Class 8 Science Notes Chapter 4 (Materials: Metals and Non Metals) – Science Book

In this revision notes, we will focus on the key concepts from NCERT Class 8 Science Chapter 4: Materials: Metals and Non-Metals, tailored for exam preparation.

Chapter 4: Materials: Metals and Non-Metals - Detailed Notes

1. Introduction:

• Materials around us can be broadly classified into Metals and Non-metals based on their physical and chemical properties.
• Understanding these properties is crucial for identifying materials and understanding their uses.

2. Physical Properties of Metals:

• Lustre: Metals have a shining surface in their pure state. This property is called metallic lustre.
  • Examples: Gold, Silver, Iron, Copper.
• Hardness: Metals are generally hard. Hardness varies from metal to metal.
  • Examples: Iron is very hard.
  • Exception: Sodium (Na) and Potassium (K) are soft metals that can be cut with a knife. Mercury (Hg) is a liquid metal at room temperature.
• Malleability: The property of metals by which they can be beaten into thin sheets.
  • Examples: Gold and Silver are highly malleable (used for foils). Aluminium foils are used for wrapping food.
• Ductility: The property of metals by which they can be drawn into thin wires.
  • Examples: Gold is the most ductile metal. Copper and Aluminium wires are used in electrical wiring.
• Sonority: Metals produce a ringing sound when struck hard. They are said to be sonorous.
  • Examples: School bells, musical instrument strings (like guitar).
• Conductivity (Heat & Electricity): Metals are good conductors of heat and electricity.
  • Heat Conductivity: Used for making cooking utensils (e.g., Copper, Aluminium).
  • Electrical Conductivity: Used for making electrical wires (e.g., Copper, Aluminium). Silver is the best conductor, but expensive.
  • Poor Conductors (among metals): Lead and Mercury are comparatively poor conductors of heat. Bismuth is a poor conductor of electricity.
• State: Metals are generally solid at room temperature.
  • Exception: Mercury (Hg) is liquid at room temperature. Gallium (Ga) and Caesium (Cs) have very low melting points and can melt in the palm.
• Density: Metals generally have high density.
  • Exception: Sodium (Na) and Potassium (K) have low densities (can float on water).

3. Physical Properties of Non-Metals:

• Lustre: Non-metals generally do not have lustre; they have a dull appearance.
  • Exception: Iodine (I) crystals are lustrous. Graphite and Diamond (allotropes of Carbon) also have lustre.
• Hardness: Non-metals are generally soft.
  • Exception: Diamond (an allotrope of Carbon) is the hardest known natural substance.
• Malleability: Non-metals are non-malleable. They are brittle, meaning they break into pieces when hammered or stretched.
  • Example: Sulphur, Coal (Carbon).
• Ductility: Non-metals are non-ductile.
• Sonority: Non-metals are non-sonorous. They do not produce a ringing sound when struck.
• Conductivity (Heat & Electricity): Non-metals are generally poor conductors of heat and electricity (insulators).
  • Exception: Graphite (an allotrope of Carbon) is a good conductor of electricity. Diamond is a good conductor of heat but a poor conductor of electricity.
• State: Non-metals exist in all three states at room temperature.
  • Solid: Carbon (C), Sulphur (S), Phosphorus (P), Iodine (I).
  • Liquid: Bromine (Br).
  • Gas: Oxygen (O₂), Hydrogen (H₂), Nitrogen (N₂), Chlorine (Cl₂).
• Density: Non-metals generally have low density.

