Changes Around Us: Physical and Chemical — NCERT Solutions

(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.

Explanation: The brown deposits on the iron handle are rust (iron oxide), formed when iron reacts with oxygen and moisture. Since a new substance (iron oxide) is formed, it is a chemical change.

(ii) Folding a handkerchief is a physical change and can be reversed.

Explanation: Folding only changes the shape of the handkerchief. No new substance is formed. The handkerchief can be unfolded to get back its original form, so it is reversible.

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

Explanation: Combustion is a chemical reaction in which a substance (fuel) reacts with oxygen to produce heat and/or light. New substances are formed, making it a chemical change.

(iv) Magnesium, when burnt in air, produces a substance called magnesium oxide. The substance formed is white (powdery) in nature. Burning of magnesium is a chemical change.

Explanation: When magnesium ribbon burns in air (oxygen), it forms magnesium oxide — a white powder. Since a new substance is formed, it is a chemical change.
$$\text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium oxide} + \text{Heat} + \text{Light}$$

Given: Two changes — (i) water to ice, and (ii) water to steam.

Concept: A physical change involves only a change in physical properties (shape, size, state) without the formation of any new substance. A chemical change involves the formation of one or more new substances.

Answer: Both changes — water to ice and water to steam — are physical changes.

Explanation:

(i) Water to Ice (Freezing):

• Water (liquid) changes to ice (solid) when cooled to $0°C$ or below.
• Only the state of water changes (liquid → solid).
• The chemical composition remains the same — both water and ice are made of $H_2O$.
• No new substance is formed.
• This change is reversible — ice can be melted back to water.
• Therefore, it is a physical change.

(ii) Water to Steam (Vaporisation/Boiling):

• Water (liquid) changes to steam (gas) when heated to $100°C$.
• Only the state of water changes (liquid → gas).
• The chemical composition remains the same — both water and steam are $H_2O$.
• No new substance is formed.
• This change is reversible — steam can be condensed back to water.
• Therefore, it is a physical change.

Conclusion: Both changes are physical changes because only the state of water changes and no new substance is formed.

Given: Curdling of milk.

Answer: Curdling of milk is a chemical change.

Justification:

1. When milk curdles, a new substance called curd is formed. Curd has different properties from milk — it is sour in taste, has a different texture, and contains lactic acid.
2. The curdling is caused by bacteria (like Lactobacillus) that convert lactose (milk sugar) into lactic acid. This acid causes the milk proteins to coagulate and form curd.
3. Since new substances are formed (lactic acid, coagulated proteins), it is a chemical change.
4. This change is irreversible — curd cannot be converted back to milk.

Conclusion: Curdling of milk is a chemical change because new substances with different properties are formed and the change cannot be reversed.

Given: Formation of soil from rocks due to natural factors like wind and rain.

Answer: The formation of soil from rocks involves both physical and chemical changes.

Explanation:

Physical Changes involved (Weathering — Physical):

• Wind, flowing water, and temperature changes cause rocks to break into smaller pieces (pebbles, gravel, sand).
• This is a physical change because only the size and shape of the rock change; no new substance is formed initially.
• Erosion by wind and water carries these broken pieces and deposits them elsewhere.

Chemical Changes involved (Weathering — Chemical):

• Rainwater (slightly acidic due to dissolved CO₂) reacts with minerals in rocks, forming new substances.
• Oxygen in air reacts with minerals in rocks (oxidation), forming new compounds.
• These chemical reactions change the chemical composition of the rock material, forming new substances that make up part of the soil.

Conclusion: The formation of soil from rocks is primarily a physical change (breaking and erosion), but chemical changes also occur when rainwater and oxygen react with rock minerals. Most of these changes take place over thousands of years and cannot be reversed.

Given: Story of Prithvi performing various activities in the kitchen and garden.

Answer with justification:

1. He chops vegetables, peels potatoes, and cuts fruits → (Physical changes)

Reason: Chopping, peeling, and cutting only change the size and shape of the vegetables and fruits. No new substance is formed. These are physical changes.

