Students can use Curiosity Class 7 Science Book Solutions Chapter 5 Changes Around Us Physical and Chemical Class 7 Question and Answer as a quick reference guide.
Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Question Answer
Science Class 7 Chapter 5 Question Answer Changes Around Us Physical and Chemical
Changes Around Us Physical and Chemical Class 7 Question Answer (InText)
Question 1.
I placed a cube of ice here half an hour ago. It has now become water!
Answer:
Ice melts and becomes water. This is physical change.
Question 2.
A bud that I saw yesterday on this rose plant has become a flower today.
Answer:
Bud blossoms to a flower, this is a chemical changes.
Question 3.
The bottle containing cold water is not cold anymore.
Answer:
Cold water becomes warm on keeping in open, this is a physical change.
Question 4.
Yesterday, I saw some brown spots on a banana, but today it has more brown spots and a strong smell.
Answer:
Rotting of banana is a chemical change.
NCERT Class 7 Science Chapter 5 Question Answer Changes Around Us Physical and Chemical (Exercise)
Let Us Enhance Our Learning (Pages 69-71)
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)
(c) (i) and (iii)
(b) (ii) and (iii)
(d) (iii) and (iv)
Answer:
(c) (i) and (iii)
(i) The state of the substance may or may not change – Physical changes can involve changes in state (like melting or freezing) but no new substance is formed.
(iii) No new substance is formed – In a physical change, the substance remains the same, only its form or state changes.
Question 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:
Stitching cloth to a shirt – Cannot be reversed: Once stitched, it is hard to undo without damage.
(ii) Twisting of straight string
Answer:
Twisting of straight string – Can be reversed: The string can be untwisted.
(iii) Making idlis from a batter
Answer:
Making idlis from a batter – Cannot be reversed: The batter cannot be returned to its original form after steaming.
(iv) Dissolving sugar in water
Answer:
Dissolving sugar in water – Can be reversed: Sugar can be recovered by evaporating water.
(v) Drawing water from a well
Answer:
Drawing water from a well – Can be reversed: Water can be returned to the well.
(vi) Ripening of fruits
Answer:
Ripening of fruits – Cannot be reversed: Once ripe, fruits cannot go back to unripe.
(vii) Boiling water in an open pan
Answer:
Boiling water in an open pan – Can be reversed: Evaporated water can be obtained back by condensation.
(viii) Rolling up a mat
Answer:
Rolling up a mat – Can be reversed: The mat can be unrolled.
(ix) Grinding wheat grains to flour
Answer:
Grinding wheat grains to flour – Cannot be reversed: Flour cannot be turned back into grains.
(x) Forming of soil from rocks
Answer:
Forming of soil from rocks – Cannot be reversed: Soil formation is a slow process and cannot form rock back.
Question 3.
State whether the following statements are True or False. In case a statement is False, write the correct statement.
1. Melting of wax is necessary for burning a candle. (True/False)
Answer:
True
2. Collecting water vapour by condensing involves a chemical change. (True/False)
Answer:
False: Collecting water vapour by condensing involves a physical change.
3. The process of converting leaves into compost is a chemical change. (True/False)
Answer:
True
Question 4.
Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer:
True
Question 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 ______ and this
is a ______ change.
Answer:
rusting, chemical
(ii) Folding a handkerchief is a ______ change and can be ______ .
Answer:
physical, reversed
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called ______, and this is a ______
change.
Answer:
combustion, chemical
(iv) Magnesium, when burnt in air, produces a substance called ______. The substance formed is ______ in nature. Burning of magnesium is a ______ change.
Answer:
magnesium oxide, basic, chemical
Question 5.
Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer:
Water to ice is called as freezing and freezing is a physical change because it can be reversed, no new substance is formed and the composition of the substance remains same. Conversion of water to steam is called boiling and boiling is a physical change because only the state changes from liquid to gas, no new substance is formed, composition of the substance remains same and it is also reversible.
Question 6.
Is curdling of milk a physical or chemical change? Justify your statement.
Answer:
Curdling of milk is a chemical change. In this new substance formed is curd. The composition of milk changes. It is irreversible as well as a permanent change.
Question 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:
The formation of soil from rocks involves both physical and chemical changes. Natural factors like wind, rain and temperature break down rocks into smaller pieces (physical change), while chemical processes, like weathering, also change the minerals in the rocks (chemical change). Both types of changes work together to form soil.
Question 8.
