Class 8 Science Notes Chapter 4 (Chapter 4) – Examplar Problems Book

Alright class, let's focus on Chapter 4, 'Materials: Metals and Non-metals' from your Science Exemplar book. This chapter is fundamental, and concepts from here frequently appear in various government exams. We need to go beyond simple definitions and understand the 'why' behind the properties and reactions.

Chapter 4: Materials: Metals and Non-metals - Detailed Notes for Exam Preparation

1. Classification of Materials:
Elements are broadly classified into Metals and Non-metals based on their physical and chemical properties. Metalloids (like Silicon, Germanium) show properties intermediate between metals and non-metals.

2. Physical Properties - A Comparative Look:

3. Chemical Properties of Metals:

• Reaction with Oxygen (O₂):

  • Metals + Oxygen → Metal Oxides
  • Nature: Generally Basic. They turn red litmus blue when dissolved in water (e.g., MgO + H₂O → Mg(OH)₂).
  • Examples:
    • 2Mg + O₂ → 2MgO (Magnesium burns with dazzling white light)
    • 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O (Rusting of Iron - requires both air and moisture)
    • 2Cu + O₂ → 2CuO (Black coating on Copper upon heating)
  • Amphoteric Oxides: Some metal oxides (e.g., Aluminium Oxide - Al₂O₃, Zinc Oxide - ZnO) show both acidic and basic behaviour. They react with both acids and bases.
    • Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (Acting as base)
    • Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (Acting as acid; forming Sodium Aluminate)

• Reaction with Water (H₂O):

  • Highly Reactive Metals (Na, K, Ca) + Cold Water → Metal Hydroxide + Hydrogen Gas (H₂) + Heat (often catches fire)
    • 2Na + 2H₂O → 2NaOH + H₂ + Heat
  • Moderately Reactive Metals (Mg) + Hot Water → Metal Hydroxide + H₂
    • Mg + 2H₂O (hot) → Mg(OH)₂ + H₂
  • Less Reactive Metals (Al, Zn, Fe) + Steam → Metal Oxide + H₂
    • 3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂
  • Least Reactive Metals (Cu, Ag, Au, Pt) → No reaction with water/steam.

• Reaction with Dilute Acids (e.g., HCl, H₂SO₄):

  • Metal (more reactive than H) + Dilute Acid → Metal Salt + Hydrogen Gas (H₂)
  • Test for H₂: Burns with a 'pop' sound.
  • Examples:
    • Zn + H₂SO₄ → ZnSO₄ + H₂
    • Fe + 2HCl → FeCl₂ + H₂
  • Metals less reactive than Hydrogen (Cu, Ag, Au, Pt) do not displace H₂ from dilute acids.
  • Note: Reaction with Nitric Acid (HNO₃) is different. It's a strong oxidizing agent and usually oxidizes the H₂ produced to H₂O, itself getting reduced to oxides of nitrogen (N₂O, NO, NO₂). Only very dilute HNO₃ reacts with Mg and Mn to produce H₂ gas.

• Reaction with Bases (e.g., NaOH):

  • Only some metals like Zinc (Zn) and Aluminium (Al) react with strong bases.
  • Metal + Base → Salt + Hydrogen Gas (H₂)
  • Example: Zn + 2NaOH → Na₂ZnO₂ (Sodium Zincate) + H₂

• Displacement Reactions:

  • A more reactive metal displaces a less reactive metal from its salt solution.
  • Based on the Reactivity Series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au > Pt (Most Reactive → Least Reactive)
  • Examples:
    • Fe (s) + CuSO₄ (aq, Blue) → FeSO₄ (aq, Greenish) + Cu (s, Red-brown deposit)
    • Zn (s) + FeSO₄ (aq, Greenish) → ZnSO₄ (aq, Colourless) + Fe (s)
    • Cu (s) + ZnSO₄ (aq, Colourless) → No Reaction (Cu is less reactive than Zn)

4. Chemical Properties of Non-metals:

• Reaction with Oxygen (O₂):

  • Non-metal + Oxygen → Non-metal Oxide
  • Nature: Generally Acidic or Neutral.
  • Acidic Oxides: Dissolve in water to form acids, turning blue litmus red.
    • S + O₂ → SO₂ ; SO₂ + H₂O → H₂SO₃ (Sulphurous Acid)
    • C + O₂ → CO₂ ; CO₂ + H₂O → H₂CO₃ (Carbonic Acid)
    • P₄ + 5O₂ → P₄O₁₀ ; P₄O₁₀ + 6H₂O → 4H₃PO₄ (Phosphoric Acid)
  • Neutral Oxides: Do not react with acids or bases, no effect on litmus (e.g., CO, N₂O, NO, H₂O).

• Reaction with Water (H₂O):

  • Generally, non-metals do not react with water (though some might dissolve slightly).

• Reaction with Dilute Acids:

  • Non-metals generally do not react with dilute acids to displace hydrogen.