Summary Table: Physical Properties

4. Chemical Properties of Metals:

• Reaction with Oxygen (O₂): Metals react with oxygen to form metal oxides, which are generally basic in nature. Basic oxides turn red litmus paper blue.
  • General Equation: Metal + Oxygen → Metal Oxide
  • Example 1 (Rusting): Iron (Fe) reacts with Oxygen (from air) and Water (moisture) to form Iron Oxide (Rust - Fe₂O₃·xH₂O).
    4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (Rust - Brown)
    Rust is basic (turns red litmus slightly blue).
  • Example 2: Magnesium (Mg) burns in air to form Magnesium Oxide (MgO).
    2Mg + O₂ → 2MgO
    MgO dissolves in water to form Magnesium Hydroxide (Mg(OH)₂), which is basic.
    MgO + H₂O → Mg(OH)₂
  • Amphoteric Oxides: Some metal oxides (like Aluminium Oxide - Al₂O₃, Zinc Oxide - ZnO) show both acidic and basic behaviour. (This is slightly advanced for Class 8 but important for exams).
• Reaction with Water (H₂O):
  • Different metals react differently with water depending on their reactivity.
  • Highly Reactive Metals (Na, K): React vigorously with cold water, producing metal hydroxide and Hydrogen gas (H₂), often catching fire.
    2Na + 2H₂O → 2NaOH + H₂ + Heat
  • Less Reactive Metals (Mg): React with hot water to form metal hydroxide and H₂.
    Mg + 2H₂O (Hot) → Mg(OH)₂ + H₂
  • Metals like Al, Zn, Fe: React with steam (not cold/hot water) to form metal oxide and H₂.
    3Fe + 4H₂O (Steam) → Fe₃O₄ + 4H₂
  • Least Reactive Metals (Cu, Ag, Au, Pt): Do not react with water/steam.
• Reaction with Acids (Dilute): Most metals react with dilute acids (like HCl, H₂SO₄) to produce Metal Salt and Hydrogen gas (H₂). Hydrogen gas burns with a 'pop' sound.
  • General Equation: Metal + Dilute Acid → Metal Salt + Hydrogen Gas
  • Example: Zinc (Zn) reacts with dilute Hydrochloric Acid (HCl).
    Zn + 2HCl → ZnCl₂ + H₂↑
  • Example: Iron (Fe) reacts with dilute Sulphuric Acid (H₂SO₄).
    Fe + H₂SO₄ → FeSO₄ + H₂↑
  • Note: Copper (Cu), Silver (Ag), Gold (Au) do not react readily with dilute acids like HCl or H₂SO₄. Nitric acid (HNO₃) behaves differently as it's a strong oxidizing agent.
• Reaction with Bases: Some metals react with strong bases like Sodium Hydroxide (NaOH) to produce Hydrogen gas.
  • Example: Aluminium (Al) reacts with NaOH solution.
    2Al + 2NaOH + 2H₂O → 2NaAlO₂ (Sodium Aluminate) + 3H₂↑
    (Zinc also reacts similarly). This shows the amphoteric nature of these metals/their oxides.

5. Chemical Properties of Non-Metals:

• Reaction with Oxygen (O₂): Non-metals react with oxygen to form non-metal oxides, which are generally acidic or neutral in nature. Acidic oxides turn blue litmus paper red.
  • General Equation: Non-metal + Oxygen → Non-metal Oxide
  • Example 1 (Acidic Oxide): Sulphur (S) burns in air to form Sulphur Dioxide (SO₂).
    S + O₂ → SO₂
    SO₂ dissolves in water to form Sulphurous Acid (H₂SO₃), which is acidic.
    SO₂ + H₂O → H₂SO₃
  • Example 2 (Acidic Oxide): Carbon (C) burns to form Carbon Dioxide (CO₂).
    C + O₂ → CO₂
    CO₂ dissolves partially in water to form Carbonic Acid (H₂CO₃), which is acidic.
    CO₂ + H₂O ⇌ H₂CO₃
  • Neutral Oxides: Some non-metal oxides like Carbon Monoxide (CO), Nitrous Oxide (N₂O), Nitric Oxide (NO), and Water (H₂O) are neutral (do not change litmus paper colour).
• Reaction with Water (H₂O): Non-metals generally do not react with water (though their acidic oxides do, as shown above).
  • Exception: Highly reactive non-metals like Phosphorus (P) are often stored in water to prevent reaction with atmospheric oxygen.
• Reaction with Acids (Dilute): Non-metals generally do not react with dilute acids.
• Reaction with Bases: Reactions of non-metals with bases are often complex.
  • Example: Chlorine (Cl₂) reacts with cold, dilute NaOH. Acidic oxides (like CO₂, SO₂) react with bases (like NaOH, Ca(OH)₂) to form salt and water (neutralization reaction).
    CO₂ + 2NaOH → Na₂CO₃ + H₂O
SO₂ + Ca(OH)₂ → CaSO₃ + H₂O

