Solutions-NCERT-Class-7-Science-Curiosity-Chapter-5-Changes Around Us: Physical and Chemical-CBSE

Burning of magnesium is a chemical change.

Reason:

• When magnesium burns, it reacts with oxygen in the air
• A new substance called magnesium oxide is formed
• Heat and bright white light are also produced

Since a new substance is formed and the change cannot be reversed, burning of magnesium is a chemical change.

• A candle needs oxygen from the air to burn (combustion).
• When the candle is covered with a glass tumbler, fresh air cannot enter.
• The limited oxygen inside the tumbler gets used up after some time.
• Once the oxygen is finished, burning cannot continue, so the flame goes out.

Carbon Dioxide Test :

• The presence of CO₂ is confirmed using lime water (calcium hydroxide).
• When CO₂ is passed through lime water, it turns milky due to the formation of calcium carbonate, an insoluble white precipitate.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Yes, both physical and chemical changes occur when a candle burns.

• Physical Change: The melting of wax, its solidification, and evaporation.
• Chemical Change: The burning of the wax vapor or the wick, which produces carbon dioxide and heat.

No, not all changes are permanent.

Some changes can be reversed like melting ice or boiling water, while others cannot be reversed, like chopping vegetables or making popcorn.

No, not all changes are desirable.

Useful changes like making curd or cooking food are desirable, but

undesirable changes like the rusting of iron are not.

Weathering is a slow natural process in which large rocks break down into smaller pieces called sediments.

These sediments collect at the base of mountains and help in the formation of soil.

(c) (i) and (iii)

This is because in a physical change, the state of the substance may or may not change and no new substance is formed.

(i) Stitching cloth to a shirt : Irreversible - Once stitched, it cannot return to original cloth easily

(ii) Twisting of straight string : Reversible - Can be untwisted back to original form

(iii) Making idlis from batter : Irreversible - Cannot convert idli back to batter

(iv) Dissolving sugar in water : Reversible - Sugar can be obtained back by evaporation

(v) Drawing water from a well : Reversible - Water can be put back into the well

(vi) Ripening of fruits : Irreversible - Cannot change ripe fruit back to raw

(vii) Boiling water in an open pan : Reversible - Steam can be cooled back into water

(viii) Rolling up a mat : Reversible -Can be unrolled easily

(ix) Grinding wheat grains to flour : Irreversible - Cannot change flour back to grains

(x) Formation of soil from rocks : Irreversible - Cannot convert soil back into rocks easily

We may be not sure in some cases (like dissolving sugar or boiling water) because special methods are needed to reverse them.

Statement is: True

Explanation:

• When a candle is lit, the wax near the flame melts.
• The melted wax is drawn up through the wick and burns.
• So, melting of wax is necessary for the candle to keep burning.

Statement is: False

Correct Statement: Collecting water vapour by condensing involves a physical change.

Explanation:

• In condensation, water vapour changes into liquid water
• No new substance is formed
• Only the state changes (gas → liquid)
• So, it is a physical change, not a chemical change.

Statement is: True

• Since a new substance is formed, it is a chemical change.

Statement is: True

• Since a new substance is formed and gas is produced, it is a chemical change.

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.

Folding a handkerchief is a physical change and can be reversed.

A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.

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.

Changes of Water to Ice and Water to Steam : Both changes are physical changes.

Explanation:

Water to ice (freezing):

• Liquid water changes into solid ice
• Only the state changes (liquid → solid)
• No new substance is formed

Water to steam (boiling):

• Liquid water changes into gas (steam)
• Only the state changes (liquid → gas)
• No new substance is formed

Since only the state changes and no new substance is formed, both changes are physical changes.

Curdling of milk is a chemical change.

Justification:

• Milk changes into curd, which is a new substance
• The properties of curd are different from milk (taste, texture, smell)
• The change cannot be reversed back to milk

Since a new substance is formed and the change is irreversible, curdling of milk is a chemical change.

This change is actually both physical and chemical, but the dominant process depends on the factor:

Physical Changes :

• Wind: Breaks rocks into smaller fragments through abrasion.
• Rainwater (mechanical action): Causes cracks, erosion, and transportation of particles.
• These are physical changes because the rock’s composition remains the same, only its size and shape are altered.

Chemical Changes :

• Rainwater (chemical action): Slightly acidic rain (due to dissolved CO₂ forming carbonic acid) reacts with minerals in rocks.
• Oxidation: Oxygen in air reacts with minerals like iron, forming oxides (rust-like substances).
• These are chemical changes because new substances are formed with different properties.

(i) Chopping vegetables, peeling potatoes, and cutting fruits are physical changes.

(ii) Collecting seeds, fruits, and vegetable peels into a clay is a physical change.

(iii) Decomposition of fruits, vegetable peels, and other materials due to the action of bacteria and fungi to form compost is a chemical change.

(iv) Germination and growth of plants are chemical changes because new substances and structures are formed during growth.

The suitable title for the story is: "composting from kitchen waste".

A : Tearing of paper, melting of ice, folding of clothes

B : Rusting, curdling of milk, ripening of fruits, burning of magnesium, mixing baking soda with vinegar

C : Burning of a candle

Lime water turned milky in the cases (a) and (d) where vinegar or lemon juice was mixed with baking soda, because carbon dioxide gas was formed which reacts with

lime water to form a white precipitate (calcium carbonate).

The chemical reaction can be represented as follows

Vinegar or lemon juice + Baking soda -> Carbon dioxide + Other substances