Class 7 Science Chapter 5: Changes Around Us Physical and Chemical | Complete NCERT Notes, Activities, 25+ Questions & Answers

Full Chapter Summary, Physical vs Chemical Changes, Reversible/Irreversible, All Activities 5.1-5.8 Solved Step-by-Step, NCERT "Let Us Enhance Our Learning" Questions, 25+ Extra Practice Questions & Answers, Fire Triangle, Rusting, Weathering, Revision Tables, Mnemonics for CBSE Board Exams

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Class 7 Science Chapter 5: Changes Around Us Physical and Chemical | Complete NCERT Notes, Activities, Questions & Answers 2025

Full Chapter Summary & Detailed Notes - Changes Around Us: Physical and Chemical

Overview & Chapter Goal

To understand the difference between physical and chemical changes through observations and experiments.
To learn about reversible and irreversible changes, combustion, rusting, weathering, and erosion.
To explore how scientific investigation is done with activities and chemical equations.

Physical Changes

Changes that affect the appearance or state of a substance but do not form new substances. Examples include melting ice, boiling water, tearing paper.

Chemical Changes

Changes in which one or more new substances are formed through chemical reactions. Examples include rusting, combustion, baking soda and vinegar reaction.

Combustion

Rapid chemical reaction with oxygen producing heat and/or light, seen in burning magnesium or candle flame.

Rusting

Reaction of iron with moist air forming iron oxide (rust), a chemical change causing damage to metals.

Natural Changes

Weathering breaks rocks physically and chemically to form soil, while erosion moves sediments by wind and water.

Text Book Activities, Questions & Extra Answers (25+ Questions)

Key NCERT Activities

Activity 5.1: Observe Changes Around You

Steps & Observations:

Observe melting ice, chopping vegetables, boiling water, burning wood, drying clothes.
Record changes in size, shape, smell, state, or formation of new substances.
Physical changes: Melting ice, boiling water; Chemical changes: Burning wood.

Activity 5.3: Lime Water Test for Carbon Dioxide

Observation:

Blowing exhaled air into lime water turns it milky due to formation of calcium carbonate.
Calcium hydroxide + Carbon dioxide → Calcium carbonate (white ppt) + Water.
This is a chemical change as new insoluble substance forms.

Activity 5.4: Vinegar + Baking Soda Reaction

Observation:

Fizzing sound and gas bubbles form; gas turns lime water milky (CO2 produced).
Vinegar + Baking soda → Carbon dioxide + Sodium acetate + Water.
Chemical change confirmed by new substances formed.

NCERT "Let Us Enhance Our Learning" Questions - Solved

1. Characteristics of physical change?

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

(i) State may change (ice to water)
(iii) No new substance formed

2. Predict reversible/irreversible changes:

Answer:

Reversible: Twisting string, dissolving sugar, boiling water, rolling mat
Irreversible: Stitching shirt, making idlis, ripening fruits, grinding wheat

3. True/False statements:

Answers:

(i) True - Wax melts before burning
(ii) False - Collecting vapour is physical change
(iii) True - Decomposition is chemical
(iv) True - Gas evolution shows chemical change

Extra Practice Questions (20+ Exam Booster)

4. What is the difference between physical and chemical changes? Give 2 examples each.

Answer:

Physical: No new substance, reversible. Ex: Melting ice, Tearing paper
Chemical: New substance formed, irreversible. Ex: Rusting iron, Burning wood

5. Why does lime water turn milky when CO2 is passed through it?

Answer:

CO2 reacts with calcium hydroxide to form insoluble calcium carbonate.
Ca(OH)₂ + CO₂ → CaCO₃ (white ppt) + H₂O

6. State the 3 requirements for combustion (Fire Triangle).

Answer:

Fuel (combustible substance)
Oxygen (from air)
Heat (to reach ignition temperature)

7. Why does a candle extinguish when covered with glass tumbler?

Answer:

Oxygen supply gets cut off inside the tumbler.
Combustion stops due to lack of one requirement from fire triangle.

8. What is ignition temperature? Give example.

Answer:

Minimum temperature at which substance catches fire.
Ex: Paper catches fire quickly as matchstick exceeds its ignition temperature.

9. Classify following as physical/chemical: Rusting, Melting wax, Burning candle, Tearing paper

Answer:

Physical: Melting wax, Tearing paper
Chemical: Rusting, Burning candle

10. What happens chemically when magnesium burns in air?

Answer:

Mg + O₂ → 2MgO (magnesium oxide - white powder)
Heat and light produced, new basic substance formed.

11. Is curdling of milk physical or chemical change?

Answer:

Chemical change - Bacteria convert lactose to lactic acid.
New substance (curd) with different properties formed.

12. Why is baking soda + vinegar a chemical change?

Answer:

Gas bubbles (CO2) produced, fizzing sound heard.
New substances: CO2, water, sodium acetate formed.

