Class 7 | ch 4 | Science part 1

 Material:-

Material is any substance or matter from which an object or thing is made.

Properties of Materials:-

1. Malleability

Definition:
Malleability is the property of a material by which it can be beaten or hammered into thin sheets without breaking.

Examples:

• Gold
• Silver

2.Ductility

Definition:

Ductility is the property of a material by which it can be drawn into thin wires.

Examples:

Copper

Aluminium

3. Sonority

Definition:

Sonority is the property of a material by which it produces a ringing sound when struck.

Examples:

Iron

Copper

Aluminium

Conduction of Heat and Electricity:

Conduction of Heat:-

Conduction of heat is the process by which heat is transferred from the hotter part of an object to the colder part without the movement of the material itself.

Examples

A metal spoon becomes hot when one end is placed in hot tea.

The handle of a metal pan becomes hot while cooking.

Good Conductors of Heat

Materials that allow heat to pass through them easily are called good conductors of heat.

Copper

Aluminium

Iron

Silver

Poor Conductors (Insulators) of Heat:

Materials that do not allow heat to pass through them easily are called poor conductors or insulators.

Wood

Plastic

Rubber

Glass

Air

Conduction of Electricity:-

Conduction of electricity is the flow of electric current through a material.

Good Conductors of Electricity

Materials that allow electric current to pass through them easily are called good conductors of electricity.

Copper

Aluminium

Silver

Iron

Poor Conductors (Insulators) of Electricity:-

Materials that do not allow electric current to pass through them easily are called insulators.

Rubber

Plastic

Wood (dry)

Glass

Air

Effect of Air and Water on Metals:-

Many metals react with oxygen in the air and moisture (water). This reaction forms a layer of compounds on the metal's surface. Over time, the metal gets damaged. This process is called corrosion.

Corrosion

Corrosion is the slow deterioration of a metal due to its reaction with air, water, or other chemicals present in the environment.

Examples:

. Rusting of iron

. Green coating on copper

. Black coating on silver

Rusting:-

Rusting is a type of corrosion that occurs only in iron and iron-containing materials (such as steel) when they are exposed to both oxygen and water.

Rust is a reddish-brown substance called hydrated iron(III) oxide (Fe₂O₃·H₂O).

Conditions necessary for rusting:-

1.Oxygen (air) 

2.Water (moisture)

If either oxygen or water is absent, iron does not rust.

Effects of Rusting and Corrosion-

1. Weakens metal structures.

2. Reduces the strength of bridges, buildings, and vehicles.

3. Damages machinery and tools.

4. Causes economic loss due to repairs and replacement.

Prevention of Rusting and Corrosion

1. Painting – Prevents air and water from reaching the metal.

2.Oiling or Greasing – Forms a protective layer.

3. Galvanization – Coating iron with zinc.

4. Electroplating – Coating with chromium, nickel, or other metals.

5. Alloying – Making alloys like stainless steel, which resist corrosion.

Reaction with Oxygen (Metal vs Non-metal Oxides) 

When elements react with oxygen, they form oxides. The properties of these oxides depend on whether the element is a metal or a non-metal.

Metal Oxides

• Formed when metals react with oxygen.
• Usually basic in nature.
• Turn red litmus blue.
• React with water to form bases (alkalis).

General Equation:
Metal + Oxygen → Metal Oxide

Examples:

2Mg + O₂ → 2MgO

4Fe + 3O₂ → 2Fe₂O₃

4Na + O₂ → 2Na₂O

Example with Water:

MgO + H₂O → Mg(OH)₂

Magnesium hydroxide is a base, so magnesium oxide is a basic oxide.

Non-metal Oxides:-

Formed when non-metals react with oxygen.

Usually acidic in nature.

Turn blue litmus red.

React with water to form acids.

General Equation:

Non-metal + Oxygen → Non-metal Oxide

Examples:

C + O₂ → CO₂

S + O₂ → SO₂

Examples with Water:

CO₂ + H₂O → H₂CO₃ (Carbonic acid)

SO₂ + H₂O → H₂SO₃ (Sulphurous acid)

These reactions show that non-metal oxides are acidic.

Exceptions-

Amphoteric oxides (show both acidic and basic properties):

    Al₂O₃ (Aluminium oxide)

    ZnO (Zinc oxide)

Neutral oxides (neither acidic nor basic):

    CO (Carbon monoxide)

     NO (Nitric oxide)

Differences between Metals and Non-metals:-

Metals	Non-metals
Usually shiny (lustrous).	Usually dull (non-lustrous).
Hard and strong.	Generally soft and brittle.
Malleable (can be beaten into sheets).	Non-malleable (cannot be beaten into sheets).
Ductile (can be drawn into wires).	Non-ductile (cannot be drawn into wires).
Good conductors of heat and electricity.	Poor conductors of heat and electricity (except graphite).
Sonorous (produce a ringing sound when struck).	Non-sonorous (do not produce a ringing sound).
Usually have high melting and boiling points.	Usually have low melting and boiling points.
Mostly solids at room temperature (except mercury).	Can be solids, liquids (bromine), or gases (oxygen, nitrogen).
React with oxygen to form basic oxides.	React with oxygen to form acidic oxides.
React with acids to produce hydrogen gas (for many metals).	Generally do not react with acids to produce hydrogen gas.
Tend to lose electrons and form positive ions (cations).	Tend to gain electrons and form negative ions (anions).