Willer Academy
Nation Builders Through Education | Founded by Er. Rahul Kumar Dubey Sir
Chapter 6: Physical and Chemical Changes
Subject: Science (Chemistry)
Class: 7
Series: Base Builder for Moderate to Hard
Learning Objectives
By the end of this chapter, you, the future scientist and nation-builder, will be able to:
- Differentiate between physical and chemical changes
- Identify examples of physical and chemical changes in daily life
- Understand the characteristics of chemical changes
- Explain rusting and crystallization processes
- Solve problems related to changes in matter
- Apply knowledge to real-life situations involving changes
6.1 Introduction: Changes Around Us
Every day, we observe various changes around us. Some changes are temporary, while others are permanent. In science, we classify these changes as physical changes and chemical changes.
Quick Check
Look around you and identify three changes happening right now. Are these changes reversible or irreversible?
6.2 Physical Changes
Physical changes are changes in which no new substance is formed. The chemical composition remains the same, only the physical properties change.
Examples
- Melting of ice
- Boiling of water
- Dissolving sugar
- Cutting paper
- Crushing a can
Characteristics
- No new substance formed
- Reversible change
- Change in state, shape, or size
- No change in weight
- Temporary change
Numerical & Application 1: State Changes
- Scenario: Ice at 0°C requires 334 J/g to melt into water at 0°C.
- Problem: How much energy is required to melt 50g of ice? If this water is then heated to 100°C (specific heat capacity 4.18 J/g°C), how much additional energy is needed?
- Solution:
- Energy to melt ice = 50g × 334 J/g = 16,700 J
- Energy to heat water = 50g × 4.18 J/g°C × 100°C = 20,900 J
- Total energy = 16,700 J + 20,900 J = 37,600 J
This shows the energy involved in physical state changes!
Task 1
Take an ice cube and observe its melting. Time how long it takes to completely melt at room temperature. Then boil the water and observe the steam. Record your observations about these physical changes.
6.3 Chemical Changes
Chemical changes are changes in which new substances with different properties are formed. The chemical composition changes completely.
Examples
- Burning of wood
- Rusting of iron
- Cooking food
- Digestion
- Photosynthesis
Characteristics
- New substance formed
- Irreversible change
- Change in composition
- Change in energy (heat/light)
- Permanent change
Experiment: Vinegar and Baking Soda
Mix vinegar (acetic acid) with baking soda (sodium bicarbonate) in a bottle. Observe the fizzing caused by carbon dioxide gas production. This is a chemical change because new substances (carbon dioxide and sodium acetate) are formed.
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
Acetic Acid + Sodium Bicarbonate → Sodium Acetate + Water + Carbon Dioxide
6.4 Comparing Physical and Chemical Changes
| Aspect | Physical Change | Chemical Change |
|---|---|---|
| Formation of new substance | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Change in composition | No | Yes |
| Energy change | Little or no energy change | Significant energy change |
| Examples | Melting, boiling, cutting | Burning, rusting, cooking |
Numerical & Application 2: Rusting of Iron
- Scenario: Rust is hydrated iron(III) oxide with the formula Fe₂O₃·xH₂O.
- Problem: If 100g of iron completely rusts to form 143g of rust, what percentage increase in mass occurs? How many grams of oxygen combined with the iron?
- Solution:
- Mass increase = 143g - 100g = 43g
- Percentage increase = (43g / 100g) × 100 = 43%
- Oxygen combined = 43g (from conservation of mass)
This shows how rusting involves combination with oxygen from air!
Task 2: Rust Observation
Place an iron nail in a jar with water and leave it exposed to air for a few days. Observe the formation of rust. Compare it with another nail kept in a dry place. Record your observations.
6.5 Rusting and Crystallization
Two important processes that demonstrate chemical and physical changes:
Rusting: A chemical change where iron reacts with oxygen and water to form rust (iron oxide).
Water (H₂O)
Oxygen (O₂)
Reaction
Rust (Fe₂O₃)
Crystallization: A physical process used to purify solids. The substance dissolves in a solvent, and then crystals form as the solution cools or evaporates.
Experiment: Growing Crystals
Dissolve as much salt as possible in hot water. Pour the solution into a shallow dish. As the water evaporates, salt crystals will form. Observe their shape under a magnifying glass.
