Materials Metals And Non Metals Class 8 Notes

Key Definitions

Malleability

The property of metals by which they can be beaten into thin sheets without breaking. Example: Aluminium foil.

Ductility

The property of metals by which they can be drawn into thin wires. Example: Copper wires.

Sonorous

The property of producing a ringing sound when struck hard. Metals are sonorous.

Lustre

The property of having a shiny surface. Metals usually possess a metallic lustre.

Conductor

A material that allows heat and electricity to pass through it easily. Metals are generally good conductors.

Insulator

A material that does not allow heat and electricity to pass through it easily. Non-metals are generally insulators.

Corrosion

The gradual destruction of materials (usually metals) by chemical reaction with their environment, often due to oxidation (e.g., rusting of iron).

Displacement Reaction

A chemical reaction in which a more reactive element displaces a less reactive element from its compound solution.

Comparison: Physical Properties of Metals and Non-Metals

Chemical Properties and Reactivity Series

Understanding the chemical properties of metals and non-metals is key to predicting their behaviour in various reactions. Most metals tend to lose electrons and form positive ions (cations), making them electropositive. Conversely, non-metals tend to gain or share electrons and form negative ions (anions) or covalent bonds, making them electronegative. This fundamental difference governs their reactions.

Reactions with Oxygen

• Metals react with oxygen to form basic oxides. These oxides turn red litmus blue. For example, when magnesium burns in air, it forms magnesium oxide (MgO), which dissolves in water to form magnesium hydroxide, a base.
• Metal + Oxygen → Metal Oxide (Basic)
• 2Mg (s) + O₂ (g) → 2MgO (s)
• Non-metals react with oxygen to form acidic or neutral oxides. Acidic oxides turn blue litmus red. For example, when carbon burns, it forms carbon dioxide (CO₂), which dissolves in water to form carbonic acid.
• Non-metal + Oxygen → Non-metal Oxide (Acidic or Neutral)
• C (s) + O₂ (g) → CO₂ (g)

Reactions with Water

• Some metals, like sodium and potassium, react vigorously with cold water to produce metal hydroxide and hydrogen gas. This reaction is often exothermic.
• 2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g) + Heat
• Less reactive metals, like iron, react slowly with steam. Non-metals generally do not react with water or steam.

Reactions with Acids and Bases

• Metals react with acids to produce salt and hydrogen gas, which burns with a 'pop' sound. This is a common test for hydrogen gas.
• Metal + Acid → Salt + Hydrogen Gas
• Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
• Most non-metals generally do not react with acids.
• Some metals like Aluminium and Zinc react with strong bases (e.g., Sodium Hydroxide) to produce hydrogen gas along with a salt. These are called amphoteric metals as their oxides can show both acidic and basic properties.
• Non-metals generally show complex reactions with bases, which are beyond the scope of Class 8.

Displacement Reactions and Reactivity Series

The Reactivity Series is an arrangement of metals in decreasing order of their reactivity. A more reactive metal can displace a less reactive metal from its salt solution. This is a crucial concept for understanding metal reactions.

• High Reactivity: Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminium (Al) > Zinc (Zn) > Iron (Fe) > Lead (Pb)
• Hydrogen (non-metal benchmark)
• Low Reactivity: Copper (Cu) > Mercury (Hg) > Silver (Ag) > Gold (Au)

For example, Zinc is more reactive than Copper. So, when a piece of zinc is placed in copper sulphate solution, zinc displaces copper:

• Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

However, Copper cannot displace Zinc from zinc sulphate solution because copper is less reactive than zinc. This principle of displacement helps us understand the relative strengths of metals.

Worked Examples of Reactions

• Reaction of Iron with Copper Sulphate When an iron nail (Fe) is dipped in copper sulphate solution (CuSO₄), the blue colour of the solution fades and a reddish-brown coating of copper (Cu) appears on the iron nail. This is because iron is more reactive than copper and displaces copper from its salt solution. Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s) (Iron) + (Copper Sulphate) → (Iron Sulphate) + (Copper)
• Burning of Sulphur in Air When sulphur powder (S), a non-metal, is heated and then reacted with oxygen (from air), it forms sulphur dioxide gas (SO₂). This gas, when dissolved in water, forms sulphurous acid, which turns blue litmus red, confirming its acidic nature. S (s) + O₂ (g) → SO₂ (g) SO₂ (g) + H₂O (l) → H₂SO₃ (aq) (Sulphurous Acid)

Key Points: Must Remember

• Exceptions are crucial: Mercury is the only metal liquid at room temperature. Carbon (in diamond form) is the hardest substance. Graphite (an allotrope of carbon) is a good conductor of electricity.
• Metals are generally good conductors of heat and electricity due to free electrons.
• Non-metals are generally poor conductors (insulators), except graphite.
• Oxides of metals are generally basic in nature (turn red litmus blue).
• Oxides of non-metals are generally acidic or neutral in nature (acidic oxides turn blue litmus red).
• Sodium and Potassium are highly reactive metals, stored in kerosene to prevent reaction with air and moisture.
• Phosphorus is a highly reactive non-metal, stored in water to prevent reaction with atmospheric oxygen.
• A more reactive metal displaces a less reactive metal from its salt solution (displacement reaction).
• Metals react with acids to produce hydrogen gas; non-metals generally do not.
• Uses of metals and non-metals are diverse: metals for machinery, electrical wires, jewellery; non-metals for fertilisers, disinfectants, fuels, and life processes (e.g., oxygen).

Exam Tip: Mastering Metals and Non-Metals

To ace questions on this chapter, focus on exceptions to general properties. Examiners often test these. For instance, while metals are hard, sodium and potassium are soft; while non-metals are dull, iodine is lustrous. Memorize the Reactivity Series as it's fundamental for predicting displacement reactions. Practice writing balanced chemical equations for reactions of metals and non-metals with oxygen, water, and acids. Pay attention to litmus paper tests for metal oxides (basic) and non-metal oxides (acidic) to distinguish them. A clear tabular comparison of properties can help you score full marks in descriptive questions. Use YoLearn AI Tools like Flashcards for quick recall of exceptions and reactivity order, and Mind Maps to link properties with examples and uses.

Practice Questions with Solutions

• Q: Give two physical properties that differentiate metals from non-metals. A: Metals are generally malleable and ductile, while non-metals are brittle and non-ductile. Metals are sonorous, non-metals are not.
• Q: What happens when a more reactive metal is placed in a salt solution of a less reactive metal? Give an example. A: A displacement reaction occurs where the more reactive metal displaces the less reactive metal from its salt solution. Example: Zinc (more reactive) displaces copper from copper sulphate solution: Zn + CuSO₄ → ZnSO₄ + Cu.
• Q: Name a metal that is liquid at room temperature and a non-metal that is a good conductor of electricity. A: Mercury is a metal liquid at room temperature. Graphite (an allotrope of carbon) is a non-metal that is a good conductor of electricity.
• Q: Why is sodium stored in kerosene? A: Sodium is a highly reactive metal that reacts vigorously with oxygen and water present in the air. Storing it in kerosene prevents its contact with air and moisture, thus preventing spontaneous ignition.

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