6. Changes Around Us
Introduction: What a fun if you suddenly get some magical powers to change things around you! While we don't have magic, many changes happen around us naturally. Fields change colour, leaves fall from trees, flowers bloom and wither. Even your body changes (nails grow, hair grows, you grow taller).
Can All Changes Always Be Reversed?
Some changes can be reversed, while others cannot. A change which can happen backward, that is, can be reversed is called a Reversible Change. If it cannot, it is an Irreversible Change.
ACTIVITY 1: Blowing a Balloon
Observation: Blow a balloon. It changes shape and size. Now let the air
escape. It comes back to original shape. This is a Reversible Change.
Note: If you burst
it with a pin, can it be reversed? No!
ACTIVITY 2: Paper Aeroplane
Observation: Fold a paper to make an aeroplane. You can unfold it back.
Reversible.
Note: If you cut an aeroplane shape with scissors, can it be reversed? No!
ACTIVITY 3: Dough Ball
Observation: Make a ball of dough and roll out a roti. If you don't like the
shape, you can make it a ball again. Reversible.
Note: If you cook the roti on a tawa, can
you get the dough ball back? No! Irreversible.
Could There Be Other Ways to Bring a Change?
We can bring a change by heating a substance or by mixing it with some other substance.
1. Expansion and Contraction (Heating): Tools used for digging soil have iron blades with a ring. The ring is mostly smaller than the wooden handle. To fix it, the ring is heated. It expands (becomes larger) and fits easily. When it cools, it contracts (becomes smaller) and fits tightly.
ACTIVITY 7: Burning Candle
Observation: Light a candle. Its length decreases. The wax burns to produce
heat, light and smoke. This cannot be reversed.
Note: Heating some wax in a pan (melting) is
reversible.
Chapter Summary
- Some changes can be reversed and some cannot be reversed.
- A change may occur by heating a substance or by mixing it with some other.
- Melting of ice into water is a reversible change.
- Cooking of food is an irreversible change.
- Expansion of metals on heating is a reversible change.
Exercise Q1
Q: To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?
Exercise Q2
Q: You accidentally dropped your favourite toy and broke it. This is a change you did not want. Can this change be reversed?
Exercise Q3: Reversible or Irreversible?
- Sawing of a piece of wood: Irreversible
- Melting of ice candy: Reversible
- Dissolving sugar in water: Reversible (by evaporation)
- The cooking of food: Irreversible
- The ripening of mango: Irreversible
- Souring of milk: Irreversible
Exercise Q4
Q: A drawing sheet changes when you draw a picture on it. Can you reverse this change?
- If drawn with a pencil, it can be erased (Reversible).
- If drawn with paint or ink, it cannot be erased (Irreversible).
Exercise Q5
Q: Give examples to explain the difference between changes that can or cannot be reversed.
- Reversible: Melting of Ice (Ice -> Water -> Ice). Folding a paper.
- Irreversible: Burning a paper (Ash cannot become paper). Growth of a plant.
Exercise Q6
Q: A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilised. Can the change in POP be reversed?
Exercise Q7
Q: A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?
Key Facts and Definitions
- Change: A noticeable difference in the shape, size, color, or state of a substance.
- Contraction: The process of becoming smaller in size (usually on cooling).
- Expansion: The process of becoming larger in size (usually on heating).
| Type | Reversibility | Examples |
|---|---|---|
| Physical Change | Often reversible | Melting of ice, Tearing paper |
| Chemical Change | Irreversible | Burning paper, Rusting iron, Curdling milk |
Principle: Metals expand on heating and contract on cooling.
Process: The iron rim (slightly smaller) is heated to expand -> Fitted onto wooden wheel -> Water poured to cool it -> Rim contracts and fits tightly.