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.