• Reaction with Bases:

  • Reactions can be complex. Example: Chlorine reacts with NaOH.
    • Cl₂ + 2NaOH (cold, dilute) → NaCl + NaOCl (Sodium Hypochlorite) + H₂O
    • Cl₂ + 6NaOH (hot, conc.) → 5NaCl + NaClO₃ (Sodium Chlorate) + 3H₂O

• Displacement Reactions:

  • A more reactive non-metal can displace a less reactive non-metal from its salt solution (common among halogens).
  • Example: Cl₂ (g) + 2NaBr (aq) → 2NaCl (aq) + Br₂ (aq) (Chlorine displaces Bromine)

5. Uses:

• Metals: Construction (Fe), Electrical wires (Cu, Al), Utensils (Al, Cu, Steel), Jewellery (Au, Ag, Pt), Thermometers (Hg), Food wrapping (Al foil), Galvanizing (Zn), Aircraft parts (Al alloys), Machinery (Fe).
• Non-metals: Oxygen (respiration, combustion), Nitrogen (fertilizers, inert atmosphere), Carbon (fuel - coal, graphite - electrodes/pencils, diamond - jewellery/cutting), Sulphur (gunpowder, vulcanization of rubber, H₂SO₄ manufacture), Phosphorus (matchsticks, fertilizers), Chlorine (water purification, bleaching), Iodine (antiseptic - Tincture of Iodine), Hydrogen (fuel, ammonia production), Helium (balloons), Argon (bulbs).

6. Corrosion:
The gradual destruction of metals by the action of air, moisture, or chemicals. Rusting of iron (formation of hydrated iron(III) oxide, Fe₂O₃.xH₂O) is a prime example. Copper forms a green coating (basic copper carbonate), Silver turns black (silver sulphide).

• Prevention: Painting, Oiling/Greasing, Galvanizing (coating with Zinc), Tinning, Chrome plating, Anodizing (for Aluminium), Alloying.

7. Alloys:
Homogeneous mixtures of two or more metals, or a metal and a non-metal, prepared to obtain desired properties (e.g., hardness, strength, corrosion resistance).

• Examples:
  • Steel: Iron (Fe) + Carbon (C) - Harder than iron.
  • Stainless Steel: Fe + Chromium (Cr) + Nickel (Ni) - Corrosion resistant.
  • Brass: Copper (Cu) + Zinc (Zn) - Used in decorative items, hardware.
  • Bronze: Copper (Cu) + Tin (Sn) - Used for statues, medals, bells.
  • Solder: Lead (Pb) + Tin (Sn) - Low melting point, used for joining electrical wires.
  • Duralumin: Aluminium (Al) + Cu + Magnesium (Mg) + Manganese (Mn) - Light, strong, used in aircraft.

Multiple Choice Questions (MCQs):

1. Which of the following pairs correctly matches a non-metal with its exceptional property?
   (a) Carbon - Liquid at room temperature
   (b) Bromine - Good conductor of electricity
   (c) Iodine - Possesses metallic lustre
   (d) Sulphur - Highly malleable

2. An element reacts with oxygen to give an oxide which dissolves in water and turns blue litmus red. The element is most likely to be:
   (a) Sodium
   (b) Calcium
   (c) Sulphur
   (d) Iron

3. Aluminium oxide (Al₂O₃) is considered amphoteric because it reacts with:
   (a) Only acids
   (b) Only bases
   (c) Both acids and bases
   (d) Neither acids nor bases

4. Consider the following reactions:
   (i) Fe(s) + ZnSO₄(aq) → No reaction
   (ii) Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s)
   (iii) Cu(s) + FeSO₄(aq) → No reaction
   Which of the following represents the correct order of reactivity?
   (a) Fe > Zn > Cu
   (b) Zn > Fe > Cu
   (c) Cu > Fe > Zn
   (d) Zn > Cu > Fe

5. Which metal is stored in kerosene to prevent its vigorous reaction with air and moisture?
   (a) Magnesium
   (b) Zinc
   (c) Iron
   (d) Sodium

6. Brass is an alloy composed primarily of:
   (a) Copper and Tin
   (b) Copper and Zinc
   (c) Iron and Carbon
   (d) Lead and Tin

7. The property of metals by which they can be drawn into thin wires is known as:
   (a) Malleability
   (b) Sonority
   (c) Ductility
   (d) Conductivity

8. Which gas is produced when Zinc metal reacts with Sodium Hydroxide solution?
   (a) Oxygen
   (b) Hydrogen
   (c) Zinc Hydroxide vapour
   (d) Sodium Zincate gas

9. Galvanization is a process of coating iron with a layer of which metal to prevent rusting?
   (a) Copper
   (b) Tin
   (c) Zinc
   (d) Aluminium

10. Which of the following statements is INCORRECT?
    (a) Mercury is the only metal which is liquid at room temperature.
    (b) Graphite, an allotrope of carbon, is a good conductor of electricity.
    (c) Metal oxides are generally acidic in nature.
    (d) Gold and Platinum are highly resistant to corrosion.

Answer Key for MCQs:

1. (c)
2. (c)
3. (c)
4. (b)
5. (d)
6. (b)
7. (c)
8. (b)
9. (c)
10. (c) (Metal oxides are generally basic or amphoteric)

Study these notes thoroughly, focusing on the comparisons, exceptions, and reaction types. Understanding the reactivity series is crucial for displacement reactions. Good luck with your preparation!