Changes around us

Changes Around Us Class 6 CBSE Notes - Reversible & Irreversible Changes

Introduction to Changes Around Us

Everything around us undergoes continuous change. From ice melting into water to milk turning into curd, changes are an integral part of our daily lives. Understanding these changes helps us appreciate the dynamic nature of matter and learn how to control beneficial changes while preventing harmful ones.

A change occurs when one or more properties of a substance become different. These properties include state (solid, liquid, or gas), position, shape, size, color, temperature, composition, or structure. Changes can be as simple as water freezing into ice or as complex as food cooking in your kitchen.

Types of Changes Around Us

Common Everyday Changes

We observe numerous changes in our daily life:

1. Formation of curd from milk - A biological change involving bacteria
2. Cooking of food - Heat transforms raw ingredients into edible meals
3. Burning of fuels - Chemical reaction producing heat and light
4. Drying of clothes - Water evaporates from wet fabric
5. Rusting of iron - Iron reacts with oxygen and moisture
6. Melting of ice - Solid ice converts to liquid water
7. Boiling/evaporation of water - Liquid water becomes steam
8. Germination of seeds - Seeds sprout into plants
9. Ripening of fruits - Fruits become sweet and soft
10. Formation of day and night - Earth's rotation causes this change
11. Seasonal changes - Earth's revolution around the sun
12. Growth of living beings - Humans, animals, and plants grow over time

Beneficial and Harmful Changes

Beneficial Changes

These changes improve our lives or environment:

• Ripening of fruits - Makes fruits tasty and nutritious
• Cooking of food - Makes food digestible and safe
• Formation of curd - Creates nutritious dairy product
• Germination of seeds - Produces new plants

We try to accelerate beneficial changes using artificial methods, such as ripening fruits faster with ethylene gas.

Harmful Changes

These changes cause damage or deterioration:

• Rusting of iron - Damages metal objects
• Spoiling of food - Makes food unsafe to eat
• Souring of milk - Makes milk unusable
• Burning of forests - Destroys ecosystems

We attempt to slow down or prevent harmful changes by using refrigeration, protective coatings, or other preservation methods.

Classification of Changes

All changes can be broadly classified into two categories based on whether they can be undone:

1. Reversible Changes

Definition: A change that can be reversed to form the original substance is called a reversible change.

Characteristics:

• The original substance can be recovered
• Usually involves physical changes
• No new substance is formed
• Can be undone by removing the causing agent

Examples of Reversible Changes

Change	How to Reverse
Melting of ice	Cool water in a freezer to get ice back
Boiling of water	Cool steam to condense it back to water
Melting of wax	Allow molten wax to cool and solidify
Stretching a rubber band	Release the force; rubber returns to original length
Stretching a spring	Remove the force; spring returns to original shape
Inflating a balloon	Release air; balloon returns to original size
Dissolving salt in water	Evaporate water to get salt crystals back
Dissolving sugar in water	Evaporate water to recover sugar
Ironing of clothes	Wrinkles can form again
Folding paper	Unfold to get the original sheet
Rolling chapatti from dough	Can reshape dough back into ball
Drying of clothes	Wet clothes again if needed
Heating milk	Hot milk can cool down
Melting ice cream	Refreeze to solidify
Melting coal tar	Cool to solidify again
Coiling a wire	Straighten to get original wire
Molding wet clay	Reshape before it dries

Note: Change of state (solid ↔ liquid ↔ gas) is typically reversible.

2. Irreversible Changes

Definition: A change that cannot be reversed to form the original substance is called an irreversible change.

