Chapter 13 - Changes – Physical and Chemical
1. Distinguish between the following.
(a) Physical change and chemical change.
Ans:
| Physical change | Chemical change |
|---|---|
A physical change is a temporary change. | Chemical change is a permanent change. |
Properties of the original substance remains the same. | The properties of the original substance change. |
No new substance is formed. | A new substance is formed. |
e.g., evaporation, boiling, etc. | e.g., ripening of a fruit, rusting of iron, etc. |
(b) Periodic change and non-periodic change.
Ans:
| Periodic change | Non - Periodic Change |
|---|---|
The changes that occur again and again after a definite interval of time are called periodic changes. | The changes that do not occur again and again after a definite interval of time are called non-periodic changes. |
Changes can easily be predicted. | It is difficult to predict the change. |
e.g., change of seasons, sunrise and sunset, etc. | e.g., floods, earthquakes, and storms. |
(c) Natural change and man-made change.
Ans:
| Natural change | Man -made Change |
|---|---|
The changes that occur due to natural causes are called natural changes. | The changes that are made by humans are called man-made changes. |
They occur on their own, according to their own accord, or naturally. | They occur due to human activity. |
e.g., Ripening of fruit, spoiling of milk, etc. | e.g., burning wood, baking bread, etc. |
2. Under which different types can the following changes be classified?
(a) Conversion of milk into yoghurt.
Ans: Chemical, irreversible, slow change.
(b) Bursting of a cracker.
Ans: Chemical, irreversible, fast change.
(c) Occurence of an earthquake.
Ans: Non – period change, natural change.
(d) Revolution of the earth around the sun.
Ans: Period change, slow, natural change.
(e) Stretching of a spring.
Ans: Physical, reversible change.
3. Give reasons.
(a) While purchasing canned food, its expiry date should be checked.
Ans:
Canned foods contain various preservatives to keep them from spoiling. after the expiration date is over. The canned food undergoes various chemical changes. The food may not remain good for consumption and may likely cause food poisoning. So while purchasing canned food, its expiration date should be checked.
(b) An iron article should be given a coat of paint.
Ans:
When an iron article is exposed to air, it is likely to get hot due to its reaction with oxygen, moisture, and vapors. A reddish-brown layer forms on it; this is called corrosion. Corrosion causes the iron to become weak. To prevent corrosion, iron articles should be given a coat of paint.
(c) A wooden article should be polished.
Ans:
(i) Wood tends to absorb moisture from the air and swell up. This reduces its durability.
(ii) Wood is also affected by fungus or termites.
(iii) So, polishing wood enhances its appearance and increases its resistance to moisture or other environmental factors.
(iv) It also seals and sanitises the pores in wood, which serve as breeding grounds for termites.
(d) Copper and brass utensils should be tinned.
Ans:
Metals like copper and brass are likely to get corroded due to the formation of a greenish layer on them, so to prevent corrosion, copper and brass utensils should be tinned.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
Ans:
A handkerchief is made of fine fabrics that tend to absorb large amounts of water, which is a fast process. whereas drying a handkerchief involves the process of evaporation, which is a slow process. Hence, a dry handkerchief gets wet at once on dipping in water, but it takes a wet handkerchief a long 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 substance following points should be taken into account.
(i) Whether the change in temporary and reversible.
(ii) The properties of the original substances remain the same and their composition remains unchanged.
(iii) No new substance is formed.
(b) A chemical change in a substance.
Ans: While identifying a chemical change in substance following points should be taken into account.
(i) Whether the change in temporary and reversible.
(ii) The properties of the original substances remain the same and their composition remains unchanged.
(iii) No new substance is formed.
5. Read the paragraph and note down the various types of changes it mentions.
It was nearing six o’clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys.Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.
Ans:
(i) Nearing six o’clock in the evening: periodic change
(ii) The sun was setting: a natural change.
(iii) Leaves on the trees were shaking: natural, non-periodic change.
(iv) Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house: a physical, man-made, reversible change.
(v) Mother made a dough from wheat flour and fried purees: man-made, use-full, non-periodic change.
(vi) Raining: natural, useful, non-periodic change.
(vii) Lightning: a natural, harmful, non-periodic change.