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Changes Around Us: Physical and Chemical

NCERT Class 7 · Science Based on NCERT Class 7 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the different types of changes that occur around us every day. The one big idea is that changes can be classified as physical (where no new substance is formed) or chemical (where one or more new substances are formed). A student must understand that physical changes are often reversible and only affect appearance, while chemical changes are usually irreversible and produce new materials with different properties.
Key Concepts (12)
Physical change
A change in which only physical properties like shape, size, and state of a substance change, and no new substance is formed.
e.g. Melting of ice cubes into water; the water can be refrozen into ice, so no new substance is formed.
Chemical change
A change in which one or more new substances are formed through a process called a chemical reaction.
e.g. Blowing exhaled air into lime water turns it milky because a new white substance (calcium carbonate) is formed.
Chemical reaction
A process in which new substances are formed. It can be represented by a chemical equation.
e.g. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water.
Combustion
A chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
e.g. Burning of magnesium ribbon produces magnesium oxide, heat, and light.
Combustible substance
A substance that can undergo combustion (burn) in the presence of oxygen.
e.g. Wood, paper, cotton, kerosene, and wax are combustible substances.
Ignition temperature
The minimum temperature at which a substance catches fire and starts burning.
e.g. Paper catches fire quickly when a lighted matchstick is brought near it because the matchstick's temperature is above paper's ignition temperature.
Rusting
A chemical change in which iron reacts with oxygen and moisture to form a new brown-coloured substance called rust (iron oxide).
e.g. Iron nails left in the open develop a brown coating of rust.
Weathering
The process of physical and chemical changes in rocks that break them down into smaller pieces, eventually forming soil.
e.g. Temperature changes, growing tree roots, and freezing water in cracks cause rocks to break. Water reacting with iron in rocks forms red-coloured iron oxide layers.
Erosion
The process by which rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water.
e.g. Fine sand collecting on riverbeds or in lakes is formed by erosion. River rocks become smoother due to constant erosion by flowing water.
Bioluminescence
The production of light (without heat) in living organisms through a chemical change.
e.g. Fireflies emit light in gardens or fields in late evenings.
Desirable change
A change that is useful and beneficial in daily life.
e.g. Changing milk into curd, ripening of fruits, cooking of food.
Undesirable change
A change that is harmful or not wanted.
e.g. Rusting of iron, decay of food during storage.
How It Works
Testing for carbon dioxide using lime water
  1. Take freshly prepared lime water in a test tube or glass tumbler.
  2. Blow exhaled air into the lime water using a straw, or pass the gas from a reaction (e.g., vinegar + baking soda) through it.
  3. Observe the lime water turning milky or cloudy.
  4. A white substance (calcium carbonate) settles at the bottom if left for some time.
💡 Lime water turns milky only in the presence of carbon dioxide. This is a chemical change because a new substance (calcium carbonate) is formed.
Combustion of a candle
  1. Light the candle. The heat of the flame melts the wax near the wick.
  2. The melted wax is drawn up the wick by capillary action.
  3. The liquid wax evaporates due to the heat.
  4. The wax vapour burns in the presence of oxygen, producing a flame, carbon dioxide, water vapour, heat, and light.
💡 Burning of a candle involves both physical changes (melting, evaporation, solidification of wax) and a chemical change (burning of wax vapour). Students often forget that melting is physical and burning is chemical.
Requirements for combustion (Fire triangle)
  1. A combustible substance (fuel) must be present.
  2. Oxygen (from air) must be available to support burning.
  3. The fuel must be heated to its ignition temperature.
💡 All three are necessary. Removing any one (e.g., covering a candle with a glass cuts off oxygen) extinguishes the fire. A blanket cuts off air supply to put out a fire on a person's clothes.
Weathering of rocks
  1. Physical changes: Temperature changes, growing tree roots, and freezing of water in cracks break large rocks into smaller pieces.
  2. Chemical changes: Water or chemicals in water react with minerals in rocks, changing their composition (e.g., iron in basalt reacts to form red iron oxide).
  3. Over time, these processes form sediments and eventually soil.
💡 Weathering includes both physical and chemical changes. The red colour in rocks is due to iron oxide (rust) formed by a chemical change.
Erosion by wind and water
  1. Natural forces like wind and flowing water break down rock pebbles, soil, and sediments.
  2. These materials are transported from one location to another.
  3. When the speed of water or wind decreases (e.g., in a lake or ocean), the transported material settles down at the bottom.
  4. Over thousands of years, these sediments harden to form new rocks.
💡 Erosion is primarily a physical change. It is a slow, irreversible process.
Exam Traps — Don't Make These Mistakes
⚠ Confusing melting of wax with burning of wax