6. Displacement Reactions:

• A more reactive metal can displace (replace) a less reactive metal from its salt solution.
• This is based on the Reactivity Series (a list of metals arranged in order of decreasing reactivity). A simplified series relevant here: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.
• Example 1: When an iron nail (Fe) is placed in a blue Copper Sulphate solution (CuSO₄), the iron displaces copper. The blue colour fades, a reddish-brown deposit of copper forms on the nail, and greenish Iron Sulphate (FeSO₄) solution is formed.
  Fe (s) + CuSO₄ (aq) (Blue) → FeSO₄ (aq) (Greenish) + Cu (s) (Red-brown)
  (Iron is more reactive than Copper).
• Example 2: Zinc (Zn) displaces Copper (Cu) from Copper Sulphate solution.
  Zn (s) + CuSO₄ (aq) (Blue) → ZnSO₄ (aq) (Colourless) + Cu (s) (Red-brown)
  (Zinc is more reactive than Copper).
• Example 3 (No Reaction): If a copper strip (Cu) is placed in Iron Sulphate solution (FeSO₄), no reaction occurs because copper is less reactive than iron.
  Cu (s) + FeSO₄ (aq) → No Reaction
• Similar displacement reactions can occur between metals and acids (displacement of Hydrogen) and between non-metals (e.g., Chlorine displacing Bromine from KBr solution).

7. Uses of Metals:

• Iron (Fe): Machinery, bridges, buildings, vehicles, tools, magnets. (Often used as steel - an alloy).
• Aluminium (Al): Utensils, foils, aircraft parts, electrical wires, window frames. (Lightweight, corrosion-resistant).
• Copper (Cu): Electrical wires, utensils, pipes, statues, alloys (brass, bronze). (Excellent conductor).
• Gold (Au) & Silver (Ag): Jewellery, coins, dentistry, electroplating. (Lustrous, malleable, ductile, less reactive).
• Zinc (Zn): Galvanizing iron (prevent rusting), batteries (dry cells), alloys (brass).
• Lead (Pb): Batteries, alloys, radiation shielding.
• Mercury (Hg): Thermometers, barometers.
• Sodium (Na) & Potassium (K): Used in chemical reactions, nuclear reactors (coolant - Na).

8. Uses of Non-Metals:

• Oxygen (O₂): Essential for respiration (life), combustion (burning).
• Nitrogen (N₂): Manufacturing fertilizers (ammonia), food packaging (inert atmosphere), major component of air.
• Hydrogen (H₂): Fuel (rockets), making ammonia, hydrogenation of oils (making vanaspati ghee).
• Carbon (C):
  • Graphite: Electrodes (in batteries, electrolysis), pencil lead, lubricant.
  • Diamond: Jewellery, cutting tools (due to hardness).
  • Coal/Coke: Fuel.
  • Charcoal: Fuel, water purification.
  • Basis of all organic compounds.
• Sulphur (S): Manufacturing sulphuric acid (H₂SO₄ - 'King of Chemicals'), gunpowder, vulcanization of rubber, skin ointments (fungicide).
• Phosphorus (P): Manufacturing fertilizers (superphosphate), matchsticks, fireworks, rat poison.
• Chlorine (Cl₂): Water purification (disinfectant), bleaching agent, making PVC plastic, pesticides.
• Iodine (I): Antiseptic (Tincture of Iodine - solution in alcohol), required by thyroid gland (added to salt - iodized salt).
• Silicon (Si): Making semiconductors (computer chips), glass, cement.
• Noble Gases (He, Ne, Ar): Helium (balloons, cooling), Neon (advertising signs), Argon (light bulbs - inert atmosphere).

Exam Focus Points:

• Memorize the key physical properties distinguishing metals and non-metals.
• Pay close attention to the exceptions in physical properties (e.g., Mercury, Graphite, Iodine, Diamond, Na/K). These are frequent targets for questions.
• Understand the nature of oxides formed by metals (basic/amphoteric) and non-metals (acidic/neutral) and their effect on litmus paper.
• Know the general patterns of reaction with water and acids for different metals based on reactivity.
• Understand the concept of displacement reactions and be able to predict the outcome based on relative reactivity (e.g., Fe + CuSO₄).
• Be familiar with common uses of important metals and non-metals.
• Remember key specific examples like rusting, burning of Mg ribbon, reaction of S with O₂.

This detailed breakdown covers the core concepts of Chapter 4 relevant for competitive exams, focusing on definitions, properties, exceptions, reactions, and applications. Remember to correlate these points with the NCERT textbook for diagrams and activities.