General Science Notes Chapter 7 Metals and Nonmetals Class 8 Maharashtra Board

Notes For All Chapters – General Science Class 8
1. Introduction

• Everything around us is made of elements.
• Elements are classified into metals, nonmetals, and metalloids based on their properties.

Types of Elements:

1. Metals – Shiny, strong, and good conductors (e.g., Iron, Copper).
2. Nonmetals – Dull, brittle, and poor conductors (e.g., Oxygen, Sulfur).
3. Metalloids – Have properties of both metals and nonmetals (e.g., Silicon, Boron).

2. Physical Properties of Metals

1. State – Most metals are solid at room temperature (except Mercury, which is liquid).

2. Lustre – Metals have a shiny surface (e.g., Gold, Silver).

3. Hardness – Metals are hard, except Sodium and Potassium, which are soft.

4. Malleability – Metals can be hammered into thin sheets (e.g., Aluminum foil).

5. Ductility – Metals can be drawn into thin wires (e.g., Copper wires).

6. Conduction of Heat – Metals are good conductors of heat (e.g., Silver, Copper).

7. Conduction of Electricity – Metals are good conductors of electricity (e.g., Aluminum, Copper).

8. Density – Most metals have high density (except Sodium, Potassium, and Lithium, which are light).

9. Melting & Boiling Point – Metals have high melting and boiling points (except Mercury, Sodium, and Potassium).

10. Sonority – Metals produce a ringing sound when struck (used in bells).

3. Physical Properties of Nonmetals

1. State – Nonmetals exist in solid, liquid, or gas form.

• Solids: Carbon, Sulfur, Phosphorus
• Liquids: Bromine
• Gases: Oxygen, Nitrogen

2. Lustre – Nonmetals are dull, except for Diamond and Iodine, which are shiny.

3. Brittleness – Nonmetals are brittle and break easily when hit.

4. Ductility & Malleability – Nonmetals cannot be drawn into wires or sheets.

5. Conduction of Heat & Electricity – Nonmetals are poor conductors, except Graphite (a form of Carbon), which conducts electricity.

6. Density – Nonmetals have low density.

7. Melting & Boiling Point – Nonmetals have low melting and boiling points, except Carbon and Boron, which have high melting points.

4. Metalloids

• Metalloids have properties of both metals and nonmetals.
• Examples: Silicon (Si), Boron (B), Arsenic (As), Germanium (Ge), Antimony (Sb).

5. Chemical Properties of Metals

1. Reaction with Oxygen

• Metals react with oxygen to form metal oxides.
• Metal + Oxygen → Metal Oxide

Example:

• • Magnesium + Oxygen → Magnesium Oxide (MgO)
• Metal oxides are basic and react with acids to form salt and water.

2. Reaction with Acids

• Metals react with acids to form metal salts and hydrogen gas.
• Metal + Acid → Salt + Hydrogen Gas

Example:

• • Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen Gas

3. Reaction with Water

• Some metals react with water to form metal hydroxides and hydrogen gas.
• Metal + Water → Metal Hydroxide + Hydrogen Gas

Example:

• • Sodium + Water → Sodium Hydroxide + Hydrogen Gas
• Sodium and Potassium react violently with water, so they are stored in kerosene.

6. Chemical Properties of Nonmetals

1. Reaction with Oxygen

• Nonmetals react with oxygen to form nonmetal oxides.
• Nonmetal + Oxygen → Nonmetal Oxide

Example:

• • Carbon + Oxygen → Carbon Dioxide (CO₂)
• Nonmetal oxides are acidic and react with bases to form salts and water.

2. Reaction with Acids

• Nonmetals do not react with acids.

3. Reaction with Water

• Nonmetals do not react with water except for a few like chlorine.

7. Corrosion

• Corrosion is the slow destruction of metals due to reaction with air, water, or chemicals.

Examples:

• • Iron rusts to form reddish-brown iron oxide.
  • Copper turns green due to the formation of copper carbonate.
  • Silver turns black due to the formation of silver sulfide.

Ways to Prevent Corrosion:

1. Painting and Greasing – Prevents air and moisture from reaching the metal.
2. Galvanization – Coating iron with zinc to prevent rusting.
3. Alloying – Mixing metals with other elements (e.g., Stainless steel = Iron + Chromium + Nickel).

8. Alloys

An alloy is a mixture of two or more metals, or a metal with a nonmetal.

Uses of Alloys:

• Brass (Copper + Zinc) – Used in musical instruments.
• Bronze (Copper + Tin) – Used for statues and medals.
• Stainless Steel (Iron + Chromium + Nickel) – Used in utensils and machinery.
• Gold Jewelry (Gold + Copper/Silver) – Increases strength of gold.

9. Noble Metals

• Gold, Silver, Platinum, Palladium, and Rhodium are noble metals.
• They are resistant to corrosion and used in jewelry, electronics, and medicine.
• Gold purity is measured in carats (24-carat is pure gold, 22-carat is mixed with copper or silver).

10. Uses of Metals and Nonmetals