✗ Burning of a candle is a chemical change because the wax melts.
→ Melting of wax is a physical change — only the state changes from solid to liquid, no new substance is formed. The chemical change is the burning of wax vapour, which produces new substances like carbon dioxide and water.
✓ Burning of a candle involves both physical and chemical changes. Melting of wax is a physical change, while the burning of wax vapour is a chemical change.
⚠ Thinking all changes that produce bubbles are chemical
✗ Boiling water produces bubbles, so it is a chemical change.
→ Boiling water produces bubbles of water vapour (gas), but no new substance is formed. The water can be condensed back to liquid. It is a physical change.
✓ Boiling water is a physical change because it only changes the state of water from liquid to gas, and no new substance is formed.
⚠ Mistaking reversible for physical and irreversible for chemical
✗ All reversible changes are physical and all irreversible changes are chemical.
→ While many physical changes are reversible (e.g., melting of ice), some physical changes are irreversible (e.g., chopping vegetables, tearing paper). Similarly, most chemical changes are irreversible, but some can be reversed by another chemical reaction.
✓ Physical changes may or may not be reversible. Chemical changes are usually irreversible. Reversibility is not the sole criterion to distinguish them.
⚠ Ignoring the need for oxygen in combustion
✗ A candle stops burning when covered because there is no air.
→ It is not just 'air' — the specific component of air that supports combustion is oxygen. The candle goes out because the oxygen inside the glass is used up and not replenished.
✓ A candle covered with a glass tumbler stops burning because the oxygen inside the glass is consumed, and no fresh oxygen can enter.
⚠ Confusing desirable and undesirable changes
✗ Rusting of iron is always undesirable.
→ While rusting is generally undesirable (damages iron objects), a change that is undesirable in one situation may be desirable in another. For example, decomposition of food is undesirable for storage but desirable for making compost.
✓ Rusting of iron is usually an undesirable change, but some changes (like decomposition) can be desirable in certain contexts (e.g., composting).
Compare & Contrast
Formation of new substances
Physical change
Chemical change
In a physical change, no new substance is formed; only the appearance (shape, size, state) changes. In a chemical change, one or more new substances with different properties are formed.
Reversibility
Physical change
Chemical change
Physical changes are often reversible (e.g., melting of ice can be reversed by freezing). Chemical changes are usually irreversible (e.g., rusting of iron cannot be reversed to get back the original iron).
Speed and heat/light production
Combustion
Rusting
Combustion is a fast chemical change that produces heat and/or light. Rusting is a slow chemical change that does not produce noticeable heat or light.
Movement of material
Weathering
Erosion
Weathering is the breaking down of rocks into smaller pieces (physical and chemical changes) without necessarily moving them. Erosion involves the transportation of broken materials from one place to another by wind or water.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks)
Most asked: Distinguishing between physical and chemical changes with examples · Combustion and the fire triangle (requirements for burning) · Rusting of iron as a chemical change · Burning of a candle as an example of both physical and chemical changes · Testing for carbon dioxide using lime water
Don't skip: Give two examples each of physical and chemical changes and explain why they are classified as such.; Explain the burning of a candle. Which changes are physical and which are chemical?; What is combustion? What are the three requirements for combustion? Explain with an activity.
One-Line Revision (read aloud before the exam)
  1. A physical change is a change in which no new substance is formed; only shape, size, or state changes.
  2. A chemical change is a change in which one or more new substances are formed through a chemical reaction.
  3. Melting of ice, boiling of water, and cutting of paper are examples of physical changes.
  4. Rusting of iron, burning of wood, and curdling of milk are examples of chemical changes.
  5. Combustion is a chemical reaction in which a substance reacts with oxygen to produce heat and/or light.
  6. The three requirements for combustion are a combustible substance, oxygen, and ignition temperature.
  7. Ignition temperature is the minimum temperature at which a substance catches fire.
  8. Lime water turns milky in the presence of carbon dioxide, indicating a chemical change.
  9. Burning of a candle involves both physical changes (melting of wax) and a chemical change (burning of wax vapour).
  10. Rusting of iron is a chemical change that forms a new brown substance called iron oxide.
  11. Weathering is the breaking down of rocks by physical and chemical changes to form soil.
  12. Erosion is the movement of broken rock and soil by wind or water.
  13. Some changes are desirable (e.g., cooking food) and some are undesirable (e.g., rusting of iron).
  14. A change that is undesirable in one situation may be desirable in another (e.g., decomposition for compost).
  15. Physical changes are often reversible, but not always (e.g., chopping vegetables is irreversible).
  16. Chemical changes are usually irreversible.