13. Differentiate reversible and irreversible changes (3 points each).

Answer:

Reversible: Can undo, same substance, ex: ice melting
Irreversible: Cannot undo, new substance, ex: cooking egg

14. What is weathering? Give 2 examples.

Answer:

Physical and chemical breakdown of rocks to form soil.
Ex: Temperature changes cracking rocks, iron in basalt turning red oxide.

15. What is erosion? How is it different from weathering?

Answer:

Erosion: Movement of sediments by wind/water.
Weathering: Breakdown of rocks at same place.

16. When burning candle, which changes are physical and chemical?

Answer:

Physical: Wax melting, evaporation
Chemical: Wax vapour burning to CO2 + H2O

17. What is bioluminescence? Give example.

Answer:

Chemical reaction producing light without heat in organisms.
Example: Fireflies glowing in gardens at night.

18. Fill blanks: Nalini saw brown deposits on cycle = _, this is change.

Answer: Rusting, chemical

19. How to reveal invisible lemon juice writing?

Answer:

Heat paper with iron or candle flame.
Organic compounds oxidize and turn brown.

20. Which experiment shows CO2 production? (Vinegar+baking soda, etc.)

Answer: Vinegar + baking soda (gas turns lime water milky).

21. Classify: Ripening fruits, dissolving sugar, burning magnesium

Answer:

Chemical: Ripening fruits, burning magnesium
Physical: Dissolving sugar

22. Why can't we reverse cooking of food?

Answer: New chemical compounds formed with different properties.

23. What happens in yeast + sugar experiment?

Answer:

Yeast ferments sugar → CO2 + alcohol
Balloon inflates, lime water turns milky.

Tip: Practice all 25 questions - covers 100% exam syllabus!

Topic	Key Points
Physical Change	Only appearance or state change, no new substance formed.
Chemical Change	New substances are formed with different properties.
Reversible Change	Can return to original state (melting ice).
Irreversible Change	Cannot return (burning paper).
Combustion	Reaction with oxygen producing heat and light.
Rusting	Formation of iron oxide in presence of air and water.

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Class 7 Science Chapter 5: Changes Around Us Physical and Chemical | Complete NCERT Notes, Activities, 25+ Questions & Answers

Full Chapter Summary, Physical vs Chemical Changes, Reversible/Irreversible, All Activities 5.1-5.8 Solved Step-by-Step, NCERT "Let Us Enhance Our Learning" Questions, 25+ Extra Practice Questions & Answers, Fire Triangle, Rusting, Weathering, Revision Tables, Mnemonics for CBSE Board Exams

Changes Around Us: Physical and Chemical

Full Chapter Summary & Detailed Notes - Changes Around Us: Physical and Chemical

Overview & Chapter Goal

Physical Changes

Chemical Changes

Combustion

Rusting

Natural Changes

Key Definitions & Terms - Complete Glossary

Physical Change

Chemical Change

Reversible Change

Irreversible Change

Combustion

Rusting

Weathering

Erosion

Ignition Temperature

Bioluminescence

Text Book Activities, Questions & Extra Answers (25+ Questions)

Key NCERT Activities

Activity 5.1: Observe Changes Around You

Activity 5.3: Lime Water Test for Carbon Dioxide

Activity 5.4: Vinegar + Baking Soda Reaction

NCERT "Let Us Enhance Our Learning" Questions - Solved

1. Characteristics of physical change?

2. Predict reversible/irreversible changes:

3. True/False statements:

Extra Practice Questions (20+ Exam Booster)

4. What is the difference between physical and chemical changes? Give 2 examples each.

5. Why does lime water turn milky when CO2 is passed through it?

6. State the 3 requirements for combustion (Fire Triangle).

7. Why does a candle extinguish when covered with glass tumbler?

8. What is ignition temperature? Give example.

9. Classify following as physical/chemical: Rusting, Melting wax, Burning candle, Tearing paper

10. What happens chemically when magnesium burns in air?

11. Is curdling of milk physical or chemical change?

12. Why is baking soda + vinegar a chemical change?

13. Differentiate reversible and irreversible changes (3 points each).

14. What is weathering? Give 2 examples.

15. What is erosion? How is it different from weathering?

16. When burning candle, which changes are physical and chemical?

17. What is bioluminescence? Give example.

18. Fill blanks: Nalini saw brown deposits on cycle = _____, this is ____ change.

19. How to reveal invisible lemon juice writing?

20. Which experiment shows CO2 production? (Vinegar+baking soda, etc.)

21. Classify: Ripening fruits, dissolving sugar, burning magnesium

22. Why can't we reverse cooking of food?

23. What happens in yeast + sugar experiment?

Key Concepts Explained

Historical and Societal Insights

Real-Life Examples

🧪 Interactive Quiz (10 Questions) - Chapter 5: Physical & Chemical Changes

Quick Revision Notes & Mnemonics

Mnemonic for Changes

Key Terms & Processes

Scientific Skills Step-by-Step

Group Discussions

18. Fill blanks: Nalini saw brown deposits on cycle = _, this is change.