import { Callout } from "@/components/Callout"; import { Checklist } from "@/components/Checklist";

Changes Around Us — Exercise Solutions

Use this page alongside the official Samacheer Kalvi textbook. Solve each exercise on your own first, then check your answers below.

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I. Choose the Appropriate Answer (MCQ)

Q1. When ice melts to form water, change occurs in its (a) position (b) colour (c) state (d) composition

(c) state — Ice (solid) changes into water (liquid). Only the physical state changes; the composition (H₂O) stays the same.

Q2. Formation of curd from milk is (a) a reversible change (b) a fast change (c) an irreversible change (d) an undesirable change

(c) an irreversible change — Once milk turns into curd, you cannot get the original milk back. A new substance (curd) is formed.

Q3. Out of the following, an example of a desirable change is (a) rusting (b) change of seasons (c) earthquake (d) flooding

(b) change of seasons — Change of seasons is useful and necessary for life. Rusting, earthquakes, and flooding are harmful or undesirable.

Q4. Air pollution leading to Acid rain is a (a) reversible change (b) fast change (c) natural change (d) human-made change

(d) human-made change — Acid rain is caused by human activities like industrial emissions and burning of fossil fuels, not by natural processes alone.

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II. Fill in the Blanks

Q1. Magnet attracts iron needle. This is \_\_\_\_\_\_\_\_ change.

reversible — The iron needle is attracted to the magnet but is not permanently altered. When removed from the magnet, it returns to its original state.

Q2. Boiling of egg results in \_\_\_\_\_\_\_\_ change.

an irreversible — The proteins in the egg denature permanently. You cannot un-boil an egg to get the raw egg back.

Q3. Changes that are harmful to us are \_\_\_\_\_\_\_\_.

undesirable — Undesirable changes cause damage or loss, such as rusting, flooding, and deforestation.

Q4. Plants convert Carbon-di-oxide and water into starch. This is an example of \_\_\_\_\_\_\_\_ change.

natural — Photosynthesis is a natural process carried out by plants without human intervention.

Q5. Bursting of fire crackers is a \_\_\_\_\_\_\_\_ change whereas germination of seeds is a \_\_\_\_\_\_\_\_ change.

fast, slow — Bursting crackers happens in seconds (fast). Germination takes days or weeks (slow).

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III. True or False (Correct the False Statements)

Q1. Growing of teeth in an infant is a slow change.

True — Teeth grow gradually over months, which qualifies as a slow change.

Q2. Burning of a match stick is a reversible change.

False — Burning of a match stick is an irreversible change. Once burnt, the match stick cannot be restored to its original form.

Q3. Change of New moon to Full moon is human-made.

False — Change of New moon to Full moon is a natural change. It occurs on its own without human involvement.

Q4. Digestion of food is a physical change.

False — Digestion of food is a chemical change. New substances are formed as food is broken down by enzymes and acids in the body.

Q5. In a solution of salt in water, water is the solute.

False — In a solution of salt in water, water is the solvent. Salt is the solute. The substance that dissolves is the solute; the substance that does the dissolving is the solvent.

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IV. Analogy

Q1. Curdling of milk : irreversible change :: Formation of clouds : \_\_\_\_\_\_\_\_ change

reversible — Clouds form from water vapour (evaporation) and can reverse back to water (condensation → rain). The water cycle is reversible.

Q2. Photosynthesis : \_\_\_\_\_\_\_\_ change :: Burning of coal : Human-made change

natural — Photosynthesis is carried out by plants without human intervention, making it a natural change.

Q3. Dissolving of glucose : reversible change :: Digestion of food : \_\_\_\_\_\_\_\_ change

irreversible — Once food is digested, it cannot be turned back into the original food. New substances are formed.

Q4. Cooking of food : desirable change :: Decaying of food : \_\_\_\_\_\_\_\_ change

undesirable — Food decay is harmful and wasteful, making it an undesirable change.

Q5. Burning of matchstick : \_\_\_\_\_\_\_\_ change :: Rotation of the Earth : Slow change

fast — Burning a matchstick is completed in seconds, which is a fast change.

