Changes Around Us: Physical and Chemical

Given: A list of changes to observe and record.

Concept: Changes can be observed through our senses — sight, smell, touch, hearing, and taste. They may involve changes in size, shape, smell, colour, state, or other properties.

| S.No. | Change | Observation(s) |
|---|---|---|
| 1. | Melting ice cubes | Ice (solid) changes to water (liquid); shape and state change; no new substance formed. |
| 2. | Chopping vegetables | Size and shape of vegetables change; no new substance formed. |
| 3. | Boiling water | Water (liquid) changes to steam (gas); state changes; bubbles are seen. |
| 4. | Making popcorn from corn | Corn kernels burst open, change in size, shape, smell, and texture; a new form is produced. |
| 5. | Cutting a piece of paper | Size and shape of paper change; no new substance formed. |
| 6. | Adding beetroot extract to water | Water turns reddish/pink in colour; colour change observed. |
| 7. | Burning wood | Wood turns to ash and smoke; heat and light are produced; new substances (ash, gases) are formed. |
| 8. | Drying wet clothes | Water evaporates from clothes; clothes become dry; state of water changes. |
| 9. | Making small balls of dough | Shape of dough changes; no new substance formed. |
| 10. | Rolling small balls of dough into chapatis | Shape of dough changes (from ball to flat disc); no new substance formed. |
| 11. | Any other (e.g., ripening of a banana) | Colour changes from green to yellow; smell and taste change; new substances are formed. |

Other changes in surroundings: Rusting of iron, curdling of milk, germination of seeds, dissolving salt in water, melting of butter, etc.

Given: Paper is folded to create new objects.

Observation: Yes, when we unfold the paper objects, we get the same paper back.

Conclusion: Only the shape of the paper changed. No new substance was formed. This is a physical change because only the physical property (shape) changed and the paper remained the same material.

Given: A balloon is inflated and then (i) deflated by loosening grip, and (ii) pricked with a pin.

(i) Loosening grip: Yes, we get the uninflated balloon back. The rubber sheet of the balloon returns to its original shape. Only the size/shape changed temporarily — this is a physical change that is reversible.

(ii) Pricking with a pin: The balloon bursts. We cannot get the uninflated balloon back in its original form because the rubber sheet is torn/damaged.

Conclusion: In both cases, the rubber material of the balloon remained the same (no new substance formed), so both are physical changes. However, pricking the balloon is an irreversible physical change, while deflating by loosening grip is a reversible physical change.

Given: A piece of chalk is crushed into powder.

Observation: The chalk breaks into fine powder. The size and shape change.

Can we get the chalk piece back? No, we cannot easily get the original chalk piece back from the powder.

Conclusion: The material (chalk) remains the same — no new substance is formed. Only the size and shape changed. This is a physical change. However, it is practically irreversible (we cannot reassemble the powder into the original chalk piece).

Similarity in A, B, and C: In all three cases — folding paper, inflating/deflating a balloon, and crushing chalk — the material remains the same. No new substance is formed. Only physical properties like shape and size change. Hence, all are physical changes.

Given: Air is blown through a straw into (A) tap water and (B) lime water.

Observation:
- Glass A (tap water): Bubbles are formed when air is blown in, but there is no change in the appearance of the water. The water remains clear.
- Glass B (lime water): Bubbles are formed and the lime water turns milky (cloudy). After some time, a white substance settles at the bottom.

Explanation: The exhaled air contains carbon dioxide (CO₂). Carbon dioxide reacts with lime water (calcium hydroxide) to form a new white-coloured substance — calcium carbonate — which is insoluble in water. This makes the lime water appear milky.

Chemical equation:
$$\text{Calcium hydroxide} + \text{Carbon dioxide} \rightarrow \text{Calcium carbonate} + \text{Water}$$

Conclusion: Since a new substance (calcium carbonate) is formed, this is a chemical change. The turning of lime water milky is used as a test for carbon dioxide. The change in tap water (only bubbles, no new substance) is a physical change.

Given: Baking soda (sodium hydrogen carbonate) is added to vinegar (or lemon juice). The gas produced is passed through freshly prepared lime water.

Observation:
- A fizzing/bubbling sound is heard when baking soda is added to vinegar.
- Gas bubbles are formed rapidly.
- When this gas is passed through lime water, the lime water turns milky.

