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ICSE Class 8 Chemistry Question 4 of 15

Chemical Reactions — Question 7

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Question 7

What do you observe when:

(a) Iron nail is kept in copper sulphate solution for sometime?

(b) Phenolphthalein is added to sodium hydroxide solution?

(c) Blue litmus paper is dipped in dilute hydrochloric acid?

(d) Lead nitrate is heated?

(e) Magnesium ribbon is burnt in oxygen?

(f) Ammonia is brought in contact with hydrogen chloride gas?

Answer

(a) When iron nail is kept in a blue coloured copper sulphate solution for sometime, a reddish brown coating is seen on the iron nail and the colour of the solution changes gradually from blue to light green. The reason for this observation is that iron being more reactive than copper, displaces copper from the solution and copper is deposited on the iron nail.
Fe + CuSO4 (aq) ⟶ FeSO4 + Cu

(b) When a few drops of phenolphthalein is added to sodium hydroxide solution, it turns pink indicating that sodium hydroxide solution is basic in nature.

(c) Blue litmus paper changes into red colour when dipped in dilute hydrochloric acid confirming its acidic nature.

(d) When solid lead nitrate is heated strongly, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas.

2Pb(NO3)2 →Δ\xrightarrow{\enspace\Delta\enspace} 2PbO + 4NO2 (g) + O2 (g)

(e) When magnesium ribbon is burnt in oxygen gas, it burns with a dazzling white light and produces a white powder of magnesium oxide.

2Mg + O2 ⟶ 2MgO

(f) When ammonia is brought in contact with hydrogen chloride gas, dense white fumes of ammonium chloride are produced.

NH3 + HCl ⟶ NH4Cl

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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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