Question Answers For All Chapters – General Science Class 7
Changes – Physical and Chemical
Exercise
1. Distinguish between the following:
(a) Physical Change and Chemical Change
| Physical Change | Chemical Change |
|---|---|
| No new substance is formed. | A new substance is formed. |
| Usually reversible. | Irreversible. |
| Only the appearance or state changes. | Chemical properties change. |
| Example: Melting ice. | Example: Burning paper. |
(b) Periodic Change and Non-Periodic Change
| Periodic Change | Non-Periodic Change |
|---|---|
| Happens at regular intervals. | Happens at irregular intervals. |
| Predictable. | Unpredictable. |
| Example: Change of seasons. | Example: Earthquake. |
(c) Natural Change and Man-Made Change
| Natural Change | Man-Made Change |
|---|---|
| Happens naturally. | Happens due to human activity. |
| Example: Ripening of fruits. | Example: Burning fuel. |
2. Classification of the following changes:
(a) Conversion of milk into yoghurt → Chemical Change, Irreversible Change.
(b) Bursting of a cracker → Chemical Change, Fast Change.
(c) Occurrence of an earthquake → Natural Change, Non-Periodic Change.
(d) Revolution of the Earth around the Sun → Natural Change, Periodic Change.
(e) Stretching of a spring → Physical Change, Reversible Change.
3. Give reasons:
(a) While purchasing canned food, its expiry date should be checked.
→ Expired food can contain harmful bacteria that cause food poisoning and health problems.
(b) An iron article should be given a coat of paint.
→ Painting prevents rusting by stopping air and moisture from coming in contact with iron.
(c) A wooden article should be polished.
→ Polishing protects wood from moisture and gives it a smooth and shiny appearance.
(d) Copper and brass utensils should be tinned.
→ Tinning prevents reaction with food, stops corrosion, and makes the utensils last longer.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
→ Water quickly soaks into dry fabric, but drying takes time because water needs to evaporate into the air.
4. What will you take into account while identifying the following?
(a) A physical change in a substance:
→ No new substance is formed, and it can be reversed (e.g., melting wax).
(b) A chemical change in a substance:
→ A new substance is formed, and the change is permanent (e.g., rusting of iron).
5. Read the paragraph and note down the various types of changes it mentions.
The sun was setting. → Natural Change, Periodic Change.
A breeze was blowing. → Natural Change, Continuous Change.
Leaves on the tree were shaking. → Physical Change.
Rolling balls of wet soil. → Physical Change.
Making dough from wheat flour. → Physical Change.
Frying puris. → Chemical Change (New substance formed).
It had started raining. → Natural Change, Fast Change.
There was lightning. → Natural Change, Fast Change.
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