By Subhash Kumar | Science Educator | UREducator
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Physical and Chemical Changes
Complete Teaching Resource for CBSE Class 7 Science Chapter 6
Class: VII
Subject: Science
Chapter: 6
Duration: 4 Hours
What Students Will Achieve
Knowledge
- Define physical and chemical changes
- Identify examples of each type of change
- Explain the process of rusting and crystallization
- Describe methods to prevent rusting
Understanding
- Differentiate between physical and chemical changes
- Analyze why certain changes are irreversible
- Understand the economic impact of rusting
- Explain the importance of crystallization
Skills
- Conduct experiments to demonstrate chemical changes
- Prepare copper sulphate crystals
- Test for chemical changes using indicators
- Design rust prevention methods
Prior Knowledge Assessment
Starter Questions:
- What happens when ice melts? Is this change reversible?
- Why does a cut apple turn brown after some time?
- What causes iron gates to develop a reddish-brown coating?
Changes in Physical Properties
Characteristics of Physical Changes
Shape/Size Change
Alters form but not composition
- Cutting paper into pieces
- Crushing chalk into powder
- Stretching a rubber band
Reversibility
Often reversible changes
- Melting and freezing water
- Evaporation and condensation
- Dissolving and recrystallizing salt
No New Substance
Same molecules before and after
- Dissolving sugar in water
- Breaking glass
- Bending metal
Examples of Physical Changes
| Example | Change Observed | Reversible? |
|---|---|---|
| Melting ice | Solid to liquid state change | Yes (by freezing) |
| Breaking glass | Size and shape change | No (cannot perfectly reassemble) |
| Dissolving salt | Solid becomes part of solution | Yes (by evaporation) |
| Inflating balloon | Shape and size change | Yes (can deflate) |
Experiment: Observing State Changes
Objective: Demonstrate reversible physical changes
- Take ice cubes in a beaker and observe
- Heat gently until ice melts completely
- Continue heating until water boils and steam forms
- Hold a cold plate above steam to observe condensation
- Place some water in freezer to observe freezing
- Record observations at each stage
Formation of New Substances
Characteristics of Chemical Changes
New Substance Formation
Irreversible changes
- Burning produces ash and gases
- Rusting forms iron oxide
- Cooking changes food composition
Energy Changes
Heat/light given off or absorbed
- Exothermic reactions release heat
- Endothermic reactions absorb heat
- Often accompanied by light/sound
Observable Indicators
Signs of chemical change
- Color change
- Gas formation (bubbles)
- Precipitate formation
- Odor change
Common Chemical Changes
| Process | New Substances Formed | Observable Signs |
|---|---|---|
| Burning wood | Ash, CO2, water vapor | Flame, heat, smoke |
| Rusting iron | Iron oxide (Fe2O3) | Color change, flaky texture |
| Digestion | Simpler molecules from food | Not directly observable |
| Photosynthesis | Glucose and oxygen | Gas bubbles (O2) |
Experiment: Vinegar and Baking Soda Reaction
Objective: Demonstrate chemical change with gas production
- Place 2 tsp baking soda in a bottle
- Mix 1/4 cup vinegar with food coloring
- Quickly pour vinegar into bottle and observe
- Pass gas through lime water to test for CO2
- Compare reactants and products
Corrosion of Iron and Prevention
Rusting Process
Requirements
- Iron or steel object
- Oxygen from air
- Water or moisture
- Electrolytes accelerate process
Chemical Equation
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Iron + Oxygen + Water → Hydrated Iron Oxide (Rust)
Economic Impact
- Global cost: ~$2.5 trillion annually
- Infrastructure damage
- Reduced lifespan of products
- Maintenance costs
Prevention Methods
| Method | How It Works | Examples |
|---|---|---|
| Barrier Protection | Prevents contact with air/water | Painting, greasing, plastic coating |
| Galvanization | Zinc coating sacrifices itself | Galvanized iron pipes, buckets |
| Alloying | Changes iron's properties | Stainless steel (iron + chromium) |
| Sacrificial Protection | More reactive metal corrodes first | Zinc blocks on ship hulls |
Project: Testing Rust Prevention Methods
Objective: Compare effectiveness of different rust prevention techniques
- Prepare 5 iron nails cleaned with sandpaper
- Treat each differently: paint, grease, zinc coating, dry environment, control
- Expose all to same humid conditions
- Observe and record rust formation over 1 week
- Analyze which method worked best
Obtaining Pure Substances
Crystallization Process
Preparation
- Create saturated solution
- Filter to remove impurities
- Allow slow cooling
- Undisturbed environment
Crystal Formation
- Molecules arrange in repeating pattern
- Pure substance separates out
- Shape reflects molecular structure
- Slow growth produces larger crystals
Applications
- Purification of substances
- Pharmaceutical production
- Food industry (sugar, salt)
- Chemical manufacturing
Comparison with Evaporation
| Aspect | Crystallization | Simple Evaporation |
|---|---|---|
| Purity | Produces very pure substances | May contain impurities |
| Crystal Shape | Well-defined geometric shapes | Irregular particles |
| Process Control | Controlled cooling rate | Direct heating |
| Applications | Pharmaceuticals, high-purity chemicals | Common salt production |
Experiment: Growing Copper Sulphate Crystals
Objective: Demonstrate crystallization process
- Dissolve copper sulphate in hot water until saturated
- Filter the solution while hot
- Allow to cool slowly in undisturbed place
- Observe crystal formation over 2-3 days
- Remove and dry perfect crystals
Evaluation Strategies
Formative Assessments
- Classify changes as physical/chemical
- Experiment report evaluation
- Diagram labeling (rusting process)
- Think-pair-share discussions
Summative Assessments
- Chapter test (MCQs + short answer)
- Crystal growing project evaluation
- Case study on rust prevention
- Practical exam on chemical tests
Extended Learning: Visit a Metal Workshop
Objective: Observe practical applications of rust prevention
- Interview workers about rust prevention methods
- Identify different coatings used on metal products
- Learn about galvanization process
- Understand economic impact of corrosion
- Prepare a report with photographs
Frequently Asked Questions
Why is rusting considered a chemical change while melting is physical?
The key differences are:
- Rusting forms a new substance (iron oxide) with different properties from iron
- Melting only changes the state of the same substance (ice to water)
- Rusting is irreversible under normal conditions
- Melting can be reversed by freezing
- Rusting involves chemical reaction with oxygen
How can we tell if a change is chemical or physical?
Look for these indicators of chemical change:
- New substance formation with different properties
- Irreversibility under normal conditions
- Energy changes (heat/light given off or absorbed)
- Color change not due to mixing
- Gas production (bubbles, odor change)
- Precipitate formation in solutions
