversibility | Usually reversible | Usually irreversible |
| Energy Change | Small | Significant |
| Examples | Melting ice, dissolving sugar | Burning 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.