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Integers
Fractions and Decimals
Simple Equations
Lines and Angles
The Triangle and Its Properties
Comparing Quantities
Rational Numbers
Perimeter and Area
Algebraic Expression
Exponents and Power
Symmetry
Visualising Solid Shape
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Nutrition in Plants
Nutrition in Animals
Heat
Acids, Bases and Salts
Physical and Chemical Changes
Respiration in Organisms
Transportation in Animals and Plants
Reproduction in Plants
Motion and Time
Electric Current and Its Effects
Light
Forests: Our Lifeline
Wastewater Story
Introduction: Tracing Changes through a Thousand Years
Kings and Kingdoms
Delhi: 12th to 15th Century
The Mughuls (16th to 17th Century)
Tribes, Nomads and Settled Communities
Devotional Paths to the Divine
The Making of Regional Cultures
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Foundation Batch For Class 7

PHYSICAL AND CHEMICAL CHANGES

Exploring the Magic of Transformations Around Us!

Introduction: The World of Changes

Imagine playing with clay. You make a pot, but then flatten it back into a lump. Now imagine burning a piece of paper. Can you reverse it?
These examples represent two kinds of changes in the world: Physical Changes and Chemical Changes.

What is a Change?

A change is a process where something becomes different from its original form. Changes can be temporary or permanent, reversible or irreversible.

Physical Changes

  1. Definition:
    A change in which the form or appearance of a substance changes, but its chemical composition remains the same.
  2. Key Characteristics:
    • No new substance is formed.
    • Usually reversible.
    • Only physical properties (shape, size, state, color) change.
  3. Examples:
    • Melting of ice into water.
    • Tearing of paper.
    • Dissolving sugar in water.
  4. Real-Life Activity:
    Stretching a Rubber Band:
    Stretch a rubber band and release it. Does it go back to its original form?

Chemical Changes

  1. Definition:
    A change in which a new substance is formed with different properties from the original substance.
  2. Key Characteristics:
    • A new substance is formed.
    • Usually irreversible.
    • Involves a change in chemical composition.
  3. Examples:
    • Burning of paper or wood.
    • Rusting of iron.
    • Cooking of food.
  4. Real-Life Activity:
    Vinegar and Baking Soda Reaction:
    Mix vinegar with baking soda. Notice the bubbles! This is due to the formation of carbon dioxide, a new substance.

Differences Between Physical and Chemical Changes

Property

Physical Change

Chemical Change

Formation of new substance

No

Yes

Reversibility

Usually reversible

Usually irreversible

Examples

Melting of ice, Dissolving salt

Burning of paper, Rusting of iron

Rusting of Iron: A Common Chemical Change

  1. What is Rusting?
    When iron reacts with oxygen and moisture (water), it forms a reddish-brown substance called rust.
    Reaction:
    Iron (Fe)+Oxygen (Oβ‚‚)+Water (Hβ‚‚O)β†’Rust (Feβ‚‚O₃.xHβ‚‚O)
  2. Prevention of Rusting:
    • Painting or Greasing: Creates a barrier.
    • Galvanization: Coating iron with zinc.
    • Alloying: Mixing iron with other metals.

Crystallization: A Special Physical Change

  1. What is Crystallization?
    The process of obtaining pure crystals from a solution.
  2. Example:
    Heating a salt solution until water evaporates and crystals form.
  3. Real-Life Use:
    • Used to make sugar and salt crystals.

Activity:
Dissolve sugar in water. Heat the solution. Watch sugar crystals form as water evaporates!

Indicators of Chemical Changes

  1. Formation of a Gas:
    Example: Effervescence in vinegar and baking soda reaction.
  2. Change in Color:
    Example: Ripening of fruits.
  3. Change in Temperature:
    Example: Heat released when quicklime reacts with water.
  4. Formation of Precipitate:
    Example: Mixing lime water with carbon dioxide to form chalk.

Applications in Daily Life

  1. Physical Changes:
    • Shaping of clay into pots.
    • Folding clothes.
  2. Chemical Changes:
    • Cooking food (new substances are formed).
    • Fermentation of milk to curd.

