CHANGES AROUND US – Class 7 - Science
SUMMARY
1. Introduction
Changes are happening all around us – burning, rusting, melting, dissolving, etc.
These changes are of two main types:
Physical changes
Chemical changes
2. Physical Changes
A change in which no new substance is formed.
Only the physical state, size, or shape may change.
Reversible (can be undone).
Examples:
Melting of ice → water → steam
Breaking of glass
Dissolving sugar or salt in water
Stretching a rubber band
3. Chemical Changes
A change in which new substances with different properties are formed.
Usually irreversible.
Heat, light, sound or gas may be produced.
Examples:
Burning of wood, paper, or candle
Rusting of iron
Cooking food
Digestion of food
Souring of milk
4. Characteristics of Physical Changes
No new substance formed
Temporary and reversible
Only physical properties (state, size, shape, colour) change
5. Characteristics of Chemical Changes
New substance formed
Permanent and irreversible
Energy may be absorbed or released
Properties of products differ from reactants
6. Examples in Daily Life
Physical: melting ice-cream, boiling water, cutting fruits
Chemical: burning crackers, rusting, fermentation, cooking rice
In short:
Physical change = No new substance, reversible
Chemical change = New substance, irreversible
Short Q&A
What is a physical change?
→ A change in which no new substance is formed, only shape, size, or state changes.
Give two examples of physical changes.
→ Melting of ice, dissolving salt in water.
What is a chemical change?
→ A change in which a new substance is formed with different properties.
Give two examples of chemical changes.
→ Burning of wood, rusting of iron.
Is melting of ice reversible or irreversible? Why?
→ Reversible, because water can be frozen again into ice.
Is burning of paper a physical or chemical change? Why?
→ Chemical change, because ash and gases are formed (new substances).
What happens to iron when left in moist air?
→ It undergoes rusting (chemical change).
Which type of change is dissolving sugar in water?
→ Physical change (no new substance formed).
What happens when milk turns sour?
→ It is a chemical change, as a new substance with different taste is formed.
What is rusting?
→ Slow chemical change where iron reacts with oxygen and moisture to form brown flaky substance (rust).
Mention one characteristic of a physical change.
→ It is usually reversible.
Mention one characteristic of a chemical change.
→ It is usually irreversible.
Does the mass change in a physical change?
→ No, mass remains the same.
Does the composition of substances change in chemical change?
→ Yes, new substances are formed with different composition.
What type of change is boiling of water?
→ Physical change.
What type of change is cooking of rice?
→ Chemical change.
Why is stretching a rubber band a physical change?
→ Because no new substance is formed, only shape changes.
Why is burning a candle both physical and chemical change?
→ Melting wax = physical change; Burning wick = chemical change.
Why is rusting harmful?
→ It weakens iron and damages structures.
Which gas is released during burning?
→ Carbon dioxide.
Why is evaporation a physical change?
→ Because water vapour can be condensed back into liquid water.
Is photosynthesis a chemical change? Why?
→ Yes, because new substances (glucose and oxygen) are formed.
Is cutting of wood a physical or chemical change?
→ Physical change, only size changes.
Why is respiration a chemical change?
→ Because glucose is broken down to release energy, carbon dioxide, and water (new products).
Why is curdling of milk a chemical change?
→ Because milk proteins change permanently to form curd.
Why is freezing of water a physical change?
→ Because only the state changes, no new substance is formed.
Give two examples of changes that are both physical and chemical.
→ Lighting a candle (wax melts + wick burns), rusting of iron (colour changes + new substance forms).
Why is frying an egg a chemical change?
→ Because proteins are permanently changed into new substances.
What type of change is grinding of wheat?
→ Physical change.
What type of change is fermentation of sugar?
→ Chemical change.
How can rusting be prevented?
→ By painting, oiling, greasing, or galvanizing iron.
Why is burning of coal considered a chemical change?
→ Because it produces carbon dioxide, heat, and ash (new substances).
Why is sharpening of a pencil a physical change?
→ Because only shape and size change, no new substance forms.
Why is digestion of food a chemical change?
→ Because food is broken into simpler substances with different properties.
Is condensation of steam a physical or chemical change? Why?
→ Physical change, as water vapour changes back to liquid water.
Why is rusting faster in coastal areas?
→ Due to more moisture and presence of salt in air.
Why is heating of iron rod till red hot a physical change?
→ Because only colour changes due to heat, no new substance is formed.
What type of change is spoiling of food?
→ Chemical change.
Give two examples of slow chemical changes.
