Class 7 Science Changes Around Us: Physical and Chemical

Class 7 Science Changes Around Us: Physical and Chemical

Chapter 5: Changes Around Us – Physical and Chemical

Introduction

Everything around us undergoes changes. These changes can be physical or chemical. Understanding them helps us identify, predict, and control changes in everyday life and in scientific experiments.

1. Physical Changes

  • Do not form a new substance.
  • Usually reversible.
  • No energy is absorbed or released significantly.
  • Examples:
    • Melting of ice → water
    • Boiling water → steam
    • Dissolving sugar in water
    • Tearing paper

Key points:

  • Shape or state may change, but chemical composition remains the same.

2. Chemical Changes

  • Produce new substances with different properties.
  • Often irreversible.
  • May release or absorb energy (heat, light, sound).
  • Examples:
    • Burning wood → ash, smoke, heat
    • Rusting of iron → iron oxide
    • Cooking an egg
    • Baking bread

Key points:

  • Chemical bonds break and form new substances.
  • Indicators: color change, gas evolution, precipitation, energy change.

3. Differences Between Physical and Chemical Changes

PropertyPhysical ChangeChemical Change
Formation of new substanceNoYes
ReversibilityUsually reversibleUsually irreversible
Energy changeUsually noneOften absorbed or released
ExamplesMelting, Boiling, DissolvingBurning, Rusting, Cooking

4. Key Points to Remember

  • Physical changes affect shape, size, and state.
  • Chemical changes result in new substances.
  • Some changes may be partly reversible (e.g., freezing water).
  • Observing changes carefully helps in identifying the type.

Questions – Changes Around Us: Physical and Chemical

1. Very Short Answer (1 mark)

  1. Define a physical change.
  2. Define a chemical change.
  3. Give an example of a reversible physical change.
  4. Give an example of an irreversible chemical change.
  5. What happens during melting of ice?
  6. Name one chemical change that produces gas.
  7. Give an example of color change in a chemical change.
  8. Is tearing paper a chemical or physical change?
  9. Is boiling water a physical or chemical change?
  10. What indicates a chemical change besides formation of a new substance?

2. Multiple Choice Questions (MCQ)

  1. Which of the following is a chemical change?
    a) Melting of butter
    b) Rusting of iron
    c) Dissolving sugar
    d) Cutting a sheet of paper
  2. Physical changes are:
    a) Irreversible
    b) Always exothermic
    c) Reversible and no new substance formed
    d) Chemical reactions
  3. Burning of wood produces:
    a) Wood unchanged
    b) Water only
    c) Ash, smoke, heat
    d) Ice
  4. Which is a physical change?
    a) Baking cake
    b) Dissolving salt in water
    c) Rusting of iron
    d) Cooking rice
  5. One indication of a chemical change is:
    a) Change of state
    b) Change of shape
    c) Gas evolution
    d) Melting

3. Fill in the Blanks

  1. Physical change does not produce a __________ substance.
  2. Chemical changes often involve the release or absorption of __________.
  3. Freezing water is an example of a __________ change.
  4. Rusting of iron is a __________ change.
  5. Burning of paper produces __________.

4. True/False Type

  1. Physical changes are usually reversible. (True/False)
  2. Chemical changes never produce energy. (True/False)
  3. Dissolving sugar in water is a physical change. (True/False)
  4. Cooking an egg is a chemical change. (True/False)
  5. Melting ice is an example of a chemical change. (True/False)

5. Short Answer Questions (2–3 marks)

  1. Give two differences between physical and chemical changes.
  2. Give two examples of physical changes in daily life.
  3. Give two examples of chemical changes in daily life.
  4. Explain why burning paper is a chemical change.
  5. Name two indicators of a chemical change.

6. Long Answer Questions (4–5 marks)

  1. Explain physical and chemical changes with examples and differences.
  2. Describe how you can identify a chemical change in everyday life.
  3. Explain reversible and irreversible changes with examples.
  4. How does energy change differ in physical and chemical changes?
  5. Discuss the importance of understanding changes around us in science.

7. Higher-Order Thinking (HOT) & Application

  1. A candle burns and melts simultaneously. Identify the physical and chemical changes involved.
  2. Rusting of iron can be prevented. Suggest methods using your knowledge of chemical changes.
  3. Explain why freezing water in a plastic bottle and breaking it is a physical change but cooking an egg is a chemical change.

8. Passage-Based Questions

Read the passage and answer:
Some changes do not produce new substances and are usually reversible, such as melting or dissolving. Other changes produce new substances and are usually irreversible, such as burning, rusting, or cooking. Observing energy change, color change, and gas evolution helps identify chemical changes.

  1. Give one example of a reversible change.
  2. Give one example of an irreversible change.
  3. What indicates a chemical change?
  4. Are melting and boiling chemical or physical changes?

Answer Key – Changes Around Us: Physical and Chemical

1. Very Short Answer

  1. A change that does not produce a new substance
  2. A change that produces a new substance
  3. Freezing/melting of water
  4. Burning paper, rusting of iron
  5. Changes from solid ice to liquid water
  6. Reaction of baking soda with vinegar
  7. Rusting of iron
  8. Physical change
  9. Physical change
  10. Gas evolution, color change, energy release

2. MCQ Answers

  1. b) Rusting of iron
  2. c) Reversible and no new substance formed
  3. c) Ash, smoke, heat
  4. b) Dissolving salt in water
  5. c) Gas evolution

3. Fill in the Blanks

  1. new
  2. energy
  3. physical
  4. chemical
  5. ash, smoke

4. True/False

  1. True
  2. False
  3. True
  4. True
  5. False

5. Short Answer

  1. Physical – no new substance, reversible; Chemical – new substance, usually irreversible
  2. Melting ice, tearing paper
  3. Burning wood, cooking an egg
  4. Paper turns into ash, smoke, and gas – new substances are formed
  5. Gas evolution, color change, energy change

6. Long Answer

  1. Physical – shape/state changes, reversible; Chemical – new substances, irreversible; Examples given
  2. Observe gas, color, energy change; e.g., burning, rusting
  3. Reversible – melting, freezing; Irreversible – cooking, rusting
  4. Physical – little/no energy change; Chemical – absorbs or releases energy
  5. Understanding changes helps in experiments, daily life, and predicting reactions

7. HOT & Application

  1. Physical – wax melts; Chemical – wax burns
  2. Prevent rust – paint coating, oiling, galvanization
  3. Freezing/breaking – state change only (physical); Cooking egg – new substance formed (chemical)

8. Passage-Based

  1. Melting of ice
  2. Burning paper
  3. Gas evolution, color change, energy change
  4. Physical changes