Inference: Since lime water turns milky, the gas produced is carbon dioxide (CO₂).

Chemical equation:
$$\text{Vinegar} + \text{Baking soda} \rightarrow \text{Carbon dioxide} + \text{Other substances}$$

Conclusion: Since a new substance (carbon dioxide) is formed, this is a chemical change.

Baking soda + Water: When baking soda is mixed with water, no bubble formation is observed. No new substance is formed. This is a physical change (dissolving).

Given: Two identical candles are lit. One is left open; the other is covered with a glass tumbler.

Observation:
- The candle left open continues to burn.
- The candle covered with the glass tumbler goes out (extinguishes) after some time.

Explanation: The candle covered with the glass tumbler uses up all the oxygen present inside the tumbler. Once the oxygen is exhausted, the candle can no longer burn and the flame goes out.

Conclusion: Oxygen is essential for combustion. Without oxygen, a combustible substance cannot burn, even if it has reached its ignition temperature.

Given: (i) A lighted matchstick is brought near paper. (ii) Sunrays are focused on paper using a magnifying glass.

Observation:
- (i) The paper quickly catches fire when a lighted matchstick is brought near it.
- (ii) When sunrays are focused on the paper using a magnifying glass, the paper first emits smoke and then catches fire, even without a direct flame.

Explanation:
- In case (i), the temperature of the lighted matchstick is already higher than the ignition temperature of paper, so it catches fire almost immediately.
- In case (ii), focusing sunrays heats the paper. The temperature of the paper gradually increases. When it reaches the ignition temperature, the paper starts burning.

Ignition Temperature: The minimum temperature at which a substance catches fire is called its ignition temperature.

Three requirements for combustion (Fire Triangle):
1. A combustible substance (fuel)
2. Oxygen (from air)
3. Heat (to reach the ignition temperature)

Conclusion: A substance can burn even without a direct flame, as long as it reaches its ignition temperature in the presence of oxygen.

Given: A candle is burning.

Analysis of changes:

| Change | Type |
|---|---|
| Wax melts (solid → liquid) | Physical change |
| Liquid wax is drawn up the wick | Physical change |
| Liquid wax evaporates (liquid → vapour) | Physical change |
| Wax vapour burns to produce flame, heat, light, CO₂, and water | Chemical change |
| Melted wax solidifies when it drips and cools | Physical change |

Conclusion: The burning of a candle involves both physical and chemical changes.
- Physical changes: Melting of wax, evaporation of wax, solidification of dripped wax.
- Chemical change: Burning of wax vapour (combustion) — new substances like carbon dioxide and water are formed, and heat and light are released.

Given: Various changes to analyse for reversibility.

| S.No. | Change | Original state can be brought back (Yes/No) | Reason |
|---|---|---|---|
| 1. | Melting ice cubes | Yes | Ice can be refrozen into ice (physical change). |
| 2. | Chopping vegetables | No | Chopped pieces cannot be rejoined (physical change, irreversible). |
| 3. | Boiling water | Yes | Steam can be condensed back to water (physical change). |
| 4. | Making popcorn from corn | No | Popcorn cannot be converted back to corn (chemical change). |
| 5. | Cutting a piece of paper | No | Cut paper cannot be rejoined to original form. |
| 6. | Adding beetroot extract to water | No | The colour cannot be easily separated back. |
| 7. | Burning wood | No | Ash and gases cannot be converted back to wood (chemical change). |
| 8. | Drying wet clothes | Yes | Water vapour can condense; clothes can be wetted again. |
| 9. | Making small balls of dough | Yes | Balls can be reshaped (physical change). |
| 10. | Rolling small balls of dough into chapatis | Yes | The dough can be reshaped (physical change, before cooking). |
| 11. | Rusting of iron | No | Rust (iron oxide) cannot easily be converted back to iron (chemical change). |
| 12. | Burning of magnesium ribbon | No | Magnesium oxide cannot be converted back to magnesium ribbon easily (chemical change). |

Conclusion: Changes that can be reversed are generally physical changes (e.g., melting, boiling, folding). Changes that cannot be reversed are generally chemical changes (e.g., burning, rusting, making popcorn) or irreversible physical changes (e.g., chopping).

