Class 8 Notes: Metals and Non-Metals CBSE

Class 8 Notes: Metals and Non-Metals

Elements

Definition: Elements are pure substances that cannot be broken down into simpler substances by chemical method.

Examples: Oxygen (O), Hydrogen (H), Carbon (C).

Classification of Elements

1. Metals: Good conductors of heat and electricity, typically solid at room temperature (except mercury).

Physical Properties:

• Density: How tightly packed particles are.
• Example: Gold is dense.
• Melting Point: Temperature at which a solid turns to liquid.
• Example: Ice melts at 0°C.
• Boiling Point: Temperature at which a liquid turns to vapor.
• Example: Water boils at 100°C.
• Malleability: Ability to be hammered into thin sheets.
• Example: Gold is malleable.
• Electrical Conductivity: Ability to conduct electricity.
• Example: Copper conducts electricity.

Chemical Properties with Examples and Equations:

• Flammability: Ability to burn.
• Example: Paper burns.
• Reactivity with Acid: Reacts with acids.
• Example: Magnesium reacts with hydrochloric acid.
  Chemical Equation: Mg + 2HCl → MgCl2 + H2
• Oxidation: Reacts with oxygen.
• Example: Iron rusts.
  Chemical Equation: 4Fe + 3O2 → 2Fe2O3
• Corrosion Resistance: Ability to resist chemical damage.
• Example: Stainless steel resists rusting.
• Reaction with Water: Reacts with water.
• Example: Sodium reacts violently with water.
  Chemical Equation: 2Na + 2H2O → 2NaOH + H2

Types of Metals

Ferrous Metals

Ferrous metals contain iron as their main component.

• Examples:
  • Steel: Strong and durable, used in construction.
  • Cast Iron: Brittle but useful in pipes and engine parts.
  • Wrought Iron: Historically used for decorative items.
• Properties:
  • Strong and magnetic.
  • Prone to rust and corrosion.

Non-Ferrous Metals

Non-ferrous metals do not contain significant amounts of iron.

• Examples:
  • Copper: Highly conductive, used in electrical wiring.
  • Aluminum: Lightweight, used in aerospace and packaging.
  • Lead: Dense, used in batteries and radiation shielding.
• Properties:
  • Non-magnetic and resistant to corrosion.
  • Malleable and lightweight.

(i) Light Metals:

Sodium (Na), Potassium (K), Magnesium (Mg), Beryllium (Be), Aluminium (Al)

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(ii) Heavy Metals:

Iron (Fe), Lead (Pb), Copper (Cu), Gold (Au), Platinum (Pt)

(iii) Protective Metals:

Zinc (Zn), Tin (Sn), Chromium (Cr), Nickel (Ni)

(iv) Noble Metals:

Silver (Ag), Gold (Au), Platinum (Pt)

(v) Rare Metals:

Lithium (Li), Beryllium (Be), Tin (Sn)

(vi) Radioactive Metals:

Radium (Ra), Uranium (U), Thorium (Th)

Occurrence of Metals

In Nature: Metals are found in the Earth's crust, often combined with other elements in ores.

Extraction: Metals are extracted from their ores through processes like smelting and electrolysis.

Noble Metals and Their Uses

Definition: Noble metals are metals that are resistant to corrosion and oxidation in moist air.

Examples: Gold (Au), Silver (Ag), Platinum (Pt).

Uses: Jewelry, currency, electronics due to their durability and conductivity.

2. Non-Metals: Poor conductors of heat and electricity, can be solid, liquid, or gas at room temperature.

Occurrence of Non-Metals

In Nature: Non-metals are found in various forms, from gases (e.g., Oxygen) to solids (e.g., Sulfur).

Physical Properties of Non- Metals:

• Low Density: Non-metals are generally less dense than metals.
  Example: Helium is less dense than iron.
• Brittle: Non-metals tend to break or shatter easily when subjected to force.
  Example: Graphite in pencils breaks into small pieces.
• Poor Conductivity: Non-metals do not conduct electricity or heat well.
  Example: Sulphur is a poor conductor of electricity.
• Low Melting and Boiling Points: Non-metals generally have lower melting and boiling points compared to metals.
  Example: Carbon (graphite) melts at around 3700°C.
• Dull Appearance: Most non-metals have a dull or matte appearance.
  Example: Phosphorus appears dull and waxy.

Chemical Properties of Non- Metals:

• Flammability: Non-metals can be highly flammable.
  Example: Hydrogen gas burns with a pop sound in the presence of oxygen.
  Chemical Equation: 2H2 + O2 → 2H2O
• Reaction with Oxygen: Non-metals react with oxygen to form oxides.
  Example: Sulphur burns in air to form sulphur dioxide gas.
  Chemical Equation: S + O2 → SO2
• Reaction with Halogens: Non-metals react with halogens to form compounds.
  Example: Chlorine reacts with phosphorus to form phosphorus trichloride gas.
  Chemical Equation: P + 3Cl2 → PCl3
• Acidic Nature: Some non-metals, like sulphur and phosphorus, form acidic oxides when they react with water.
  Example: Sulphur dioxide dissolves in water to form sulphurous acid.
  Chemical Equation: SO2 + H2O → H2SO3
• Reaction with Water: Non-metals do not react with water as readily as metals, but some can react slowly.
  Example: Phosphorus reacts slowly with water to form phosphoric acid.
  Chemical Equation: P4 + 5O2 + 6H2O → 4H3PO4

Purity of Gold

Purity: Gold purity is measured in carats (24-carat gold is pure gold).

Uses: Jewelry, investment, and industrial applications due to its resistance to corrosion and malleability.

Alloy

Definition: An alloy is a mixture of metals or a metal combined with other elements.

Examples: Brass (Copper and Zinc), Bronze (Copper and Tin).

Uses: Increased strength, durability, and specific properties for various applications.

Corrosion

Corrosion is the gradual destruction of materials (usually metals) by chemical reactions with the environment.

Prevention:

1. Oil Paint and Grease: Applying oil-based paints or greases to create a barrier against moisture and corrosive substances.
2. Electroplating: Coating a metal object with a thin layer of another metal using electrolysis to prevent corrosion and enhance appearance.
3. Alloying: Mixing a metal with other elements to form alloys that are more resistant to corrosion than pure metals.
4. Galvanization: Coating iron or steel with a layer of zinc to protect against corrosion by sacrificial protection.
5. Anodizing: Creating an oxide layer on the surface of metals (like aluminum) through electrolytic passivation to enhance corrosion resistance.
6. Tinning: Coating iron or steel with a thin layer of tin to prevent corrosion and improve solderability.