Characteristics:

• Original substance cannot be recovered
• Usually involves chemical changes
• New substances are formed
• Cannot be undone even after removing the causing agent

Examples of Irreversible Changes

Change	Why Irreversible
Burning of paper	Produces ash and smoke; cannot recombine
Burning of fuels (wood, coal, LPG)	Forms gases and ash permanently
Formation of curd from milk	Bacterial action changes milk proteins
Cooking of food	Chemical changes make it impossible to reverse
Rusting of iron	Iron oxide (rust) has different properties
Grinding wheat into flour	Cannot reassemble grains from flour
Baking chapatti/cake	Heat causes permanent chemical changes
Growth of plants and animals	Biological processes move forward
Ripening of fruits	Biochemical changes are permanent
Ageing of humans	Cannot reverse time or cellular changes
Boiling an egg	Proteins denature permanently
Breaking a toy	Cannot reassemble perfectly
Cutting paper	Pieces cannot rejoin exactly
Bursting a balloon	Damaged structure cannot be restored
Burning of wax (candle)	Forms CO₂, water vapor, and soot
Burning incense stick	Produces fragrant gases and ash
Setting of cement	Hardens permanently after mixing with water
Setting of Plaster of Paris	Forms hard mass irreversibly
Sawing wood	Cut pieces cannot be rejoined
Printing on paper	Ink marks are permanent
Souring of milk	Lactic acid formation is permanent
Making cheese (paneer)	Protein coagulation is irreversible

Same Material, Different Changes

Interestingly, the same material can undergo both reversible and irreversible changes under different conditions:

1. Paper Changes

Reversible: Folding paper → Can be unfolded back to original sheet

Irreversible: Cutting paper → Cannot rejoin pieces to form exact original sheet

2. Roti (Chapatti) Changes

Reversible: Rolling dough into roti → Can reshape into dough ball

Irreversible: Baking roti on tawa → Cannot convert baked roti back to dough

3. Clay Pot Changes

Reversible: Shaping wet clay into pot → Can reshape wet clay before drying

Irreversible: Baking clay pot in oven → Baked pot cannot become clay again

4. Balloon Changes

Reversible: Inflating a balloon → Deflate to return to original size

Irreversible: Bursting a balloon → Cannot restore to original shape

5. Wax Changes

Reversible: Melting wax → Cool to solidify back

Irreversible: Burning wax in candle → Produces CO₂, water vapor, and soot permanently

Physical Changes vs Chemical Changes

Understanding the difference between physical and chemical changes is fundamental to chemistry:

Physical Changes

Definition: Changes that affect only the physical properties of a substance without changing its chemical composition.

Characteristics:

• No new substance is formed
• Usually reversible
• Changes in state, shape, size, or physical properties
• Molecular structure remains the same

Examples:

• Melting of ice (H₂O remains H₂O)
• Boiling of water
• Dissolving sugar in water
• Cutting wood into pieces
• Crushing a can

Chemical Changes

Definition: Changes that result in the formation of one or more new substances with different chemical properties.

Characteristics:

• New substances are formed
• Usually irreversible
• Changes in chemical composition
• Often accompanied by heat, light, or gas production
• Molecular structure changes

Examples:

• Burning of paper (forms ash, CO₂, and water)
• Rusting of iron (forms iron oxide)
• Cooking food (proteins and carbohydrates undergo chemical reactions)
• Formation of curd (lactose converts to lactic acid)
• Digestion of food

Comparison Table

Aspect	Physical Change	Chemical Change
New substance	No	Yes
Reversibility	Usually reversible	Usually irreversible
Composition	Remains same	Changes
Energy change	Minimal	Significant
Examples	Melting, freezing	Burning, rusting

Causes of Changes

Changes don't occur spontaneously; there's always a cause or agent that brings about the change.

Common Causes of Changes

1. Heat - Melting ice, cooking food, evaporating water
2. Light - Fading of colors, photosynthesis in plants
3. Electricity - Electroplating, charging batteries
4. Force - Breaking objects, stretching rubber bands
5. Mixing substances - Dissolving salt in water, making cement
6. Biological agents - Bacteria converting milk to curd
7. Time - Rusting, aging, ripening

Expansion and Contraction: A Reversible Change

Understanding Expansion

Expansion: The increase in size of an object when heated.

Contraction: The decrease in size of an object when cooled.

This principle is reversible - objects expand when heated and contract when cooled.