MCQs — 10 Questions with Answers

Q1. Which of the following is a physical change?

  • A. Melting of ice cube into water ✓
  • B. Burning of paper
  • C. Rusting of iron nails
  • D. Ripening of banana

Answer: A — Melting ice is a physical change because ice remains water (same substance) just in a different state; no new substance forms.

Q2. What colour does lime water turn when carbon dioxide is passed through it?

  • A. Yellow
  • B. Red
  • C. Milky white or cloudy ✓
  • D. Green

Answer: C — Carbon dioxide reacts with lime water to form calcium carbonate, which is insoluble and makes the liquid appear milky.

Q3. Which of the following is a combustible substance?

  • A. Iron
  • B. Water
  • C. Wood ✓
  • D. Sand

Answer: C — Wood catches fire and burns in the presence of oxygen, making it a combustible substance; iron, water, and sand do not burn easily.

Q4. What happens when you fold a piece of paper and then unfold it?

  • A. A new substance is formed
  • B. The paper becomes smaller permanently
  • C. You get the same paper back ✓
  • D. The paper turns into a different material

Answer: C — Folding paper is a physical change where only the shape changes; unfolding returns it to the original form with no new substance created.

Q5. In your kitchen, you mix vinegar and baking soda. Bubbles form and the mixture fizzes. What gas is being produced?

  • A. Oxygen
  • B. Carbon dioxide ✓
  • C. Nitrogen
  • D. Hydrogen

Answer: B — Vinegar and baking soda react chemically to produce carbon dioxide gas, which can be confirmed by the lime water test turning milky.

Q6. Why does a candle stop burning when covered with a glass tumbler?

  • A. The temperature decreases
  • B. The supply of oxygen gets cut off ✓
  • C. The wax melts
  • D. The wick becomes wet

Answer: B — Combustion requires oxygen; covering the candle removes the continuous supply of air, causing the flame to extinguish.

Q7. A banana you bought yesterday has developed more brown spots today and smells stronger. Is this a physical or chemical change?

  • A. Physical change because the banana is still a banana
  • B. Chemical change because new substances with different properties are formed ✓
  • C. No change is occurring
  • D. Both physical and chemical change at the same time

Answer: B — Banana ripening involves chemical changes where new flavour compounds and pigments form; it is not just a size or appearance change.

Q8. When magnesium ribbon burns in air, which of the following is true?

  • A. Magnesium changes its shape but remains magnesium
  • B. A new white substance (magnesium oxide) is formed along with heat and light ✓
  • C. Only heat is produced, no new substance forms
  • D. The magnesium disappears completely

Answer: B — Burning magnesium is a chemical change where magnesium reacts with oxygen to form a new substance, magnesium oxide, with heat and light.

Q9. For a fire to continue burning, which component of air is essential?

  • A. Nitrogen
  • B. Carbon dioxide
  • C. Oxygen ✓
  • D. Argon

Answer: C — Oxygen is the component of air that supports combustion; without it, fire cannot continue and will get extinguished.

Q10. A piece of chalk is crushed into powder. Can you recover the original chalk piece from this powder?

  • A. Yes, by adding water to the powder
  • B. Yes, by applying heat to the powder
  • C. No, because a new substance has been formed
  • D. No, because only the size and shape have changed, but the substance is still chalk ✓

Answer: D — Crushing chalk is a physical change; the chalk remains chalk whether it is one piece or powder, so it cannot be recovered as a piece but the substance is unchanged.

Flashcards

What is a physical change?

A change in which only the appearance (shape, size, or state) changes but the substance remains the same and no new substance forms.

What is a chemical change?

A change in which one or more new substances are formed through a chemical reaction, and the original substance no longer exists in the same form.

Name the three things needed for combustion.

Combustible substance (fuel), oxygen, and heat (at or above the ignition temperature).

What is ignition temperature?