Correct Answer: (c) (i) and (iii)

Justification:
- Statement (i) is correct: In a physical change, the state of the substance may or may not change (e.g., melting changes state; cutting does not).
- Statement (iii) is correct: In a physical change, no new substance is formed — only physical properties like shape, size, or state change.
- Statement (ii) is incorrect: Formation of a substance with different properties is a characteristic of a chemical change.
- Statement (iv) is incorrect: A chemical reaction occurs in a chemical change, not a physical change.

Given: Various changes to classify as reversible or irreversible.

Concept: A change is reversible if the original substance/object can be obtained back. Physical changes are generally reversible; chemical changes are generally irreversible.

| S.No. | Change | Reversible/Irreversible | Reason |
|---|---|---|---|
| (i) | Stitching cloth to a shirt | Reversible | The stitches can be removed (unstitched) to get back the cloth pieces. |
| (ii) | Twisting of straight string | Reversible | The string can be untwisted to get the straight string back. |
| (iii) | Making idlis from a batter | Irreversible | Idlis cannot be converted back to batter; a chemical change (fermentation and cooking) has occurred. |
| (iv) | Dissolving sugar in water | Reversible | Sugar can be recovered by evaporating the water (physical change). |
| (v) | Drawing water from a well | Reversible | Water can be poured back into the well; no change in the water itself. |
| (vi) | Ripening of fruits | Irreversible | Ripened fruits cannot be made unripe; chemical changes have occurred. |
| (vii) | Boiling water in an open pan | Irreversible (practically) | Water vapour escapes into the atmosphere and cannot be easily collected back. If the pan is closed, it is reversible. Not sure — if the steam is collected and condensed, it is reversible; but in an open pan, the water is lost. |
| (viii) | Rolling up a mat | Reversible | The mat can be unrolled back to its original flat form. |
| (ix) | Grinding wheat grains to flour | Irreversible | Flour cannot be converted back to wheat grains. |
| (x) | Forming of soil from rocks | Irreversible | Soil formed from rocks over thousands of years cannot be converted back to rocks easily. |

Uncertainty: For (vii) — boiling water in an open pan — one may be unsure because if the steam is captured and condensed, the water is recovered (reversible). But if the steam escapes freely, the water is lost (practically irreversible). The answer depends on the conditions.

(i) Melting of wax is necessary for burning a candle.
Answer: True
Explanation: When a candle burns, the wax first melts due to the heat of the flame, is drawn up the wick, and then evaporates. The wax vapour then burns. So, melting of wax is indeed necessary for the candle to burn.

(ii) Collecting water vapour by condensing involves a chemical change.
Answer: False
Correct statement: Collecting water vapour by condensing involves a physical change. Condensation is the change of state from gas (water vapour) to liquid (water). No new substance is formed; it is a physical change.

(iii) The process of converting leaves into compost is a chemical change.
Answer: True
Explanation: When leaves decompose to form compost, new substances are formed due to the action of microorganisms (bacteria and fungi). Since new substances are formed, it is a chemical change.

(iv) Mixing baking soda with lemon juice is a chemical change.
Answer: True
Explanation: When baking soda (sodium hydrogen carbonate) is mixed with lemon juice (which contains citric acid), a reaction occurs and carbon dioxide gas is produced (seen as bubbles). Since a new substance (CO₂) is formed, it is a chemical change.

(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:

| Activity | Type of Change | Reversible? |
|---|---|---|
| Melting of butter/ghee | Physical (change of state) | Yes — can be solidified by cooling |
| Dissolving salt or sugar in water | Physical (dissolving) | Yes — water can be evaporated to recover salt/sugar |
| Cutting/chopping vegetables | Physical (change in size/shape) | No — irreversible physical change |
| Boiling water | Physical (change of state) | Yes — steam can be condensed back to water |
| Freezing water to make ice | Physical (change of state) | Yes — ice can be melted back to water |
| Kneading dough | Physical (change in shape) | Yes — dough can be reshaped |

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:

| Change | Type |
|---|---|
| Dissolving sugar in water | Physical change |
| Balloon inflating due to gas production | Physical change |
| Fermentation of sugar by yeast (producing CO₂ and ethanol) | Chemical change |
| Lime water turning milky (CO₂ + lime water → CaCO₃) | Chemical change |

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.