2. He collects the seeds, fruits, and vegetable peels into a clay pot → (Physical change)

Reason: Collecting and placing materials into a pot only involves a change in position/location. No new substance is formed. This is a physical change.

3. The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost → (Chemical change)

Reason: Decomposition by bacteria and fungi involves the breakdown of organic matter into new substances (compost/humus). New substances with different properties are formed. This is a chemical change.

4. The seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers → (Chemical change)

Reason: Germination and growth of plants involve complex chemical reactions (like photosynthesis, respiration, and biosynthesis of new compounds). New substances are continuously formed. These are chemical changes.

Suggested title for the story: "From Kitchen Waste to Garden Treasure" or "Prithvi's Green Cycle"

Given: A list of changes to classify as physical (A), chemical (B), or both physical and chemical (C).

Concept:

• Physical change (A): Only physical properties change; no new substance formed.
• Chemical change (B): New substance(s) formed; involves a chemical reaction.
• Both (C): The process involves both physical and chemical changes.

Classification:

Area A — Physical Changes only:

• Tearing of paper — Only the size and shape of paper change; no new substance formed.
• Melting of ice — Only the state changes (solid → liquid); no new substance formed.
• Folding of clothes — Only the shape changes; no new substance formed.

Area B — Chemical Changes only:

• Rusting — Iron reacts with oxygen and moisture to form iron oxide (rust); new substance formed.
• Curdling of milk — Lactic acid and coagulated proteins are formed; new substances formed.
• Ripening of fruits — Chemical composition changes; new substances (sugars, pigments) formed.
• Burning of magnesium — Magnesium oxide (new substance) is formed along with heat and light.
• Mixing baking soda with vinegar — Carbon dioxide gas (new substance) is produced.

Area C — Both Physical and Chemical Changes:

• Process of burning a candle — Melting of wax, evaporation of wax (physical changes) + burning of wax vapour to form CO₂ and water (chemical change).

Given: Four experiments (a, b, c, d) are performed with lime water. (Note: The figures show different setups — based on the context of the chapter, the experiments likely involve: (a) blowing exhaled air into lime water, (b) passing gas from vinegar + baking soda reaction through lime water, (c) passing some other gas or air through lime water, (d) another variation. The common theme is testing for carbon dioxide.)

Concept: Lime water turns milky when carbon dioxide (CO₂) is passed through it. The reaction is:
$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O}$$
Calcium carbonate (CaCO₃) is insoluble in water and appears as a white precipitate, making the lime water milky.

Based on the chapter context, the likely setups are:

• (a) Blowing exhaled air (which contains CO₂) into lime water → Lime water turns milky because exhaled air contains CO₂.
• (b) Passing gas produced from vinegar + baking soda reaction (CO₂) through lime water → Lime water turns milky because CO₂ is produced in this reaction.
• (c) Passing normal air (inhaled air, which has very little CO₂) through lime water → Lime water may not turn milky (or turns very slightly milky) because normal air contains only about 0.04% CO₂, which is insufficient to cause a visible change.
• (d) Passing a gas that does not contain CO₂ (e.g., oxygen or nitrogen) through lime water → Lime water does not turn milky because no CO₂ is present.

Conclusion: Lime water turns milky in the cases where carbon dioxide is present — i.e., in experiments (a) (exhaled air) and (b) (gas from vinegar + baking soda). The key reason is that CO₂ reacts with Ca(OH)₂ to form insoluble CaCO₃, which makes the solution appear milky/cloudy.

Given: Lemon juice is used as invisible ink. When heat is applied, the letters turn dark brown.

Explanation of changes:

1. Writing with lemon juice and drying: Lemon juice (which contains citric acid and water) is applied to paper. When it dries, the water evaporates and the lemon juice becomes transparent/invisible. The drying is a physical change (evaporation of water).