Read the following story titled ‘Eco-friendly Prithin’, 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:
Prithvi’s Green Kitchen
-
- Prithvi chops vegetables, peels potatoes, and cuts fruits – S physical changes
- He collects the seeds, fruits, and vegetable peels into a clay pot – V physical change
- He collects the seeds, fruits, and vegetable peels into a clay pot — V physical change
- The fruits, vegetable peels, and other materials decompose into compost — V chemical change
- Seeds germinate and grow into plants V chemical change
Question 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:
(i) The physical changes in the area marked A are—Tearing of paper, Melting of ice, Folding of clothes.
(ii) The chemical changes in the area marked B are—Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium and Mixing baking soda with vinegar.
(iii) Physical changes and chemical changes in the area marked as C— Process of burning a candle.
Question 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 turned milky in cases (a) and (d). Lime water turns milky in the presence of CO2 . The reaction between vinegar or lemon juice and baking soda produces CO2 gas.
Class 7 Changes Around Us Physical and Chemical Question Answer (Activities)
Activity 5.2: Let Us Create and Discuss (Page 59)
A. Creating some objects with paper
• Take a few sheets of paper and fold them to create new objects (Fig.).
• Do you get the same paper back when you unfold these objects?
Answer:
Yes, we get the same paper back when we unfold these objects.
B. Playing with a balloon
• Take a balloon and inflate it. Now, loosen your grip and let the air escape out.
• Do you get the uninflated balloon back?
• Take another balloon; inflate it and grip the opening tightly. Now, prick it with a pin.
• What happens? Will you be able to get the uninflated balloon back?
Answer:
(i) Yes, we get uninflated balloon back.
(ii) Balloon gets deflated and burst on pricking it with pin. We do not get the uninflated balloon back as its appearance (size, shape and form) changes.
C. Crushing a piece of chalk
• Crush a small piece of chalk into powder.
• Can you get the chalk piece back from the powder?
Answer:
No, we can’t get back the chalk piece from powder. The appearance of chalk piece changes.
Is there any similarity in the changes listed in A, B and C?
Answer:
Yes:
A. Physical change (Reversible)
B. Physical change (Irreversible)
C. Physical change (Irreversible) They all are physical changes.
Activity 5.3: Let Us Create and Discuss (Page 60)
• Take two glass tumblers or small transparent bottles. Mark them A and B.
• Fill one-fourth of glass tumbler A with tap water and one-fourth of glass tumbler B with lime water.
• Now, blow air (exhale) into each glass tumbler, one at a time, using separate straws (Fig.) and observe them.
Do you notice any changes?
Observations
| Experiment | Glass Tumbler | Observations |
| Blow air in A | Tap Water | Only bubbles are formed. |
| Blow air in B | Lime Water | (1) Lime water turns milky, (2) Bubbles are also formed. |
Conclusion:
• Blowing air in tap water is a physical change. This is because no new substance is formed. Composition of water remains same.
• Blowing air in lime water is a chemical change because new substance calcium carbonate (white insoluble) is formed.
Activity 5.5: Let Us Investigate (Pages 62-63)
• Place two identical candles on two separate petri dishes and light them,
• Cover one of these with a glass tumbler, as shown in Fig.
• What happens to the candle flames in the two cases?
| Experiment | Observation | Inference |
| Burning candle uncovered in Petri dish | Candle continues to burn | Oxygen is required for combustion |
| Cover burning candle with glass tumbler in a Petri dish | Flame gets extinguished | CO2 formed in the inverted glass extinguishes fire |
Activity 5.6: Let Us Investigate (Page 64)
• Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it. It quickly catches fire. Do we say that we need a fire to start the burning process?
Answer:
Yes
• Take another piece of paper. Using a magnifying glass, focus the sunrays„ to make the smallest and brightest spot on the paper, as shown in the Fig. Hold it therefor some time. What do you observe?
Answer:
Paper catches fire because ignition temperature of paper is low. Yes, we can start burning process without fire. Sunrays focussed on the paper heat the paper. The temperature of the paper increases with time and reaches ignition temperature, so it starts burning.
Activity 5.8: Let Us Think (Page 66)
• Think again about all the changes that we have discussed or talked about so far. In which of these can we get back the object or substance in the form we started with? Record your observations in Table
Answer:
| Change | The Original State can be brought back (Yes/No) |
| Melting ice cubes | Yes |
| Chopping vegetables | No |
| Boiling water | Yes |
| Making popcorn from corn | No |
| Growth of a plant | No |
| Dissolution of sugar in water | Yes |
| Condensation of water vapours | Yes |