Maharashtra State Board
Standard VII – Science
Chapter 13 – Changes: Physical and ChemicaTextbook Solutions
1. Distinguish between the following.
(a) Physical change and chemical change.
Ans:
| Physical Change | Chemical Change |
|---|---|
| A physical change is usually a temporary change. | A chemical change is usually a permanent change. |
| The properties of the original substance remain the same. | The properties of the original substance change. |
| No new substance is formed. | A new substance is formed. |
| Example: evaporation, melting, boiling. | Example: rusting of iron, ripening of fruits. |
(b) Periodic change and non-periodic change.
Ans:
| Periodic Change | Non-Periodic Change |
|---|---|
| Changes that occur again and again after a definite interval of time are called periodic changes. | Changes that do not occur at regular intervals are called non-periodic changes. |
| These changes can be predicted easily. | These changes are difficult to predict. |
| Example: change of seasons, sunrise and sunset. | Example: floods, earthquakes, storms. |
(c) Natural change and man-made change.
Ans:
| Natural Change | Man-made Change |
|---|---|
| Changes that occur due to natural causes are called natural changes. | Changes that are caused by human activities are called man-made changes. |
| These changes occur on their own in nature. | These changes occur due to human effort or activity. |
| Example: ripening of fruits, spoiling of milk. | Example: burning wood, baking bread. |
2. Under which different types can the following changes be classified?
(a) Conversion of milk into yoghurt.
Ans: This is a chemical, irreversible, and slow change.
(b) Bursting of a cracker.
Ans: This is a chemical, irreversible, and fast change.
(c) Occurrence of an earthquake.
Ans: This is a natural and non-periodic change.
(d) Revolution of the earth around the sun.
Ans: This is a periodic, slow, and natural change.
(e) Stretching of a spring.
Ans: This is a physical and reversible change.
3. Give reasons.
(a) While purchasing canned food, its expiry date should be checked.
Ans: Canned foods contain preservatives to prevent them from spoiling. However, after the expiry date, the food may undergo chemical changes. Such food may become unsafe to eat and can cause food poisoning. Therefore, the expiry date should always be checked before purchasing canned food.
(b) An iron article should be given a coat of paint.
Ans: When iron articles are exposed to air and moisture, they react with oxygen and form a reddish-brown layer called rust. This process is known as corrosion and it weakens the iron. Painting the iron article forms a protective layer and prevents rusting.
(c) A wooden article should be polished.
Ans: Wood can absorb moisture from the air and swell, which reduces its durability. It may also be damaged by fungus or termites. Polishing wood improves its appearance and protects it from moisture and environmental damage. It also seals the pores of the wood and helps prevent termite attacks.
(d) Copper and brass utensils should be tinned.
Ans: Copper and brass utensils react with air and moisture and may develop a greenish layer on their surface. This layer is harmful if it mixes with food. Tinning the utensils forms a protective coating and prevents corrosion.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
Ans: A dry handkerchief quickly absorbs water when dipped in water because the cloth fibers easily soak up the liquid. However, drying involves evaporation, which is a slower process. Therefore, a wet handkerchief takes a longer time to dry.
4. What will you take into account while identifying the following?
(a) A physical change in a substance.
Ans: While identifying a physical change in a substance, the following points should be considered:
- The change is temporary and reversible.
- The properties of the original substance remain the same.
- The composition of the substance does not change.
- No new substance is formed.
(b) A chemical change in a substance.
Ans: While identifying a chemical change in a substance, the following points should be considered:
- The change is usually permanent and irreversible.
- The properties of the original substance change.
- The composition of the substance changes.
- A new substance is formed.
5. Read the paragraph and note down the various types of changes it mentions.
Ans:
- Nearing six o’clock in the evening – A periodic change.
- The sun was setting – A natural change.
- Leaves on the tree were shaking – A natural, non-periodic change.
- Sahil rolling balls of wet soil and shaping them into toys – A physical, man-made, reversible change.
- Mother making dough from wheat flour and frying puris – A man-made, useful, non-periodic change.
- Rain starting – A natural, useful, non-periodic change.
- Lightning – A natural, harmful, non-periodic change.