→ Rusting of iron, fermentation of sugar.
Give two examples of fast chemical changes.
→ Burning of crackers, burning of paper.
LONG QUESTION AND ANSWER
1. Define physical change. Explain with five examples.
Answer:
A physical change is a change in which no new substance is formed.
Only the state, shape, size, or form of matter changes.
It is usually reversible.
Examples:
Melting of ice → water.
Boiling of water → steam.
Dissolving salt in water.
Cutting paper.
Stretching rubber.
2. Define chemical change. Explain with five examples.
Answer:
A chemical change is a change in which new substances are formed with new properties.
It is usually irreversible.
Examples:
Burning of wood.
Rusting of iron.
Cooking of food.
Souring of milk.
Digestion of food.
3. Write the differences between physical and chemical changes.
Answer:
Physical Change Chemical Change
No new substance formed New substance formed
Usually reversible Irreversible
Only shape, size, or state changes Properties change
Temporary Permanent
Examples: melting ice, boiling water Examples: rusting, burning
4. Explain rusting of iron with conditions required.
Answer:
Rusting is a slow chemical change where iron reacts with oxygen and moisture to form reddish-brown flaky substance called rust.
Conditions:
Presence of oxygen (air).
Presence of water (moisture).
Formula: Iron + Oxygen + Water → Hydrated Iron Oxide (rust).
5. What are the methods of preventing rusting?
Answer:
Painting – covers iron surface.
Oiling/Greasing – prevents contact with air and water.
Galvanizing – coating with zinc.
Alloying – mixing iron with chromium and nickel to form stainless steel.
Plastic coating – prevents exposure to air and moisture.
6. Why is burning of candle both physical and chemical change? Explain.
Answer:
Physical change: Wax melts to liquid; no new substance is formed.
Chemical change: Wax burns to form carbon dioxide, water vapour, and heat.
Hence, candle burning involves both physical and chemical changes.
7. Describe five examples of physical changes in daily life.
Answer:
Freezing of water.
Melting butter.
Breaking glass.
Cutting fruits.
Dissolving sugar in tea.
8. Describe five examples of chemical changes in daily life.
Answer:
Digestion of food.
Cooking rice.
Souring of milk.
Burning crackers.
Rusting of iron.
9. Why is burning of paper a chemical change but tearing paper a physical change?
Answer:
Tearing paper only changes shape and size → physical change.
Burning produces ash and gases (new substances) → chemical change.
10. Explain how curd is formed from milk. What type of change is it?
Answer:
Curd is formed when lactobacillus bacteria ferment lactose sugar of milk into lactic acid.
It changes taste and texture permanently.
It is a chemical change.
11. Explain why rusting is faster in coastal areas.
Answer:
Coastal areas have more moisture and salt in the air.
Moisture accelerates oxidation, while salt acts as an electrolyte, increasing the speed of rusting.
12. What are slow and fast changes? Give examples.
Answer:
Slow changes: Take place gradually over time.
Examples: rusting of iron, germination of seeds.
Fast changes: Occur in a short span of time.
Examples: burning paper, explosion of crackers.
13. Explain the role of chemical changes in our daily life.
Answer:
Chemical changes are essential in:
Digestion of food.
Respiration for energy.
Cooking food.
Rusting and corrosion of metals.
Burning fuels for transport and electricity.
14. Is boiling of egg a physical or chemical change? Explain.
Answer:
Boiling an egg changes the structure of proteins permanently.
A new substance is formed which cannot be reversed.
Hence, it is a chemical change.
15. Explain respiration. Why is it a chemical change?
Answer:
In respiration, glucose reacts with oxygen to form carbon dioxide, water, and energy.
Since new substances are formed, respiration is a chemical change.
16. Explain photosynthesis as a chemical change.
Answer:
In photosynthesis, plants prepare food by combining carbon dioxide and water in the presence of sunlight and chlorophyll to form glucose and oxygen.
New substances are formed → chemical change.
17. Describe an activity to show that burning of magnesium ribbon is a chemical change.
Answer:
Clean a magnesium ribbon and burn it.
It burns with a dazzling white flame and forms white ash (magnesium oxide).
Since a new substance is formed, it is a chemical change.
18. Differentiate between reversible and irreversible changes with examples.
Answer:
Reversible: Can be undone. Example: melting ice, dissolving sugar.
Irreversible: Cannot be undone. Example: burning paper, cooking rice.
19. Why is spoilage of food a chemical change?
Answer:
Microorganisms act on food and produce harmful substances with bad smell and taste.
