Page 178 - Let us Enhance our Learning
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1. Assertion (A): The flame of a burning candle covered with a glass tumbler goes out after some time.
Reason (R): Oxygen inside the glass tumbler gets gradually used up.Answer: A) Both A and R are true, and R is the correct explanation of A. -
2. Abdul focuses sunlight on a piece of paper using a magnifying glass, and the paper starts burning. The same paper does not burn when kept in sunlight without the magnifying glass. What is the most appropriate explanation?
Answer: D) Focusing sunlight raises the paper to its ignition temperature.
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3. Which of the following correctly represents the sequence of changes involved in the burning of a candle, along with their nature?
Answer: D) Wax melts (physical change) → wax evaporates at wick (physical change) → wax vapour burns (chemical change)
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4. Which of the following is an example of a physical change?
Answer: D: evaporation of water
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5. 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.Answer: (c) (i) and (iii) -
6. Define the following.
a) Physical change: A physical change is one in which a substance or object undergoes a change in its physical properties (such as shape, size, or state) and no new substance is formed.
b) Chemical change: A chemical change is one in which one or more new substances are formed through a chemical reaction.
c) Rusting: The process of formation of a brown-coloured substance (rust or iron oxide) on objects made of iron when exposed to moist air is called rusting.
d) Combustion: A chemical reaction in which a substance reacts with oxygen and produces heat and light is called combustion.
e) Weathering: The physical and chemical changes in rocks caused by climatic conditions, temperature changes, growing roots, freezing water, or reactions with water and air, which eventually lead to the formation of soil, are collectively called weathering.
f) Erosion: The process in which rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water is called erosion.
g) Ignition temperature: The lowest temperature at which a substance catches fire is called its ignition temperature. -
7. Write any two characteristics each of physical and chemical changes.
Characteristics of Physical Changes:
1. No new substance is formed.
2. The change can be easily reversed by simple physical methods.
Characteristics of Chemical Changes:
1. One or more new substances with entirely different properties are formed.
2. The change is permanent and cannot be reversed by simple methods. -
8. What are the three essential requirements for a combustion process to occur?
The three essential requirements for combustion are:
1. A combustible substance (fuel).
2. Oxygen (supplied from air).
3. Heat to raise the fuel to its ignition temperature.
Page 182 - Let us Enhance our Learning (Continued)
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9. 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. -
10. 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
(ii) Twisting of straight string
(iii) Making idlis from a batter
(iv) Dissolving sugar in water
(v) Drawing water from a well
(vi) Ripening of fruits
(vii) Boiling water in an open pan
(viii) Rolling up a mat
(ix) Grinding wheat grains to flour
(x) Forming of soil from rocks* (i) Stitching cloth to a shirt: Cannot be reversed (irreversible physical change as cutting/stitching alters the fabric permanently).
* (ii) Twisting of straight string: Can be reversed.
* (iii) Making idlis from a batter: Cannot be reversed (chemical change).
* (iv) Dissolving sugar in water: Can be reversed (by evaporation of water).
* (v) Drawing water from a well: Can be reversed.
* (vi) Ripening of fruits: Cannot be reversed.
* (vii) Boiling water in an open pan: Cannot be reversed completely in an open pan as steam escapes, though the process of evaporation/condensation is physically reversible.
* (viii) Rolling up a mat: Can be reversed.
* (ix) Grinding wheat grains to flour: Cannot be reversed.
* (x) Forming of soil from rocks: Cannot be reversed (takes thousands of years). -
11. State whether the following statements are True or False. In case a statement is False, write the correct statement.
Statement Ans (i) Melting of wax is necessary for burning a candle. True (ii) Collecting water vapour by condensing involves a chemical change. False (Condensation of water vapour is a physical change) (iii) The process of converting leaves into compost is a chemical change. True (iv) Mixing baking soda with lemon juice is a chemical change. True -
12. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Both changes are physical changes. This is because only the state of water changes (from liquid to solid as ice, or from liquid to gas as steam), and no new substance is formed; water retains its chemical identity in all three states.
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13. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
The formation of soil from rocks involves both physical and chemical changes (weathering). Physical changes break large rocks into smaller sediment pieces through temperature variations, freezing water, or wind action, while chemical changes occur when water or air reacts chemically with rock minerals (such as iron in basalt turning into iron oxide).
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14. 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.
Title: Prithvi's Green Journey (or Eco-Friendly Prithvi)
Story with correct options:
Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (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 flowers (chemical change). His efforts are appreciated by all his family members. -
15. 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'.
Processes: Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.* A (Physical Changes): Tearing of paper, Melting of ice, Folding of clothes.
* B (Chemical Changes): Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar.
* C (Both Physical and Chemical Changes): Process of burning a candle. -
16. 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?
Lime water turned milky in (a) (Vinegar and baking soda) and (d) (Lemon juice and baking soda). This happens because mixing an acid (vinegar or lemon juice) with baking soda produces carbon dioxide gas, which turns lime water milky. In cases (b) and (c), no carbon dioxide gas is produced, so the lime water does not turn milky.
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17. Is curdling of milk a physical or chemical change? Justify your statement.
Curdling of milk is a chemical change. This is because a new substance (curd) with entirely different properties, taste, and composition is formed, and the change cannot be reversed to get back original milk by simple methods.