Quick Revision

Type of Change

Examples

Key Feature

Physical Change

Melting of ice, Boiling water

No new substance formed

Chemical Change

Burning wood, Rusting of iron

New substance is formed

Practice Questions

  1. Is dissolving sugar in water a physical or chemical change? Why?
  2. What are the two ways to prevent rusting?
  3. Write the reaction for rusting of iron.
  4. Give two examples each of physical and chemical changes.

Fun Activity: Test for a Chemical Change

  1. Take a glass of water. Add a spoon of turmeric powder.
  2. Now add some soap solution. Watch the color change!
  3. What type of change is this?

Summary

Physical and chemical changes are all around us. Physical changes only affect the appearance, while chemical changes create entirely new substances. Learning to identify and understand these changes makes science both fun and practical!

Β 

Basic Understanding

  1. A change in which no new substance is formed is called:
    A) Chemical change
    B) Physical change
    C) Nuclear change
    D) Biological change
  2. A change that forms a new substance is called:
    A) Physical change
    B) Temporary change
    C) Chemical change
    D) Reversible change
  3. Which of the following is a physical change?
    A) Burning paper
    B) Rusting iron
    C) Melting ice
    D) Cooking food
  4. Which of the following is a chemical change?
    A) Tearing paper
    B) Dissolving salt
    C) Burning wood
    D) Cutting cloth
  5. Physical changes are usually:
    A) Irreversible
    B) Reversible
    C) Permanent
    D) Harmful
  6. Chemical changes are usually:
    A) Reversible
    B) Temporary
    C) Irreversible
    D) Simple
  7. In a physical change, what remains unchanged?
    A) Shape
    B) Size
    C) Chemical composition
    D) Color
  8. Which of the following involves change in state only?
    A) Burning
    B) Rusting
    C) Melting
    D) Cooking
  9. Which change is reversible?
    A) Burning paper
    B) Rusting iron
    C) Melting ice
    D) Cooking food
  10. Which is NOT a physical change?
    A) Boiling water
    B) Freezing water
    C) Cooking food
    D) Dissolving sugar

11–25: Conceptual Questions

  1. Which of the following produces a new substance?
    A) Cutting wood
    B) Dissolving salt
    C) Burning fuel
    D) Breaking glass
  2. Rusting of iron is a:
    A) Physical change
    B) Chemical change
    C) Temporary change
    D) Reversible change
  3. Which condition is necessary for rusting?
    A) Heat only
    B) Oxygen only
    C) Water only
    D) Oxygen and moisture
  4. Rust is chemically:
    A) Iron oxide
    B) Copper oxide
    C) Zinc oxide
    D) Carbon
  5. Which of the following prevents rusting?
    A) Cutting
    B) Painting
    C) Heating
    D) Cooling
  6. Galvanization involves coating iron with:
    A) Copper
    B) Aluminium
    C) Zinc
    D) Silver
  7. Which is a sign of chemical change?
    A) Change in shape
    B) Formation of gas
    C) Change in size
    D) Change in state
  8. Which of the following shows color change due to chemical reaction?
    A) Melting ice
    B) Ripening fruit
    C) Cutting paper
    D) Boiling water
  9. Formation of a precipitate indicates:
    A) Physical change
    B) Chemical change
    C) Temporary change
    D) No change
  10. Dissolving sugar in water is a:
    A) Chemical change
    B) Physical change
    C) Permanent change
    D) Irreversible change
  11. Which of the following is irreversible?
    A) Freezing water
    B) Melting wax
    C) Burning paper
    D) Boiling water
  12. Cooking food is a:
    A) Physical change
    B) Chemical change
    C) Temporary change
    D) Reversible change
  13. Which change involves only appearance?
    A) Chemical
    B) Physical
    C) Nuclear
    D) Thermal
  14. Which of the following is not reversible?
    A) Folding paper
    B) Stretching rubber
    C) Burning wood
    D) Melting ice
  15. Which property changes in physical change?
    A) Chemical composition
    B) Internal structure
    C) External appearance
    D) Molecular identity