Practical Applications

1. Fixing Iron Rim to Wooden Wheel

Process:

1. Iron rim is made slightly smaller than the wooden wheel
2. Heat the iron rim uniformly until it expands
3. Quickly fit the expanded rim around the wooden wheel
4. Pour cold water to cool the rim
5. As the rim cools, it contracts and fits tightly on the wheel

Scientific principle: Thermal expansion (heating) followed by thermal contraction (cooling)

2. Fixing Iron Blade to Wooden Handle

Process:

1. Make a hole in the wooden handle slightly smaller than the blade base
2. Heat the iron blade until it expands
3. Insert the expanded blade into the hole
4. Cool the blade with water
5. The blade contracts and fits tightly in the handle

Experiments to Demonstrate Changes for Class 6 Students

Experiment 1: Reversible Change - Melting and Freezing

Materials needed: Ice cubes, beaker, heat source, freezer

Procedure:

1. Place ice cubes in a beaker
2. Observe the solid state
3. Heat gently and observe melting
4. Water forms (liquid state)
5. Cool the water in a freezer
6. Observe ice forming again

Observation: Ice → Water → Ice (reversible change)

Conclusion: Change of state is reversible

Experiment 2: Irreversible Change - Burning Paper

Materials needed: Small piece of paper, matchstick, metal plate

Procedure:

1. Place paper on a metal plate
2. Light it carefully with adult supervision
3. Observe the burning process
4. Note ash and smoke formation
5. Try to reverse the change

Observation: Paper burns to form ash and smoke

Conclusion: Burning is an irreversible change; original paper cannot be recovered

Experiment 3: Dissolving Salt in Water

Materials needed: Salt, water, beaker, stirrer, heat source

Procedure:

1. Add salt to water in a beaker
2. Stir until salt dissolves completely
3. Heat the solution to evaporate water
4. Observe salt crystals forming

Observation: Salt dissolves in water and can be recovered by evaporation

Conclusion: Dissolving salt is a reversible physical change

Experiment 4: Formation of Curd

Materials needed: Warm milk, small quantity of curd (starter)

Procedure:

1. Take warm milk in a clean container
2. Add a teaspoon of curd
3. Stir and keep aside for 5-6 hours
4. Observe the change
5. Try to convert curd back to milk

Observation: Milk changes to curd; cannot be reversed

Conclusion: Curd formation is an irreversible chemical change

Experiment 5: Expansion of Metals

Materials needed: Metal ball and ring apparatus, heat source

Procedure:

1. Show that the cold metal ball passes through the ring
2. Heat the metal ball
3. Try to pass it through the ring again
4. It won't pass (expanded)
5. Cool the ball
6. It passes through again (contracted)

Observation: Metal expands on heating and contracts on cooling

Conclusion: Expansion is a reversible change

Causes of Slow and Fast Changes with Examples

Fast Changes

Changes that occur rapidly, often in seconds or minutes:

Examples:

• Burning of paper - Occurs in seconds
• Bursting of a balloon - Instantaneous
• Burning of fireworks - Few seconds
• Breaking of glass - Immediate
• Boiling of water - Minutes
• Lighting a matchstick - Instant
• Dissolving sugar in hot water - Quick

Causes:

• High energy input (heat, force)
• Rapid chemical reactions
• Sudden physical changes

Slow Changes

Changes that occur gradually over extended periods:

Examples:

• Rusting of iron - Takes days to months
• Ripening of fruits - Takes days
• Formation of day and night - 24-hour cycle
• Change of seasons - Months
• Growth of plants - Weeks to years
• Ageing of humans - Years to decades
• Weathering of rocks - Centuries
• Formation of curd - 5-8 hours
• Drying of clothes - Hours

Causes:

• Low or gradual energy changes
• Slow chemical reactions
• Natural biological processes
• Environmental factors

Methods of Separation in Mixtures

When discussing changes, we often need to separate mixtures back to their original components. Here are common separation methods:

1. Handpicking

Description: Manually removing unwanted particles from a mixture

When used: When particles are large and easily distinguishable

Example: Removing stones from rice, separating good fruits from spoiled ones

2. Threshing

Description: Beating harvested crops to separate grains from stalks

When used: For separating grains from agricultural crops

Example: Separating wheat grains from wheat stalks

3. Winnowing

Description: Using wind to separate lighter particles from heavier ones

When used: When components have different weights

Example: Separating husk from wheat, chaff from rice

4. Sieving (Sifting)

Description: Using a sieve with holes to separate particles of different sizes

When used: When components have different particle sizes

Example: Separating flour from bran, sand from gravel, stones from soil

5. Sedimentation

Description: Allowing heavier insoluble particles to settle at the bottom

When used: When solid particles are suspended in liquid

Example: Muddy water settling, sand in water

Time required: Minutes to hours depending on particle size

6. Decantation

Description: Carefully pouring out the clear liquid without disturbing the sediment

When used: After sedimentation, to separate liquid from settled solid

Example: Separating water from settled mud, oil from water

7. Filtration

Description: Passing mixture through filter paper or cloth to separate solids from liquids

When used: When solid particles are very small and need to be removed from liquid

Example: Tea leaves from tea, coffee grounds from coffee, dirt from water

Process:

• Solid particles remain on filter (residue)
• Liquid passes through (filtrate)

8. Evaporation

Description: Heating a liquid mixture to convert liquid into vapor, leaving solid behind

When used: To separate dissolved solid from liquid

Example: Salt from seawater, sugar from sugar solution

Requirement: Heat source and time

9. Condensation

Description: Cooling vapor to convert it back into liquid

When used: To collect liquid from a vapor

Example: Distilled water collection, perfume extraction

Comparison of Sedimentation, Decantation, and Filtration

Aspect	Sedimentation	Decantation	Filtration
Process	Settling of particles	Pouring clear liquid	Passing through filter
Time required	Minutes to hours	Quick	Quick to moderate
Particle size	Larger, heavier	After sedimentation	Very small particles
Separation	Partial	Partial	Complete
Equipment	Container only	Container only	Filter paper/cloth needed
Result	Sediment + liquid	Separated layers	Residue + filtrate
Example	Muddy water settling	Pouring clear water off mud	Removing tea leaves

Which Methods Separate Solids from Liquids in Everyday Life

Kitchen Applications

1. Filtration - Making tea, coffee, or soup; straining pasta
2. Sieving - Sifting flour, separating lumps from powder
3. Decantation - Separating oil from water, ghee from milk solids
4. Handpicking - Removing stones from lentils or rice
5. Evaporation - Making salt from seawater, concentrating sugar syrup

Household Applications

1. Winnowing - Separating rice from husk
2. Filtration - Water purifiers, vacuum cleaner filters
3. Sedimentation - Allowing tank water to settle before use
4. Magnetic separation - Separating iron nails from sawdust

Industrial Applications

1. Filtration - Water treatment plants, air purifiers
2. Evaporation - Salt pans, sugar refineries
3. Sedimentation - Sewage treatment, water purification
4. Centrifugation - Washing machines (spin dry), cream separation

How to Choose the Best Separation Method for a Given Mixture

Follow this decision-making process:

Step 1: Identify the Components

• Are they solid-solid, solid-liquid, or liquid-liquid?
• What are their physical properties?

Step 2: Consider Physical Differences

• Size difference → Use sieving
• Weight difference → Use winnowing
• Magnetic property → Use magnetic separation
• Solubility difference → Use dissolution + filtration

Step 3: Consider State of Matter

• Insoluble solid in liquid → Use sedimentation, decantation, or filtration
• Soluble solid in liquid → Use evaporation
• Two immiscible liquids → Use separating funnel or decantation

Step 4: Practical Considerations

• Equipment availability
• Time constraints
• Quantity of mixture
• Safety requirements
• Cost effectiveness

Decision Chart

Is the solid soluble in liquid?
├─ NO → Sedimentation → Decantation or Filtration
└─ YES → Evaporation to recover solid
Are particles of different sizes?
└─ YES → Sieving
Are particles of different weights?
└─ YES → Winnowing
Are particles magnetic?
└─ YES → Magnetic separation
Is quick separation needed?
└─ YES → Centrifugation

Simple Experiments to Demonstrate Filtration and Evaporation

Experiment: Filtration - Separating Tea Leaves from Tea

Objective: To demonstrate filtration as a separation technique

Materials needed:

• Tea leaves
• Hot water
• Tea strainer or filter paper
• Beaker or cup
• Funnel

Procedure:

1. Boil water and add tea leaves
2. Let it brew for 2-3 minutes
3. Set up the funnel with filter paper over a cup
4. Pour the tea mixture through the filter
5. Observe tea leaves caught on filter (residue)
6. Observe clear tea passing through (filtrate)

Observation:

• Solid tea leaves are retained on the filter
• Liquid tea passes through as filtrate
• Filtration successfully separates solid from liquid

Conclusion: Filtration is effective for separating insoluble solids from liquids

Experiment: Evaporation - Recovering Salt from Saltwater

Objective: To demonstrate evaporation as a separation technique

Materials needed:

• Salt
• Water
• Beaker or shallow dish
• Heat source (optional)
• Stirrer

Procedure:

1. Dissolve 2 tablespoons of salt in 100ml water
2. Stir until completely dissolved
3. Pour the solution into a shallow dish
4. Keep in sunlight or heat gently
5. Observe water evaporating over time
6. Note salt crystals forming

Observation:

• Water gradually evaporates
• Salt crystals appear as water evaporates
• Eventually, only salt remains

Conclusion: Evaporation separates dissolved solids from liquids effectively

Experiment: Combined Separation - Sand and Salt Mixture

Objective: To use multiple separation techniques

Materials needed:

• Sand and salt mixture
• Water
• Filter paper and funnel
• Beaker
• Heat source

Procedure:

1. Add water to the sand-salt mixture
2. Stir well (salt dissolves, sand doesn't)
3. Filter the mixture using filter paper
4. Sand remains on filter (collect it)
5. Salt solution passes through as filtrate
6. Heat the filtrate to evaporate water
7. Collect salt crystals

Observation:

• Filtration separates sand
• Evaporation recovers salt
• Both components successfully separated

Conclusion: Complex mixtures may need multiple separation methods

Lesson Plan on Changes Around Us Chapter

Lesson Plan for Class 6 Science - Changes Around Us

Duration: 5-6 class periods (40-45 minutes each)

Period 1: Introduction to Changes

Objectives:

• Understand what constitutes a change
• Identify different types of changes in daily life
• Distinguish between beneficial and harmful changes

Activities:

1. Warm-up (5 min): Show pictures of various changes; students identify them
2. Discussion (15 min): Explain properties that change, causes of changes
3. Group activity (15 min): Students list 10 changes they observed today
4. Conclusion (10 min): Classify changes as beneficial or harmful

Materials: Pictures, chart paper, markers

Assessment: Oral questioning, group participation

Period 2: Reversible Changes

Objectives:

• Define reversible changes
• Identify examples from daily life
• Understand the concept of change of state

Activities:

1. Recap (5 min): Quick review of previous class
2. Demonstration (15 min): Ice melting and refreezing experiment
3. Explanation (15 min): Discuss characteristics and examples
4. Worksheet (10 min): Identify reversible changes from given list

Materials: Ice, beaker, refrigerator access, worksheet

Assessment: Worksheet completion, concept questions

Period 3: Irreversible Changes

Objectives:

• Define irreversible changes
• Distinguish between reversible and irreversible changes
• Understand chemical nature of irreversible changes

Activities:

1. Demonstration (15 min): Burning paper (with safety precautions)
2. Discussion (15 min): Why can't we reverse certain changes?
3. Comparison chart (10 min): Create reversible vs irreversible chart
4. Quiz (5 min): Quick classification quiz

Materials: Paper, matchbox, metal plate, safety equipment, chart paper

Assessment: Chart completion, quiz performance

Period 4: Physical vs Chemical Changes

Objectives:

• Differentiate between physical and chemical changes
• Relate to reversible and irreversible changes
• Understand molecular-level differences

Activities:

1. Concept introduction (10 min): Explain physical vs chemical changes
2. Demonstration (20 min): Multiple experiments (dissolving, burning, melting)
3. Group work (15 min): Create comparison table

Materials: Sugar, water, candle, matchbox, ice, various objects

Assessment: Table accuracy, group presentation

Period 5: Expansion, Contraction, and Practical Applications

Objectives:

• Understand thermal expansion and contraction
• Learn practical applications in daily life
• Relate to reversible changes

Activities:

1. Demonstration (15 min): Ball and ring experiment
2. Video (10 min): Fixing iron rim to wooden wheel
3. Discussion (15 min): Other applications of expansion
4. Activity (5 min): Open tight jar lids using hot water

Materials: Ball-ring apparatus, burner, video setup

Assessment: Observation skills, application understanding

Period 6: Separation Methods and Revision

Objectives:

• Learn common separation methods
• Apply appropriate methods to given scenarios
• Review entire chapter

Activities:

1. Demonstration (20 min): Filtration and evaporation experiments
2. Group discussion (10 min): When to use which method
3. Revision game (15 min): Quiz competition on entire chapter

Materials: Separation equipment, question cards

Assessment: Practical skills, quiz performance

Additional Teaching Strategies

1. Use analogies: Compare irreversible changes to one-way streets
2. Real-life connections: Relate to cooking, weather, seasons
3. Hands-on learning: Maximum practical demonstrations
4. Visual aids: Charts, models, videos
5. Differentiated instruction: Extra support for struggling learners
6. Formative assessment: Regular checks for understanding

Homework Assignments

• Day 1: List 20 changes observed at home
• Day 2: Draw and explain one reversible change
• Day 3: Draw and explain one irreversible change
• Day 4: Compare physical and chemical changes with examples
• Day 5: Research one practical application of expansion
• Day 6: Create a concept map of the entire chapter

Concepts Summary Table

Concept	Definition	Example
Change	Alteration in properties of a substance	Ice melting, paper burning
Reversible Change	Change that can be undone	Water ↔ Ice ↔ Steam
Irreversible Change	Change that cannot be undone	Burning paper, cooking
Physical Change	No new substance formed	Melting, dissolving
Chemical Change	New substances formed	Burning, rusting
Expansion	Increase in size on heating	Metal rim expanding
Contraction	Decrease in size on cooling	Metal rim contracting
Beneficial Change	Improves our lives	Ripening fruits, cooking
Harmful Change	Causes damage	Rusting, food spoiling

Important Terms Glossary

• Change: Alteration in one or more properties of a substance
• State: Physical form - solid, liquid, or gas
• Expansion: Increase in size when heated
• Contraction: Decrease in size when cooled
• Reversible: Can be undone to get original substance
• Irreversible: Cannot be undone to get original substance
• Physical change: Change in physical properties without forming new substances
• Chemical change: Change that produces new substances
• Evaporation: Conversion of liquid to vapor
• Condensation: Conversion of vapor to liquid
• Melting: Solid changing to liquid
• Freezing: Liquid changing to solid
• Filtration: Separating solids from liquids using filter
• Sedimentation: Settling of heavier particles at bottom
• Decantation: Pouring clear liquid without disturbing sediment

Exam Preparation Tips

For Short Answer Questions

1. Define terms clearly and concisely
2. Always provide relevant examples
3. Mention the type of change (reversible/irreversible)

For Long Answer Questions

1. Break answer into clear points
2. Use comparisons and contrasts
3. Draw diagrams where applicable
4. Give multiple examples

For Practical Questions

1. Understand the purpose of each step
2. Know the expected observations
3. Be able to explain the scientific principle
4. Relate to real-life applications

Common Mistakes to Avoid

• Confusing reversible with physical changes (not all physical changes are reversible)
• Thinking all irreversible changes are chemical (breaking glass is physical but irreversible)
• Forgetting that same material can undergo both types of changes
• Not understanding the difference between melting wax and burning wax

Conclusion

Understanding changes around us helps us:

• Appreciate the dynamic nature of matter
• Make informed decisions in daily life
• Control beneficial changes
• Prevent or slow harmful changes
• Apply scientific knowledge practically

Changes are fundamental to life and the universe. From the smallest molecular transformations to massive geological processes, changes shape our world. By learning to observe, classify, and understand these changes, we develop scientific thinking and problem-solving skills that serve us throughout life.

Remember: Every change has a cause, and understanding this cause helps us predict, control, and utilize changes for our benefit.