Which of the following statements are the characteristics of a physical change?
- The state of the substance may or may not change.
- A substance with different properties is formed.
- No new substance is formed.
- The substance undergoes a chemical reaction.
- (i) and (ii)
- (ii) and (iii)
- (i) and (iii)
- (iii) and (iv)
Answer
(i) and (iii)
Reason — In a physical change, only physical properties such as shape, size and state of a substance change, and no new substance is formed. The state of the substance may or may not change [statement (i)], and no new substance is formed [statement (iii)]. Statements (ii) and (iv) describe a chemical change, in which a new substance with different properties is formed through a chemical reaction.
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?
- Stitching cloth to a shirt
- Twisting of straight string
- Making idlis from a batter
- Dissolving sugar in water
- Drawing water from a well
- Ripening of fruits
- Boiling water in an open pan
- Rolling up a mat
- Grinding wheat grains to flour
- Forming of soil from rocks
Answer
| Change | Can be reversed / Cannot be reversed |
|---|---|
| Stitching cloth to a shirt | Can be reversed (the stitches can be removed to get the cloth back) |
| Twisting of straight string | Can be reversed (the string can be untwisted) |
| Making idlis from a batter | Cannot be reversed (the idlis cannot be changed back into batter) |
| Dissolving sugar in water | Can be reversed (sugar can be recovered by evaporating the water) |
| Drawing water from a well | Can be reversed (the water can be poured back) |
| Ripening of fruits | Cannot be reversed (a ripe fruit cannot be changed back into an unripe fruit) |
| Boiling water in an open pan | Can be reversed (the water vapour can be condensed back into water) |
| Rolling up a mat | Can be reversed (the mat can be unrolled) |
| Grinding wheat grains to flour | Cannot be reversed (the flour cannot be turned back into whole grains) |
| Forming of soil from rocks | Cannot be reversed (soil cannot be turned back into the original rock) |
State whether the following statements are True or False. In case a statement is False, write the correct statement.
- Melting of wax is necessary for burning a candle. (True/False)
- Collecting water vapour by condensing involves a chemical change. (True/False)
- The process of converting leaves into compost is a chemical change. (True/False)
- Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer
(i) True
(ii) False
Corrected statement — Collecting water vapour by condensing involves a physical change, as water vapour simply changes into liquid water and no new substance is formed.
(iii) True
(iv) True
Fill in the blanks in the following statements:
- Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ..............., and this is a ............... change.
- Folding a handkerchief is a ............... change and can be ................
- A chemical process in which a substance reacts with oxygen with evolution of heat is called ..............., and this is a ............... change.
- Magnesium, when burnt in air, produces a substance called ............... . The substance formed is ............... in nature. Burning of magnesium is a ............... change.
Answer
(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.
Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer
Both changes are physical changes.
When water changes into ice or steam, only its physical state changes and no new substance is formed. Ice can melt back into water, and steam can condense back into water. Hence, both changes are physical and reversible.
Is curdling of milk a physical or chemical change? Justify your statement.
Answer
Curdling of milk is a chemical change.
During curdling, substances with properties different from those of milk are formed, and the original milk cannot be obtained back. Hence, curdling of milk is a chemical change.
Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Answer
The formation of soil from rocks involves both physical and chemical changes, a process collectively called weathering.
Physical changes break large rocks into smaller pieces. Temperature changes due to climatic conditions, growing roots of trees, and freezing of water within cracks in the rocks cause them to break. Water or chemicals present in water can also react with the rocks and cause chemical changes in their composition. These physical and chemical changes together lead to the formation of soil.
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 flowers (physical change/chemical change). His efforts are appreciated by all his family members.
Answer
- Chopping vegetables, peeling potatoes, and cutting fruits — physical changes
- Collecting the seeds, fruits, and vegetable peels into a clay pot — physical change
- Decomposition of materials by bacteria and fungi to form compost — chemical change
- Germination of seeds and growth of plants into flowers — chemical change
A suitable title for the story — "From Waste to Wealth"
The other suitable titles such as "Nature's Recycling" or "Prithvi's Green Kitchen" are equally acceptable.
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 and Mixing baking soda with vinegar.
Answer
In the burning of a candle, melting, flowing, solidification and evaporation of wax are physical changes, while the burning of wax vapour is a chemical change. Hence, burning a candle involves both physical and chemical changes.
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
The experiments in the four cases involve passing the gas produced from the following mixtures through lime water:
(a) Vinegar and baking soda
(b) Lemon juice and vinegar
(c) Vinegar and common salt
(d) Lemon juice and baking soda
Lime water turns milky in cases (a) and (d).
In case (a), vinegar reacts with baking soda, and in case (d), lemon juice reacts with baking soda. In both these reactions, carbon dioxide gas is produced. When this carbon dioxide is passed through lime water, it reacts with the lime water to form a white insoluble substance (calcium carbonate), which makes the lime water turn milky.
In cases (b) and (c), no carbon dioxide is produced, so the lime water does not turn milky.