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ICSE Class 8 Chemistry Question 14 of 15

Chemical Reactions — Question 5

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Question 9

Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide.

(a) Limestone

(b) Lead carbonate

(c) Calcium nitrate

(d) Calcium hydroxide.

Answer

(a) Thermal decomposition of limestone

CaCO3 →Δ\xrightarrow\Delta CaO + CO2

(b) Thermal decomposition of lead carbonate

PbCO3 →Δ\xrightarrow\Delta PbO + CO2

(c) Thermal decomposition of calcium nitrate

2Ca(NO3)2 →Δ\xrightarrow\Delta 2CaO + 4NO2 + O2

(d) Thermal decomposition of calcium hydroxide.

Ca(OH)2 →Δ\xrightarrow\Delta CaO + H2O

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Chemistry | Chapter 5: Chemical ReactionsWeb Content / Study Guide

Chemical Reactions — Interactive Study Guide

Quick Concept Map

CHANGES: Physical (reversible, no new substance) vs Chemical (irreversible, new substance)
REACTION TYPES: Combination | Decomposition | Displacement | Double Displacement
ENERGY: Exothermic (releases heat) vs Endothermic (absorbs heat)
REDOX: Oxidation (gain O / lose H) + Reduction (lose O / gain H) happening together

Four Types of Reactions — Quick Reference

TypePatternEasy Memory AidExample
CombinationA + B → ABJoining togetherMg + O2 → MgO
DecompositionAB → A + BBreaking apartCaCO3 → CaO + CO2
DisplacementA + BC → AC + BBully kicks out the weakFe + CuSO4 → FeSO4 + Cu
Double DisplacementAB + CD → AD + CBExchange partnersNaCl + AgNO3 → AgCl + NaNO3

Signs of a Chemical Reaction

  1. Change in colour
  2. Evolution of gas (bubbles)
  3. Change in temperature (hot or cold)
  4. Formation of precipitate (insoluble solid)
  5. Change in smell
  6. Change in state
  7. Sound or light produced

Oxidation vs Reduction

OxidationReduction
Gain of oxygenLoss of oxygen
Loss of hydrogenGain of hydrogen
Example: C + O2 → CO2 (C is oxidised)Example: CuO + H2 → Cu + H2O (CuO is reduced)

Memory aid: OIL RIG — Oxidation Is Loss (of hydrogen), Reduction Is Gain (of hydrogen)

Self-Check Questions

  1. Is dissolving salt in water a physical or chemical change? Why?
  2. Classify: Iron + Copper sulphate → Iron sulphate + Copper. What type of reaction is this?
  3. What is the difference between exothermic and endothermic reactions? Give one example of each.
  4. In the reaction CuO + H2 → Cu + H2O, which substance is oxidised and which is reduced?
  5. Write word equations for one example of each: combination, decomposition, displacement, double displacement.

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