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V. Circle the Odd One Out (Give Reason)

Q1. Growth of a child, Blinking of eye, Rusting, Germination of a seed

Rusting — Rusting is an undesirable change. The other three (growth of a child, blinking of eye, germination) are desirable changes.

Q2. Glowing of a bulb, Lighting of a candle, Breaking of a coffee mug, Curdling of milk

Breaking of a coffee mug — Breaking a mug is an undesirable change. The other three are desirable changes.

Q3. Rotting of an egg, Condensation of water vapour, Trimming of hair, Ripening of fruit

Rotting of an egg — Rotting of an egg is an undesirable change. The other three are desirable changes.

Q4. Inflating a balloon, Popping a balloon, Fading of wall paint, Burning of kerosene

Inflating a balloon — Inflating is a reversible change. The other three (popping, fading, burning) are irreversible changes.

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VI. Very Short Answers

Q1. What kind of change is associated with the decaying of plants?

Decaying of plants is a slow change and a chemical change. It takes place over a long period and produces new substances.

Q2. You are given some candle wax. Can you make a candle doll from it? What kind of change is this?

Yes, you can shape candle wax into a doll by melting and moulding it. This is a physical (reversible) change because no new substance is formed, and the wax can be melted back into its original form.

Q3. Define a slow change.

Changes that take place over a long period of time — hours, days, months, or years — are called slow changes. Examples: growth of nails and hair, change of seasons, germination of seeds.

Q4. What happens when cane sugar is strongly heated? Mention any two changes in it.

When cane sugar (C₁₂H₂₂O₁₁) is strongly heated, it loses all water molecules and decomposes to form carbon (black residue). Two types of change: 1. Chemical change — new substance (carbon) is formed 2. Irreversible change — the original sugar cannot be recovered

Q5. What is a solution?

When a solute is dissolved in a solvent, it forms a solution. Solute + Solvent → Solution Example: Sugar (solute) dissolved in water (solvent) gives sugar solution.

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VII. Short Answers

Q1. What happens when paper is burnt? Explain.

When paper is burnt, it catches fire and turns into ash and smoke within seconds or minutes. This is a fast change because it takes place within a short period. It is also an irreversible change and a chemical change because new substances (ash, smoke, gases) are formed that cannot be converted back to paper.

Q2. Can deforestation be considered a desirable change? Explain.

No, deforestation is an undesirable and human-made change. It leads to:

Deforestation harms the environment and cannot be considered desirable.

• Decreasing rainfall
• Global warming
• Loss of biodiversity
• Soil erosion

Q3. What type of change is associated with the germination of a seed? Explain.

Germination of a seed is associated with slow change because the changes take place over a long period of time (days to weeks). The seed absorbs water, swells, develops a radicle and plumule, and gradually grows into a seedling. It is also a chemical change as new substances are formed during the process.

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VIII. Detailed Answers

Q1. Give one example in each case that happens around you:

• a. Slow and Fast change: Growth of nails (slow), burning of paper (fast)
• b. Reversible and Irreversible change: Melting of ice (reversible), change of milk into curd (irreversible)
• c. Physical and Chemical change: Drying of cloth (physical), rusting of iron (chemical)
• d. Natural and Human-made change: Rotation of the Earth (natural), construction of a building (human-made)
• e. Desirable and Undesirable change: Cooking of food (desirable), deforestation (undesirable)

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IX. HOTS (Higher Order Thinking Skills)

Q1. When a candle is lit the following changes are observed: a. Wax melts b. Candle keeps burning c. The size of the candle decreases d. The molten wax solidifies

Which of the changes can be reversed? Justify your answer.

Changes (a) wax melting and (d) molten wax solidifying are reversible. When wax melts on heating, it changes from solid to liquid; when it cools, it solidifies back to solid wax. These are physical changes where no new substance is formed, and the original state can be restored.

Changes (b) and (c) involve burning (chemical change) and permanent loss of material — these are irreversible.