New substances are formed → irreversible → chemical change.
20. Why is evaporation of water considered a physical change?
Answer:
Because water vapour can be condensed back into water.
No new substance is formed.
21. Explain with examples: changes that are both physical and chemical.
Answer:
Burning candle: Melting wax (physical) + burning wick (chemical).
Rusting: Colour change (physical) + new substance (chemical).
22. Why is cutting of vegetables a physical change but cooking them a chemical change?
Answer:
Cutting only changes size and shape → physical.
Cooking produces new substances with different taste and smell → chemical.
23. Describe how chemical changes help in industry.
Answer:
Production of fertilizers.
Manufacturing of cement.
Production of medicines.
Extraction of metals.
Fuel burning in factories.
24. Explain with an activity that dissolving salt in water is a physical change.
Answer:
Add salt to water; it dissolves.
Evaporate water; salt is recovered.
No new substance formed → physical change.
25. Is respiration a slow or fast chemical change? Explain.
Answer:
Respiration is a fast chemical change as glucose breaks down quickly with oxygen to release energy, carbon dioxide, and water.
26. Why is corrosion of metals considered a harmful chemical change?
Answer:
Because it damages metals, weakens structures, causes loss of money and safety risks (like bridges, ships).
27. How is physical change useful in daily life?
Answer:
Cutting wood for furniture.
Melting ice for cooling.
Stretching rubber bands for packing.
Boiling water for sterilizing.
28. How is chemical change useful in daily life?
Answer:
Respiration gives energy.
Cooking makes food edible.
Burning fuels provides energy.
Fermentation produces alcohol and medicines.
29. Why is burning of coal a chemical change?
Answer:
Coal reacts with oxygen to produce carbon dioxide, water, and ash.
New substances are formed → chemical change.
30. Explain how formation of clouds is a physical change.
Answer:
Water vapour condenses into tiny droplets.
No new substance is formed.
Hence, it is a physical change.
31. What is fermentation? Why is it a chemical change?
Answer:
Fermentation is conversion of sugar into alcohol and carbon dioxide by yeast.
New substances are formed → chemical change.
32. What type of change is germination of seeds? Why?
Answer:
Germination produces new plant with new properties.
Irreversible change → chemical change.
33. Why is respiration essential though it is a chemical change?
Answer:
Respiration releases energy required for life activities like growth, movement, and repair of cells.
34. Explain with an activity that heating of iron is a physical change.
Answer:
Heat an iron rod; it becomes red hot.
On cooling, it returns to normal state.
No new substance formed → physical change.
35. Why is cracking of crackers a chemical change?
Answer:
Crackers release heat, light, sound, and gases.
New substances are formed.
Hence, it is a chemical change.
