Study Notes of Physical and Chemical Change

Grade VII Science Notes

Physical and Chemical Changes

1. What is a Change?

A change is any process in which a substance becomes different in appearance, shape, size, or composition.

Examples:

  • Ice melting into water
  • Burning of paper
  • Rusting of iron
  • Cooking food

Changes are mainly of two types:

  1. Physical Change
  2. Chemical Change

2. Physical Change

A physical change is a change in which only the physical properties (shape, size, colour, or state) of a substance change. No new substance is formed.

Characteristics

  • No new substance is produced.
  • Usually temporary.
  • Often reversible.
  • Chemical composition remains the same.
  • Only physical properties change.

Examples

  • Melting of ice
  • Boiling of water
  • Freezing of water
  • Tearing paper
  • Breaking glass
  • Dissolving sugar in water
  • Stretching a rubber band
  • Cutting vegetables

3. What Happens in a Physical Change?

During a physical change:

  • The particles remain the same.
  • Only the arrangement or movement of particles changes.
  • The substance keeps its original identity.
  • The change may be reversed.

Example

Ice → Water → Steam

All three are forms of water (Hâ‚‚O). No new substance is formed.

4. Crystallization

Crystallization is the process of obtaining pure crystals of a substance from its solution.

It is used to purify solids.

Steps of Crystallization

  1. Dissolve the impure solid in hot water.
  2. Filter the solution.
  3. Allow the solution to cool slowly.
  4. Pure crystals are formed.
  5. Dry the crystals.

Examples

  • Copper sulphate crystals
  • Alum crystals
  • Sugar crystals
  • Salt crystals

Uses

  • Purifying chemicals
  • Making medicines
  • Producing beautiful crystals for laboratories

5. Chemical Change

A chemical change is a change in which one or more new substances with different properties are formed.

Characteristics

  • New substance is formed.
  • Usually permanent.
  • Usually irreversible.
  • Heat, light, sound, gas, or colour change may be produced.
  • Chemical composition changes.

Examples

  • Burning wood
  • Cooking food
  • Digestion of food
  • Rusting of iron
  • Curdling of milk
  • Burning a candle (chemical as well as physical)

6. Signs of a Chemical Change

A chemical change may show one or more of these signs:

  • Formation of a new substance
  • Colour change
  • Gas released
  • Heat produced or absorbed
  • Light produced
  • Smell changes
  • Formation of a solid (precipitate)

7. Physical Change vs Chemical Change

Physical Change

Chemical Change

No new substance is formed.

New substance is formed.

Usually reversible.

Usually irreversible.

Physical properties change.

Chemical properties change.

Temporary.

Permanent.

Example: Melting ice

Example: Rusting iron

8. Useful Chemical Changes

These chemical changes are beneficial.

Examples

  • Cooking food
  • Digestion of food
  • Respiration
  • Photosynthesis
  • Making curd
  • Burning fuel for energy
  • Manufacture of medicines
  • Baking bread and cakes

9. Undesirable Chemical Changes

These changes cause damage or loss.

Examples

  • Rusting of iron
  • Food spoilage
  • Corrosion of metals
  • Burning of forests
  • Rancidity of oils
  • Decay of fruits and vegetables

10. Corrosion

Corrosion is the slow destruction of metals due to the action of air, water, moisture, or chemicals.

It weakens the metal and causes economic loss.

Examples

  • Rusting of iron
  • Green coating on copper
  • Black coating on silver

11. Rusting

Rusting is a type of corrosion that occurs only in iron.

When iron reacts with oxygen and moisture, it forms a reddish-brown substance called rust.

Conditions Needed for Rusting

  • Oxygen (air)
  • Water or moisture

Without either oxygen or water, rusting does not occur.

Word Equation

Iron + Oxygen + Water → Rust (Hydrated Iron Oxide)

Effects of Rusting

  • Weakens iron objects
  • Damages bridges, vehicles, and machines
  • Increases maintenance costs

12. Prevention of Rusting

Rusting can be prevented by keeping iron away from air and moisture.

Methods

1. Painting

A coat of paint prevents air and water from reaching iron.

Examples: Gates, bridges, railings.

2. Oiling and Greasing

Oil or grease forms a protective layer.

Examples: Bicycle chains, machines.

3. Galvanization

Iron is coated with a layer of zinc.

Examples: Water pipes, iron sheets, buckets.

4. Electroplating

A thin layer of another metal such as chromium or nickel is coated on iron.

Examples: Bathroom fittings, car parts.

5. Alloying

Iron is mixed with other metals to form stainless steel, which resists rusting.

Examples: Kitchen utensils, surgical instruments.

13. Importance of Physical and Chemical Changes

Importance of Physical Changes

  • Recycling materials
  • Making ice
  • Shaping metals
  • Cutting and polishing stones

Importance of Chemical Changes

  • Cooking food
  • Digestion
  • Respiration
  • Manufacture of medicines, fertilizers, and plastics
  • Production of electricity in batteries

Key Differences (Quick Revision)

  • Physical Change: No new substance is formed.
  • Chemical Change: A new substance is formed.
  • Crystallization: Process of obtaining pure crystals from a solution.
  • Corrosion: Damage to metals by air, water, or chemicals.
  • Rusting: Corrosion of iron due to oxygen and moisture.
  • Galvanization: Coating iron with zinc to prevent rusting.

Keywords to Remember

  • Change
  • Physical Change
  • Chemical Change
  • Reversible Change
  • Irreversible Change
  • Crystallization
  • Crystal
  • Corrosion
  • Rust
  • Rusting
  • Galvanization
  • Electroplating
  • Alloy
  • Moisture
  • Oxygen
  • Zinc
  • Stainless Steel

Exam Points

Very Short Questions

  1. What is a physical change?
  2. What is a chemical change?
  3. Define crystallization.
  4. What is corrosion?
  5. What is rusting?
  6. Name two conditions necessary for rusting.
  7. Name one method of preventing rusting.
  8. Give one example each of a physical and a chemical change.

Short Answer Questions

  1. Differentiate between physical and chemical changes.
  2. Explain the process of crystallization.
  3. What are the characteristics of a chemical change?
  4. Explain corrosion with examples.
  5. Describe four methods to prevent rusting.

Long Answer Questions

  1. Explain physical and chemical changes with suitable examples.
  2. Describe rusting, its causes, effects, and methods of prevention.
  3. Explain crystallization and its uses with neat labelled steps.