Changes Around Us: Physical and Chemical Class 7 (Curiosity)

Q1
Question
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)
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Concept:
5.1 A Substance May Change in Appearance but Remain the Same!
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Q2
Question
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
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Concepts:
5.1 Physical Change
5.5 Are Changes Permanent?
5.7.1 Weathering of Rocks
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Q3
Question
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)
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Concepts:
5.1 Physical Change
5.2 Chemical Change
5.4 Both Physical and Chemical
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Q4
Question
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.
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Concepts:
5.1 Physical Change
5.3.1 Rusting
5.3.2 Combustion
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Q5
Question
Are the changes of water to ice and water to steam, physical or chemical? Explain.
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Concept:
5.1 A Substance May Change in Appearance but Remain the Same!
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Q6
Question
Is curdling of milk a physical or chemical change? Justify your statement.
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Concept:
5.2 A Substance May Change in Appearance and Not Remain the Same!
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Q7
Question
Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
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Concept:
5.7.1 Weathering of Rocks
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Q8
Question
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.
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Concepts:
5.1 Physical Change
5.2 Chemical Change
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Q9
Question
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.
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Concepts:
5.1 Physical Change
5.2 Chemical Change
5.4 Can Both Occur Together?
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Q10
Question
The experiments shown below (a, b, c, and d) were performed. Find out in which case(s) did lime water turn milky and why?
(a) Vinegar + baking soda → lime water
(b) Lemon juice + vinegar → lime water
(c) Vinegar + common salt → lime water
(d) Lemon juice + baking soda → lime water
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Concept:
5.2 A Substance May Change in Appearance and Not Remain the Same!
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Caution
— Perform this activity under the supervision of an adult.
Objective
— Discover a chemical change that "writes" with heat.
What to do
— Write a message on a piece of paper using lemon juice as ink and let it dry. The message will become invisible.
Reveal step
— Now use a warm iron over the paper (or hold the paper carefully over a candle flame, taking care that it does not catch fire).
Observation
— The invisible letters turn dark brown as the paper warms.
Reflect
— Lemon juice is an acid. When heated, it reacts with the paper (which is mainly carbon-rich) and "browns" before the rest of the paper. Can these changes be reversed?
Type of change
— The browning is a chemical change (new substances formed in the paper); it cannot be reversed.

Objective
— Connect what you've learnt about weathering and erosion to real-world challenges.
What to do
— Read recent news about landslides in Indian hilly regions (Uttarakhand, Himachal Pradesh, Sikkim, the Western Ghats).
Discussion points
— What can be done to reduce landslides and erosion?
Plant more trees (roots hold soil in place).
Build retaining walls along roads.
Reduce deforestation and unplanned construction on steep slopes.
Use terracing in agriculture to slow water runoff.
Maintain proper drainage to prevent water pooling in slopes.
Deliverable
— Write a one-page report or make a presentation with photos and your suggestions.

Objective
— See physical and chemical changes happening in a busy kitchen.
What to do
— Spend 30 minutes observing the kitchen at home (with permission). List every change you see.
For each change
— Decide:
Reversible or irreversible?
Physical or chemical?
Examples to look for
— Boiling water (physical, reversible); Kneading dough (physical, reversible); Frying onions (chemical, irreversible); Cooking
dal
(chemical, irreversible); Dissolving salt (physical, reversible).
Reflect
— Are there more chemical or physical changes happening in a typical kitchen?

Objective
— See the chemical change that makes bread rise.
Materials
— A small bottle, two teaspoons of sugar, fresh yeast, water, and a balloon.
Step 1
— Make a sugar solution in the bottle by dissolving sugar in water. Add a spoonful of yeast. Cover the mouth of the bottle with the balloon. Leave it undisturbed for about an hour.
Observation 1
— The balloon inflates on its own — yeast is producing a gas.
Step 2
— Carefully take off the balloon while pinching its mouth shut. Attach it to another small bottle containing freshly prepared lime water. Shake so the gas mixes with the lime water.
Observation 2
— The lime water turns milky — the gas is carbon dioxide.
Conclusion
— Yeast feeds on sugar and produces carbon dioxide (and a small amount of alcohol). This gas makes bread rise during baking.
Identify the changes
— Sugar dissolving (physical); yeast fermenting sugar (chemical); CO₂ turning lime water milky (chemical).

Chameleons (
Girgits
) change colour to blend in with their surroundings — and also when they are angry or sense danger.
Question to explore
— Is this a change that can be reversed?
Find out from
— the internet, your school library, or a biology textbook.
Hint
— Chameleons have special cells with tiny crystals; the cells expand or contract to reflect different colours of light. Yes, it's reversible — the chameleon can change back!
Every kitchen, garden, beach, and mountainside is a laboratory full of physical and chemical changes. Once you can spot which is which — and which can be reversed — you start seeing the chemistry of everyday life everywhere you look.