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Additional Practice Questions

Additional MCQs

Q1. Growth of seed into sapling is \_\_\_\_\_\_\_\_ change (a) slow (b) chemical (c) reversible (d) undesirable

(a) slow — Germination takes days or weeks, making it a slow change.

Q2. The softening of vegetables by cooking is a (an) (a) Fast change (b) Chemical change (c) Artificial change (d) Reversible change

(c) Artificial change — Cooking is done by humans, making it an artificial (human-made) change.

Q3. Which of the following is not a man-made change? (a) Burning of fuels (b) Drying of clothes (c) Tearing of paper (d) Change of day and night

(d) Change of day and night — Day and night cycle is a natural change caused by the Earth's rotation.

Q4. Sublimation of Iodine is a (a) Chemical change (b) Physical change (c) Natural change (d) Undesirable change

(b) Physical change — Sublimation changes only the state (solid to gas). No new substance is formed, so it is a physical change.

Q5. Raining is a \_\_\_\_\_\_\_\_ change (a) human-made (b) undesirable (c) slow (d) natural

(d) natural — Rainfall occurs naturally through the water cycle without human intervention.

Additional Fill in the Blanks

Q1. \_\_\_\_\_\_\_\_ changes take place within a short period of time.

Fast — Fast changes are completed quickly, like bursting a balloon.

Q2. When the 'Touch me not' plant is touched, \_\_\_\_\_\_\_\_ changes occur.

reversible — The leaves fold when touched but reopen after some time, making it a reversible change.

Q3. Tearing a sheet of paper into pieces is \_\_\_\_\_\_\_\_ change.

physical — Only the shape and size change; no new substance is formed.

Q4. By the process of heating water, \_\_\_\_\_\_\_\_ is obtained.

steam — Water changes to steam (vapour) on heating.

Q5. \_\_\_\_\_\_\_\_ is the substance that is dissolved in a solvent.

Solute — The solute dissolves in the solvent. Example: salt (solute) dissolves in water (solvent).

Q6. The changes in which new substances with new chemical properties are formed are \_\_\_\_\_\_\_\_ changes.

chemical — Chemical changes always produce new substances with different properties.

Q7. Deforestation is \_\_\_\_\_\_\_\_ change.

human-made — Deforestation is caused by human activity, not by natural processes.

Additional True or False

Q1. The difference between the initial state and the final state is called change.

True — Change is defined as the difference between the initial and final states of an object or process.

Q2. Gas exists in three states as solid, liquid, and gas.

False — Water exists in three states as solid (ice), liquid (water), and gas (steam). Not all gases exist in three states.

Q3. Water turning into steam on heating is called freezing.

False — Water turning into steam on heating is called vaporisation (or evaporation). Freezing is the conversion of liquid to solid by cooling.

Q4. When a solute is dissolved in a solvent, it forms a solution.

True — Solute + Solvent = Solution. Example: Salt + Water = Salt solution.

Q5. Chemical change is a temporary change.

False — Chemical change is a permanent change. Physical change is a temporary change because the original substance can be recovered.

Q6. Natural changes that take place in nature on their own are beyond the control of human beings.

True — Natural changes like earthquakes, volcanic eruptions, and the water cycle happen without human intervention and cannot be controlled by humans.

Q7. Construction of a building is a natural change.

False — Construction of a building is a human-made change. It is carried out by people using tools and materials.

Additional Analogy

Q1. Germination of seed : slow change :: Breaking of glass : \_\_\_\_\_\_\_\_

fast change — Glass breaks in an instant, making it a fast change.

Q2. Irreversible change : Making idly from batter :: Reversible change : \_\_\_\_\_\_\_\_

Stretching of a rubber band — A stretched rubber band returns to its original shape when released, making it reversible.

Q3. Melting of ice : Physical change :: Popping of popcorn : \_\_\_\_\_\_\_\_

chemical change — Popping popcorn creates new substances (starch gelatinisation, flavour compounds), making it a chemical change.

Q4. Melting : ice into water on heating :: Freezing : \_\_\_\_\_\_\_\_

water into ice on cooling — Freezing is the conversion of liquid to solid by cooling, the reverse of melting.