The minimum temperature at which a combustible substance catches fire and starts burning.

What is the test for carbon dioxide gas?

Pass the gas through freshly prepared lime water; if it turns milky or cloudy, carbon dioxide is present.

Give one example of a physical change.

Melting of ice, boiling of water, folding paper, or crushing chalk into powder.

Give one example of a chemical change.

Burning of wood, rusting of iron nails, ripening of banana, or burning of magnesium ribbon.

What happens when a burning candle is covered with a glass tumbler?

The flame stops burning after some time because the supply of oxygen (air) is cut off.

What new substance forms when magnesium ribbon burns?

Magnesium oxide, which is a white powder.

What is combustion?

A chemical reaction in which a substance reacts with oxygen and produces heat and/or light.

Important Board Questions

Define physical change with one example. [1 mark]

Physical change = only appearance changes, no new substance. Example: melting ice, folding paper, or crushing chalk.

When vinegar and baking soda are mixed, a gas is produced. How would you test this gas to confirm it is carbon dioxide? [2 marks]

Pass the gas through lime water. If lime water turns milky or cloudy, carbon dioxide is present. This is the lime water test.

Explain with an example why oxygen is necessary for combustion. What happens when a burning candle is covered with a glass tumbler? [3 marks]

Activity 5.5: candle without cover burns, with cover stops burning. Reason: oxygen is cut off. Example: fire extinguishers work by removing oxygen supply.

What is ignition temperature? List the three requirements for combustion to occur and draw a labelled diagram of the fire triangle showing all three components. [5 marks]

Ignition temperature = minimum temperature to start burning. Three requirements: combustible substance (fuel), oxygen, heat. Fire triangle diagram must show three sides labelled: Fuel, Oxygen, Heat.

Assertion-Reason Questions for Changes Around Us: Physical and Chemical

Q1. Assertion (A):

Melting of ice is a physical change.

Reason (R):

During melting, ice changes its state from solid to liquid, but the substance remains water with no new substance formed.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why melting is a physical change. The key point is that water's identity remains unchanged whether it's solid ice or liquid water.

Q2. Assertion (A):

Rusting of iron is a chemical change.

Reason (R):

Rusting produces a new brown substance called rust (iron oxide) that has different properties from the original iron.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains the chemical change. The formation of a new substance (iron oxide) with completely different properties proves this is a chemical change.

Q3. Assertion (A):

Combustion always requires oxygen to occur.

Reason (R):

When a burning candle is covered with a glass tumbler, the flame eventually goes out because oxygen inside the tumbler gets depleted.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason directly supports the assertion. The candle flame extinguishing when oxygen is unavailable proves oxygen is essential for combustion.

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Case-Based Questions (CBQ) for Changes Around Us: Physical and Chemical

Case Study:

A science teacher set up an experiment in the school laboratory. She took three transparent glass bottles labeled X, Y, and Z. In bottle X, she mixed vinegar and baking soda and immediately covered it. In bottle Y, she heated water until it boiled and observed the steam. In bottle Z, she placed iron nails in a beaker containing water mixed with salt to accelerate the process. After one week, she observed that bottle X had a residue at the bottom, bottle Y had condensation on the glass, and bottle Z had brown rust on the nails.

Q1. What type of change occurred in bottle X when vinegar and baking soda were mixed?

  • A. Physical change because a gas was produced
  • B. Chemical change because a new substance was formed ✓
  • C. Combustion because bubbles were produced
  • D. Rusting because iron particles were present

Answer: B

Q2. Which observation from bottle Y indicates a physical change?

  • A. The water completely disappeared
  • B. Steam condensed back into water on cooling, showing the substance remained water ✓
  • C. Heat was required to produce the change
  • D. The water turned into a completely new substance

Answer: B

Q3. Why did the teacher add salt to the water in bottle Z to accelerate rusting?

  • A. Salt provides oxygen needed for combustion
  • B. Salt is a fuel that helps iron burn
  • C. Salt increases the rate of chemical reaction between iron and oxygen ✓
  • D. Salt converts iron into a new element

Answer: C

Q4. If the teacher wanted to test whether the gas produced in bottle X was carbon dioxide, which method would be most appropriate?

  • A. Heat the gas and observe if it burns
  • B. Dissolve the gas in water and check the pH
  • C. Cool the gas and see if it condenses
  • D. Pass the gas through lime water and observe if it turns milky ✓

Answer: D

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