26–40: Application-Based Questions

  1. Why is rusting harmful?
    A) Changes color
    B) Weakens iron
    C) Increases weight
    D) Makes shiny
  2. Why do we paint iron objects?
    A) Decoration
    B) Prevent rusting
    C) Increase weight
    D) Reduce size
  3. Why is burning of fuel useful?
    A) Physical change
    B) Produces energy
    C) Reversible
    D) Temporary
  4. Which method prevents rusting?
    A) Cooling
    B) Heating
    C) Greasing
    D) Cutting
  5. Why is crystallization used?
    A) To change color
    B) To purify substances
    C) To burn ΰ€ͺΰ€¦ΰ€Ύΰ€°ΰ₯ΰ€₯
    D) To melt solids
  6. Which is an example of crystallization?
    A) Burning salt
    B) Forming sugar crystals
    C) Boiling milk
    D) Freezing water
  7. Why is dissolving salt reversible?
    A) Heating
    B) Evaporation
    C) Cooling
    D) Freezing
  8. Which of the following shows gas formation?
    A) Melting
    B) Boiling
    C) Vinegar + baking soda
    D) Freezing
  9. Which change releases heat?
    A) Physical
    B) Chemical
    C) Temporary
    D) Reversible
  10. Which change absorbs heat?
    A) Burning
    B) Freezing
    C) Melting
    D) Rusting
  11. Why is rusting faster in humid areas?
    A) Less oxygen
    B) More moisture
    C) Less heat
    D) Less iron
  12. Which is NOT a method to prevent rusting?
    A) Painting
    B) Galvanizing
    C) Alloying
    D) Cutting
  13. Which process forms new substance?
    A) Freezing
    B) Boiling
    C) Fermentation
    D) Melting
  14. Why is milk turning into curd a chemical change?
    A) Color change
    B) New substance formed
    C) Temperature change
    D) Shape change
  15. Which of the following is physical change?
    A) Burning coal
    B) Rusting iron
    C) Breaking glass
    D) Cooking food

41–50: Higher Thinking Questions

  1. Which change cannot be reversed easily?
    A) Melting
    B) Freezing
    C) Burning
    D) Dissolving
  2. Which is fastest rusting condition?
    A) Dry air
    B) Moist air
    C) No oxygen
    D) Vacuum
  3. Which change involves energy release and new substance?
    A) Physical
    B) Chemical
    C) Temporary
    D) Reversible
  4. Which of the following is both physical and chemical?
    A) Boiling water
    B) Burning candle
    C) Cutting wood
    D) Freezing
  5. Which is an indicator of chemical change?
    A) Shape change
    B) Size change
    C) Gas formation
    D) State change
  6. Why is crystallization a physical change?
    A) New substance forms
    B) No new substance forms
    C) Color changes
    D) Heat releases
  7. Which process involves oxidation?
    A) Melting
    B) Rusting
    C) Freezing
    D) Dissolving
  8. Which factor affects rusting?
    A) Oxygen
    B) Water
    C) Air
    D) All of these
  9. Which is an example of reversible change?
    A) Cooking
    B) Burning
    C) Melting wax
    D) Rusting
  10. Which statement is correct?
    A) All physical changes are irreversible
    B) All chemical changes are reversible
    C) Physical changes do not form new substances
    D) Chemical changes are temporary

βœ… Answer Key (1–50)

1-B, 2-C, 3-C, 4-C, 5-B, 6-C, 7-C, 8-C, 9-C, 10-C
11-C, 12-B, 13-D, 14-A, 15-B, 16-C, 17-B, 18-B, 19-B, 20-B
21-C, 22-B, 23-B, 24-C, 25-C
26-B, 27-B, 28-B, 29-C, 30-B, 31-B, 32-B, 33-C, 34-B, 35-C
36-B, 37-D, 38-C, 39-B, 40-C
41-C, 42-B, 43-B, 44-B, 45-C, 46-B, 47-B, 48-D, 49-C, 50-C

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