MCQS
1. Which of the following is a physical change?
a) Burning of coal
b) Rusting of iron
c) Melting of ice
d) Souring of milk
✅ Answer: c) Melting of ice
2. Which of the following is a chemical change?
a) Evaporation of water
b) Condensation of steam
c) Burning of wood
d) Melting of butter
✅ Answer: c) Burning of wood
3. Rusting of iron requires
a) Oxygen only
b) Water only
c) Oxygen and water both
d) Sunlight only
✅ Answer: c) Oxygen and water both
4. Formation of clouds is a
a) Chemical change
b) Physical change
c) Both
d) None
✅ Answer: b) Physical change
5. Which is an irreversible change?
a) Freezing of water
b) Cutting paper
c) Cooking food
d) Melting wax
✅ Answer: c) Cooking food
6. Which of the following is NOT a physical change?
a) Boiling of water
b) Tearing paper
c) Burning paper
d) Melting ice
✅ Answer: c) Burning paper
7. Which of the following is a fast change?
a) Rusting
b) Burning of crackers
c) Germination of seed
d) Ripening of fruit
✅ Answer: b) Burning of crackers
8. Fermentation of sugar is a
a) Physical change
b) Chemical change
c) Reversible change
d) None
✅ Answer: b) Chemical change
9. Respiration is a
a) Physical change
b) Chemical change
c) Reversible change
d) None
✅ Answer: b) Chemical change
10. Which of the following is both a physical and chemical change?
a) Dissolving sugar in water
b) Melting of ice
c) Burning of candle
d) Rusting
✅ Answer: c) Burning of candle
11. Souring of milk is a
a) Physical change
b) Chemical change
c) Reversible change
d) Temporary change
✅ Answer: b) Chemical change
12. Which of the following is a reversible change?
a) Boiling water
b) Burning wood
c) Rusting iron
d) Cooking rice
✅ Answer: a) Boiling water
13. The chemical change in which iron reacts with oxygen and water is called
a) Burning
b) Evaporation
c) Rusting
d) Condensation
✅ Answer: c) Rusting
14. Which change releases energy in the form of heat and light?
a) Melting ice
b) Burning fuel
c) Dissolving sugar
d) Boiling water
✅ Answer: b) Burning fuel
15. Digestion of food is a
a) Physical change
b) Chemical change
c) Reversible change
d) None
✅ Answer: b) Chemical change
16. Which of the following is NOT a chemical change?
a) Respiration
b) Rusting
c) Photosynthesis
d) Melting butter
✅ Answer: d) Melting butter
17. The change in which no new substance is formed is called
a) Physical change
b) Chemical change
c) Irreversible change
d) None
✅ Answer: a) Physical change
18. Which of the following is a slow change?
a) Explosion of cracker
b) Growth of a tree
c) Burning candle
d) Melting ice
✅ Answer: b) Growth of a tree
19. Which of the following is a temporary change?
a) Rusting
b) Burning
c) Melting wax
d) Cooking
✅ Answer: c) Melting wax
20. What type of change is the germination of seeds?
a) Physical
b) Chemical
c) Reversible
d) None
✅ Answer: b) Chemical
21. Heating of iron rod till it is red hot is
a) Physical change
b) Chemical change
c) Both
d) None
✅ Answer: a) Physical change
22. Which is a permanent change?
a) Melting ice
b) Freezing water
c) Burning wood
d) Evaporation of water
✅ Answer: c) Burning wood
23. Formation of curd from milk is caused by
a) Air
b) Heat
c) Bacteria
d) Water
✅ Answer: c) Bacteria
24. Which of these shows chemical change?
a) Breaking chalk
b) Dissolving sugar
c) Burning candle wick
d) Cutting cloth
✅ Answer: c) Burning candle wick
25. The gas released during respiration is
a) Oxygen
b) Hydrogen
c) Nitrogen
d) Carbon dioxide
✅ Answer: d) Carbon dioxide
26. Which is NOT required for rusting?
a) Iron
b) Water
c) Oxygen
d) Sunlight
✅ Answer: d) Sunlight
27. Which process is reversible?
a) Cooking
b) Melting
c) Rusting
d) Burning
✅ Answer: b) Melting
28. Crystallization of sugar is a
a) Physical change
b) Chemical change
c) Irreversible change
d) None
✅ Answer: a) Physical change
29. Which change is useful to our body?
a) Rusting
b) Respiration
c) Burning wood
d) Spoiling food
✅ Answer: b) Respiration
30. Which is an irreversible change?
a) Stretching rubber band
b) Breaking glass
c) Cutting vegetables
d) Cooking rice
✅ Answer: d) Cooking rice
31. Which is a natural chemical change?
a) Dissolving salt
b) Freezing water
c) Photosynthesis
d) Evaporation
✅ Answer: c) Photosynthesis
32. Why is cutting of paper a physical change?
a) Because shape changes only
b) Because new substance is formed
c) Because heat is produced
d) None
✅ Answer: a) Because shape changes only
33. Which of the following is a harmful chemical change?
a) Cooking food
b) Respiration
c) Rusting
d) Photosynthesis
✅ Answer: c) Rusting
34. Which is a desirable chemical change?
a) Spoiling food
b) Rusting
c) Digestion
d) Burning of forest
✅ Answer: c) Digestion
35. What type of change is the growth of hair and nails?
a) Physical change
b) Chemical change
c) Both
d) None
✅ Answer: b) Chemical change
36. The process of conversion of sugar into alcohol is
a) Evaporation
b) Fermentation
c) Rusting
d) Respiration
✅ Answer: b) Fermentation
37. Which of the following changes involves both heat and light production?
a) Melting ice
b) Burning candle
c) Evaporation
d) Cutting paper
✅ Answer: b) Burning candle
38. Why is melting of ice reversible?
a) Because energy is released
b) Because water can freeze again
c) Because new substance is formed
d) Because oxygen is used
✅ Answer: b) Because water can freeze again
39. Which is NOT a characteristic of chemical change?
a) New substance is formed
b) Irreversible
c) Heat/light may be produced
d) No new substance formed
✅ Answer: d) No new substance formed
40. Which change is used in refrigerator cooling?
a) Evaporation
b) Burning
c) Rusting
d) Cooking
✅ Answer: a) Evaporation
