Changes Around Us: Physical and Chemical
1. Which of the following statements are the characteristics of a physical change?
(i) The state of the substance may or may not change.
(ii) A substance with different properties is formed.
(iii) No new substance is formed.
(iv) The substance undergoes a chemical reaction.
(a) (i) and (ii) (b) (ii) and (iii)
(c) (i) and (iii) (d) (iii) and (iv)
Answer: (c) (i) and (iii)
Explanation: A physical change involves changes in physical properties like shape, size, or state without forming a new substance. Statement (i) is true as the state may change (e.g., ice to water), and (iii) is true as no new substance is formed. Statements (ii) and (iv) describe chemical changes.
2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these?
(i) Stitching cloth to a shirt
Answer: Can be reversed.
Explanation: The cloth can be unstitched to separate it from the shirt, restoring the original cloth.
(ii) Twisting of straight string
Answer: Can be reversed.
Explanation: The twisted string can be untwisted to return to its straight form.
(iii) Making idlis from a batter
Answer: Cannot be reversed.
Explanation: Cooking the batter into idlis forms a new substance that cannot be turned back into batter.
(iv) Dissolving sugar in water
Answer: Can be reversed.
Explanation: Sugar can be recovered by evaporating the water, leaving the sugar behind.
(v) Drawing water from a well
Answer: Can be reversed.
Explanation: The water can be returned to the well, restoring its original state.
(vi) Ripening of fruits
Answer: Cannot be reversed.
Explanation: Ripening is a chemical change that permanently alters the fruit; it cannot become unripe.
(vii) Boiling water in an open pan
Answer: Cannot be reversed.
Explanation: The water evaporates into the air and cannot be easily collected back in the pan. Not sure because: If the water vapor is condensed and collected, it could be reversed, but in an open pan, it escapes.
(viii) Rolling up a mat
Answer: Can be reversed.
Explanation: The mat can be unrolled to return to its original flat state.
(ix) Grinding wheat grains to flour
Answer: Cannot be reversed.
Explanation: Wheat grains cannot be reformed from flour as the structure is permanently changed.
(x) Forming of soil from rocks
Answer: Cannot be reversed.
Explanation: Weathering breaks rocks into soil over a long time, and rocks cannot be reformed from soil.
3. State whether the following statements are True or False. In case a statement is False, write the correct statement.
(i) Melting of wax is necessary for burning a candle. (True/False)
Answer: True
Explanation: Melting of wax is a physical change needed to allow the liquid wax to be drawn up the wick for burning.
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
Answer: False
Correct Statement: Collecting water vapour by condensing involves a physical change.
Explanation: Condensation changes water vapor back to liquid water without forming a new substance.
(iii) The process of converting leaves into compost is a chemical change. (True/False)
Answer: True
Explanation: Decomposition of leaves into compost involves chemical reactions, forming new substances.
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer: True
Explanation: Mixing baking soda with lemon juice produces carbon dioxide gas, indicating a chemical change.
4. Fill in the blanks in the following statements:
(i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to rusting, and this is a chemical change.
(ii) Folding a handkerchief is a physical, change and can be reversed.
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.
(iv) Magnesium, when burnt in air, produces a substance called magnesium oxide, The substance formed is basic, in nature. Burning of magnesium is a chemical change.
5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer: Physical
Explanation: Changing water to ice (freezing) or steam (evaporation/boiling) involves changes in state (solid to liquid or liquid to gas) without forming a new substance. No chemical reaction occurs, so these are physical changes.
6. Is curdling of milk a physical or chemical change? Justify your statement.
Answer: Chemical
Justification: Curdling of milk involves bacteria or acids reacting with milk to form curd, a new substance with different properties. This chemical reaction cannot be reversed, indicating a chemical change.
7. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Answer: Both physical and chemical
Explanation: Soil formation involves physical changes like the breaking of rocks into smaller pieces by wind, rain, or temperature changes (weathering). It also involves chemical changes when water or chemicals react with rock minerals, altering their composition (e.g., iron in rocks forming iron oxide). Thus, both types of changes occur.
8. Read the following story titled ‘Eco-friendly Prithvi’, and tick the most appropriate option(s) given in the brackets. Provide a suitable title of your choice for the story.
Prithvi is preparing a meal in the kitchen. He chops vegetables,
peels potatoes, and cuts fruits (physical changes/chemical
changes). He collects the seeds, fruits, and vegetable peels
into a clay pot (physical change/chemical change). The fruits,
vegetable peels, and other materials begin to decompose
due to the action of bacteria and fungi, forming compost
(physical change/chemical change). He decides to plant
seeds in the compost and water them regularly. After a few
days, he notices that the seeds begin to germinate and small
plants start to grow, eventually blooming into colourful
fl owers (physical change/chemical change). His eff orts are
appreciated by all his family members.
Story Answers:
- Chops vegetables, peels potatoes, and cuts fruits: physical changes (changes in shape and size, no new substance formed).
- Collects seeds, fruits, and vegetable peels into a clay pot: physical change (only collecting, no new substance formed).
- Fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost: chemical change (decomposition forms new substances like compost).
- Seeds begin to germinate and small plants start to grow, eventually blooming into colorful flowers: chemical change (germination and growth involve chemical reactions forming new tissues).
Suitable Title: “Prithvi’s Green Journey”
9. Some changes are given here. Write physical changes in the area marked ‘A’ and chemical changes in the area marked ‘B’. Enter the changes which are both physical and chemical in the area marked ‘C’. Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium; Mixing baking soda with vinegar.
Answer:
A.(Physical Changes): Tearing of paper, Melting of ice, Folding of clothes
Explanation: These involve changes in shape, size, or state without forming new substances.
B.(Chemical Changes): Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar
Explanation: These involve chemical reactions forming new substances.
C.(Both Physical and Chemical Changes): Process of burning a candle
Explanation: Burning a candle involves physical changes (melting and evaporation of wax) and chemical changes (burning of wax vapor to form carbon dioxide and water).
10. The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why?
Answer: Lime water turns milky in cases (a) and (d).
Explanation:
(a) Vinegar and baking soda: This reaction produces carbon dioxide gas, which reacts with lime water (calcium hydroxide) to form calcium carbonate, making it milky.
(d) Lemon juice and baking soda: Lemon juice (acidic) reacts with baking soda to produce carbon dioxide, which turns lime water milky.
(b) Vinegar and common salt: No carbon dioxide is produced, so lime water does not turn milky.
(c) Lemon juice and vinegar: Both are acids and do not produce carbon dioxide, so lime water remains unchanged.
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