CHANGES AROUND US
We have observed many changes are taking place around us.
Example –
- Leaves fall from trees.
- Your nails grow.
- You go taller and so on
Can all changes always be reversed?
Some changes are reversible while some are irreversible in nature.
⇒ Reversible change –
It can be changed back to what it was before.
Example-
- Melting is turning of ice into water.
- Stretching of a rubber band.
We can explain reversible change by taking an example of boiling of water as follow:-
when we boil water by heating, then it changes into steam. Now, if we cool the steam, then water is formed again. Thus, the boiling of water is a reversible change.
⇒ Irreversible changes –
In this type of change, a substance cannot return to its original state, and the entire new substance will form.
Example-
- The cooking of food.
- Rusting of iron
Irreversible changes can be explained as –
When we burn a piece of wood, it turns to ash and we cannot obtain wood back from that ash, so the burning of wood is an irreversible change.
Chemical changes Vs Physical changes –
| Physical change | Chemical change |
|---|---|
| It includes the changes in physical properties like shape, size,hardness & nature. | It includes the changes in chemical properties like reactivity,flammability. |
| May or may not there are reversible. | These are irreversible in nature. |
| No new substance is formed. | New substance will formed. |
EXERCISE
1-To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?
Answer:
Yes, the process can be reversed by unfolding the dress.
2. You accidentally dropped your favorite toy and broke it. This is a change you did not want. Can this change be reversed?
Answer:
No, this is not reversible.
3. Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not.
| Sl No | Change | Can be reversed Yes/No |
| 1 | The sawing of a piece of wood | |
| 2 | The melting of ice candy | |
| 3 | Dissolving sugar in water | |
| 4 | The cooking of food | |
| 5 | The ripening of a mango | |
| 6 | Souring of milk |
Answer:
| Sl No | Change | Can be reversed Yes/No |
| 1 | The sawing of a piece of wood | No |
| 2 | The melting of ice candy | Yes |
| 3 | Dissolving sugar in water | Yes |
| 4 | The cooking of food | No |
| 5 | The ripening of a mango | No |
| 6 | Souring of milk | No |
4. A drawing sheet changes when you draw a picture on it. Can you reverse this change?
Answer:
If we draw with a pen than we cannot change it. If we use a pencil for drawing then we can change it by erasing. So change is reversible for a pencil while irreversible for the pen.
5. Give examples to explain the difference between changes that can or cannot be reversed.
Answer:
Changes that can be reversed:
-melting of ice.
– boiling of water
Changes that cannot be reversed
-Making of tea.
-Rusting of iron.
6. 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 immobilized. Can the change in POP be reversed?
Answer:
No, the change is irreversible.
7. 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?
Answer:
No, it is an irreversible change.
