In this article, you will find the answers and detailed solutions for the NCERT questions of class 7 science chapter 5 "Changes Around us : Physical and Chemical".
Changes around us : Physical and Chemical - NCERT Solutions
Q1. 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)
Q2. 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?
Q3. 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)
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
(iii) The process of converting leaves into compost is a chemical change. (True/False)
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer:
(i) True
(ii) False – Collecting water vapour by condensation involves a physical change.
(iii) True
(iv) True
Q4. 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 ________, and this is a________ change.
(ii) Folding a handkerchief is a _________________ change and can be________.
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called _________, and this is a __________ change.
(iv) Magnesium, when burnt in air, produces a substance called __________. The substance formed is _________ in nature. Burning of magnesium is a _________ change.
Answer:
(i) rusting, chemical.
(ii) physical, reversed.
(iii) combustion, chemical.
(iv) magnesium oxide, basic, chemical.
Q5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer: Both changes are physical because only the state of water changes, no new substance is formed. A change is called chemical only iff some new substance is formed.
Q6. Is curdling of milk a physical or chemical change? Justify your statement.
Answer: Curdling of milk is a chemical change because a new substance (curd) with different properties is formed, and it cannot be reversed.
Q7. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Answer: It is a physical change because rocks break down into smaller pieces to form soil, but no new substance is formed.
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). 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.
Suitable Title: Prithvi’s Green Effort
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 and Mixing baking soda with vinegar.
Answer:
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): Process of burning a candle
Q10. The experiments shown were performed. Find out in which case(s) did lime water turn milky and why?
Answer: Lime water turned milky in the experiments where carbon dioxide gas was released. This is because carbon dioxide reacts with lime water to form calcium carbonate, which is white and makes the lime water appear milky.