Class 6 Science Ch 6: Changes Around Us Notes CBSE

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Changes Around Us - Science Notes for CBSE Class 6

Chapter 6: Changes Around Us

Science Notes for CBSE Class 6

Chapter Summary

  • Change is a natural phenomenon: Everything around us is constantly undergoing some kind of change. These changes can happen slowly or quickly.

  • Examples of changes we observe daily:

    • Growth of plants and animals.
    • Ripening of fruits.
    • Changing seasons.
    • Melting of ice.
    • Cooking of food.
    • Rusting of iron.

  • Reversible Changes: These are changes that can be undone or reversed. The material returns to its original state after the change.

    • Examples:
    • Melting of ice (water can be frozen back into ice).
    • Folding a piece of paper (it can be unfolded).
    • Blowing up a balloon (air can be released, and the balloon returns to its original size).
    • Stretching a rubber band (it goes back to its original shape).
    • Dissolving salt in water (water can be evaporated to get salt back).
    • Dough can be rolled into chapati and then rolled back into dough before cooking.

  • Irreversible Changes: These are changes that cannot be undone or reversed. Once the change has occurred, the material cannot return to its original state.

    • Examples:
    • Burning of a candle (wax melts and also burns, producing new substances).
    • Burning of paper (it turns into ash and cannot become paper again).
    • Cooking of food (raw vegetables cannot be turned back into raw form after cooking).
    • Milk turning into curd (curd cannot be changed back to milk).
    • Growth of a plant or animal.
    • Rusting of iron.
    • Baking a cake or clay pot.
    • Cutting a piece of paper or wood.

  • Changes due to Heating: Many changes happen when a substance is heated.

    • Some substances melt (e.g., ice, wax, metals). Melting is a reversible change.
    • Some substances burn (e.g., paper, wood, fuel). Burning is an irreversible change, as new substances are formed.
    • Some substances expand on heating and contract on cooling (e.g., metals, air). This property is used in fixing metal rims on wooden wheels or bullock carts.
    • Cooking food involves heating, leading to irreversible changes in ingredients.
    • Potters bake clay pots in a kiln to make them hard and durable, which is an irreversible change.

  • Changes due to Mixing: Mixing different substances can also cause changes.

    • Dissolving sugar or salt in water is a change. While the sugar/salt can be recovered, the mixture itself is a change from separate components.
    • Mixing ingredients to make dough or batter for cooking leads to changes.

  • Many changes can be classified as either reversible or irreversible based on whether the original state of the substance can be recovered.

Questions and Answers

Q1. List five changes you have seen around you that can be turned back.

Answer:

  1. Melting of ice.
  2. Stretching of a rubber band.
  3. Blowing up a balloon.
  4. Folding a piece of paper.
  5. Dissolving sugar in water (water can be evaporated to get sugar back).

Q2. List five changes you have seen around you that cannot be turned back.

Answer:

  1. Cooking of food.
  2. Burning of wood/paper.
  3. Milk turning into curd.
  4. Ripening of a fruit.
  5. Growth of a plant or child.

Q3. Give reasons:

  1. A piece of wood burns to ashes.
  2. Milk turns into curd.
  3. An ice cream melts.

Answer:

  1. When a piece of wood burns, it changes into ash, smoke, and heat. These new substances cannot be turned back into wood. This is an irreversible chemical change.
  2. Milk turns into curd due to a chemical reaction caused by bacteria. The composition of curd is different from milk, and curd cannot be converted back into milk. This is an irreversible change.
  3. When an ice cream melts, it changes its state from solid to liquid. However, the liquid ice cream can be refrozen back into solid ice cream. So, this is a reversible physical change.

Q4. To fix a metal rim on a wooden wheel, the metal rim is usually heated. Explain why.

Answer: The metal rim is heated because metals expand on heating. When the metal rim is heated, it expands and becomes slightly larger than the wooden wheel. This allows it to easily slip over the wooden wheel. After placing it on the wheel, cold water is poured over the rim. The metal rim contracts on cooling and tightly fits onto the wooden wheel, holding it firmly.

Q5. What kind of change is:

  1. Burning of a candle?
  2. Stretching of a rubber band?
  3. Melting of ice?
  4. Dissolving sugar in water?
  5. Burning of a paper?
  6. Baking of a cake?

Answer:

  1. Burning of a candle: Mostly irreversible (wax changes, wick burns), though melting of wax is reversible.
  2. Stretching of a rubber band: Reversible change.
  3. Melting of ice: Reversible change.
  4. Dissolving sugar in water: Reversible change (water can be evaporated to recover sugar).
  5. Burning of a paper: Irreversible change.
  6. Baking of a cake: Irreversible change.

Q6. Explain why a potter bakes his pots in a kiln.

Answer: A potter bakes his pots in a kiln to make them hard, strong, and durable. When raw clay pots are heated in a kiln, chemical changes occur within the clay particles, making them lose water and bond together more strongly. This process is an irreversible change that transforms the soft, mouldable clay into a rigid, waterproof, and much stronger ceramic material.

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