2. Heating the paper (ironing or holding over flame): When heat is applied, the organic compounds in lemon juice (citric acid and sugars) undergo combustion/oxidation at a lower temperature than the paper itself. They turn dark brown (char). This is a chemical change — new brown-coloured substances are formed.

Can these changes be reversed?

• The drying of lemon juice (physical change) is reversible in principle — the water that evaporated could be condensed, but practically the message would not become invisible again.
• The browning/charring of lemon juice on heating (chemical change) is irreversible — the dark brown substances formed cannot be converted back to the original lemon juice compounds.

Conclusion: The physical change (drying) is reversible, but the chemical change (browning due to heat) is irreversible.

Given: Landslides and rock erosion cause damage to life and property in hilly regions.

Steps to reduce landslides and rock erosion:

1. Planting trees and vegetation (Afforestation): Tree roots bind the soil and rocks together, preventing them from sliding. Vegetation cover reduces the impact of rainwater on the soil.

2. Building retaining walls: Constructing strong walls along slopes can hold back soil and rocks, preventing them from sliding down.

3. Terracing of slopes: Converting steep slopes into step-like terraces reduces the speed of water runoff and prevents soil erosion.

4. Avoiding deforestation: Cutting trees on hillsides removes the natural binding of soil, making it prone to erosion and landslides. Deforestation should be strictly controlled.

5. Proper drainage systems: Building channels to divert rainwater away from slopes reduces water pressure on hillsides.

6. Avoiding construction on unstable slopes: Human settlements and roads should not be built on steep or geologically unstable slopes.

7. Rock bolting and netting: In areas prone to rockfalls, metal nets and bolts can be used to hold rocks in place.

8. Early warning systems: Installing sensors to detect ground movement can provide early warnings to people living in landslide-prone areas.

Conclusion: A combination of natural methods (afforestation) and engineering solutions (retaining walls, terracing) can significantly reduce the damage caused by landslides and rock erosion.

Given: Kitchen activities to observe for reversible changes.

Reversible changes observed in the kitchen:

Conclusion: Most reversible changes in the kitchen are physical changes — they involve changes in state, shape, or size without forming new substances. Chemical changes (like cooking, frying, baking) are generally irreversible.

Given: Sugar solution + yeast in a bottle, covered with a balloon. After ~1 hour, the balloon is removed and its contents are mixed with lime water.

Observation:

1. After about an hour, the balloon inflates — a gas is produced inside the bottle.
2. When the gas from the balloon is mixed with lime water, the lime water turns milky.

Inference/Conclusion:

• The gas produced is carbon dioxide (CO₂), because it turns lime water milky.
• Yeast (a microorganism) breaks down sugar in the absence of sufficient oxygen through a process called fermentation, producing carbon dioxide and alcohol (ethanol).

$$\text{Sugar} \xrightarrow{\text{Yeast}} \text{Carbon dioxide} + \text{Ethanol (Alcohol)}$$

• This is how yeast makes bread fluffy — the CO₂ bubbles get trapped in the dough, making it rise and become soft.

Identification of changes:

Conclusion: The fermentation of sugar by yeast is a chemical change because new substances (CO₂ and ethanol) are formed. The inflation of the balloon is a physical change.

Given: Chameleons (Girgits) change colour in response to their surroundings, mood, or danger.

Answer: Yes, the colour change in chameleons is reversible.

Explanation:

• Chameleons have special cells in their skin called chromatophores (and iridophores) that contain pigments and reflect light differently.
• When a chameleon changes its mood, temperature, or environment, these cells expand or contract, changing the way light is reflected and thus changing the apparent colour of the skin.
• This is a physical change — the structure of the skin cells changes temporarily, but no new substance is permanently formed.
• When the chameleon returns to its normal state (calm, normal temperature, familiar surroundings), the cells return to their original arrangement and the colour reverts.
• Therefore, the colour change is reversible.

Conclusion: The colour change in chameleons is a reversible physical change controlled by the nervous system and hormones. It is not a chemical change because no new permanent substance is formed in the skin.