Section B: MCQ Challenge (20 Questions)
Test your knowledge and find where you need to improve!
1. Which of these is a physical change?
a) Burning of paper
b) Rusting of iron
c) Melting of ice
d) Cooking food
2. Rusting of iron requires:
a) Oxygen only
b) Water only
c) Both oxygen and water
d) Neither oxygen nor water
3. Which process is used to purify solids?
a) Rusting
b) Crystallization
c) Combustion
d) Photosynthesis
Section B: MCQ Challenge (20 Questions)
Test your knowledge and find where you need to improve!
1. Which of these is a physical change?
a) Burning of paper
b) Rusting of iron
c) Melting of ice
d) Cooking food
2. Rusting of iron requires:
a) Oxygen only
b) Water only
c) Both oxygen and water
d) Neither oxygen nor water
3. Which process is used to purify solids?
a) Rusting
b) Crystallization
c) Combustion
d) Photosynthesis
4. Cutting a piece of paper is an example of:
a) Physical change
b) Chemical change
c) Nuclear change
d) Biological change
5. Cooking an egg is a:
a) Physical change
b) Reversible change
c) Sublimation
d) Chemical change
6. Which of the following is reversible?
a) Burning wood
b) Melting butter
c) Rusting
d) Cooking meat
7. Photosynthesis is an example of:
a) Physical change
b) Reversible change
c) Chemical change
d) Mechanical change
8. Freezing of water involves:
a) Physical change
b) Chemical change
c) Combustion
d) Rusting
9. Which of these is an irreversible change?
a) Boiling water
b) Melting wax
c) Dissolving salt
d) Burning paper
10. Which of the following is caused by chemical change?
a) Cutting fruits
b) Rusting of iron
c) Melting ice
d) Evaporation of water
11. Dissolving sugar in water is a:
a) Chemical change
b) Burning reaction
c) Physical change
d) Nuclear reaction
12. Melting wax is an example of:
a) Physical change
b) Chemical change
c) Rusting
d) Photosynthesis
13. Which of these is a chemical change?
a) Boiling water
b) Melting ice
c) Cutting wood
d) Burning candle
14. Photosynthesis requires sunlight, water, and:
a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Hydrogen
15. Heating sugar until it caramelizes is a:
a) Physical change
b) Reversible change
c) Chemical change
d) Mechanical change
16. Evaporation of water is an example of:
a) Physical change
b) Chemical change
c) Rusting
d) Cooking
17. Combustion of petrol in a car engine is:
a) Physical change
b) Melting
c) Crystallization
d) Chemical change
18. Which of the following is NOT a chemical change?
a) Rusting of iron
b) Melting of ice
c) Cooking rice
d) Burning paper
19. Boiling of water involves:
a) Chemical reaction
b) Nuclear reaction
c) Physical change
d) Photosynthesis
20. Formation of rust involves:
a) Combination of iron and oxygen
b) Melting of iron
c) Boiling of water
d) Cutting iron
Answers: 1-c, 2-c, 3-b, 4-a, 5-d, 6-b, 7-c, 8-a, 9-d, 10-b, 11-c, 12-a, 13-d, 14-b, 15-c, 16-a, 17-d, 18-b, 19-c, 20-a
Section C: Answer in Detail (For Answer Writing Practice)
Q1. Differentiate between physical and chemical changes with three examples of each.
Hint: Focus on reversibility, formation of new substances, and energy changes.
Q2. Explain the process of rusting. How can it be prevented?
Hint: Discuss the conditions necessary for rusting and methods of prevention.
Q3. What is crystallization? Describe how you would grow copper sulfate crystals in the laboratory.
Hint: Include the steps involved in the crystallization process.
Section D: Tackle These! (Higher Order Thinking Skills - HOTS)
Q1. When a candle burns, both physical and chemical changes occur. Explain these changes.
Think about: Melting of wax vs. burning of wax.
Q2. Why is rusting considered a chemical change but melting of iron is a physical change?
Think about: Formation of new substances and reversibility.
Q3. Design an experiment to show that both air and water are necessary for rusting.
Think about: Controlled variables and experimental setup.
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