Q5. Changing of phases of the moon : Natural change :: Cultivation of paddy : \_\_\_\_\_\_\_\_

human-made change — Paddy cultivation requires human effort — ploughing, sowing, irrigation, and harvesting.

Match the Following

• i → (e) Fast change — Lightning strikes in an instant.
• ii → (d) Reversible change — The leaves fold and then reopen.
• iii → (a) Physical change — Sugar dissolves without forming a new substance.
• iv → (b) Camphor — Camphor sublimes directly from solid to gas.
• v → (c) Water — Water dissolves a wide range of substances, earning it the name "universal solvent."

Additional Short Answers

Q1. Mention any three examples of fast change.

1. Bursting of a balloon 2. Breaking of glass 3. Burning of paper

Q2. What is a reversible change? Give an example.

Changes that can be reversed to get back the original state are called reversible changes. Example: Melting of ice — ice melts to water on heating, and water freezes back to ice on cooling.

Q3. Give some examples of physical change.

1. Melting of ice 2. Dissolving of salt or sugar in water 3. Stretching of a rubber band 4. Tearing of paper (only shape changes)

Q4. What is freezing? Give an example.

The conversion of a liquid into a solid by cooling is called freezing. Example: Water turning into ice cubes when kept in a freezer.

Q5. Define sublimation.

The change of state from solid directly to gas without passing through the liquid state is called sublimation. Example: Camphor and iodine sublime on heating.

Q6. Why is water called the universal solvent?

Water dissolves a wide range of substances — more than any other solvent. Because of this unique ability, it is called the universal solvent.

Q7. What are natural changes? Give examples.

Changes that take place in nature on their own, without human intervention, and are beyond the control of human beings are called natural changes. Examples: Rotation of the Earth, rainfall, change of seasons.

Q8. What are artificial (human-made) changes? Give examples.

Changes brought about by human beings are called human-made or artificial changes. Examples: Construction of bridges, road building, cooking of food.

Additional Detailed Answer

Q1. Tabulate the differences between physical and chemical changes.

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Common Mistakes

Students often make these errors in exams. Make sure you avoid them:

1. Confusing reversible with physical and irreversible with chemical — Not all physical changes are reversible (e.g., tearing paper is physical but irreversible). Not all chemical changes are irreversible in principle, but in Class 6, treat chemical changes as irreversible.

2. Calling digestion a physical change — Digestion is a chemical change because food is broken down into entirely new substances by enzymes and acids.

3. Saying "water is the solute" in a salt solution — Water is always the solvent in aqueous solutions. The substance that dissolves (salt, sugar) is the solute.

4. Mixing up natural and human-made changes — Raining, earthquakes, and day-night cycles are natural. Cooking, construction, and deforestation are human-made.

5. Calling burning a reversible change — Burning is always irreversible. You cannot get the original substance back after it has been burnt to ash and smoke.

6. Confusing slow change with chemical change — Slow and fast refer to the duration of a change. Physical and chemical refer to whether a new substance is formed. A slow change can be physical (drying of clothes) or chemical (rusting of iron).

7. Treating all undesirable changes as human-made — Some natural changes are also undesirable, such as earthquakes and volcanic eruptions. Desirability and origin (natural vs. human-made) are separate classifications.

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Exam Checklist

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• [ ] I can define and give examples of reversible and irreversible changes
• [ ] I can distinguish physical changes from chemical changes with at least 3 examples each
• [ ] I can classify changes as slow or fast with examples
• [ ] I can identify natural vs. human-made changes
• [ ] I can classify changes as desirable or undesirable
• [ ] I know the definitions of solute, solvent, and solution
• [ ] I can define sublimation and give examples (camphor, iodine)
• [ ] I can define freezing and vaporisation
• [ ] I can explain why water is called the universal solvent
• [ ] I can tabulate differences between physical and chemical changes
• [ ] I can identify the odd one out with proper reasoning
• [ ] I can complete analogy questions using correct change classifications
• [ ] I can explain the candle experiment (HOTS question) — reversible melting vs. irreversible burning