Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical notes and book solutions from the NCERT Curiosity textbook with easy explanations, important concepts, examples and complete textbook question answers for revision and exam preparation.
Changes Around Us
Physical and Chemical Changes
Grade 7 Science – Curiosity
Colourful, simple and easy-to-understand chapter notes with activities, questions and short solutions.
🌈 Introduction – Changes Around Us
We see many changes around us every day.
- Ice melts and becomes water.
- Cold water becomes warmer.
- A flower bud becomes a flower.
- A banana develops brown spots and may develop a strong smell.
These changes may affect the size, shape, state, colour, smell or other properties of a substance.
🤔 Think About It
Activity 5.1: Let us think and reflect
You might have observed various changes happening around you. Some of them are listed below. Observe each change carefully.
| S.No. | Change | Observation |
|---|---|---|
| 1 | Melting ice cubes | Ice changes from solid to liquid water. |
| 2 | Chopping vegetables | The size and shape of vegetables change. |
| 3 | Boiling water | Water changes into water vapour. |
| 4 | Making popcorn from corn | Corn changes into popcorn and a new form is produced. |
| 5 | Cutting a piece of paper | The size and shape of paper change. |
| 6 | Adding beetroot extract to water | The colour of water changes. |
| 7 | Burning wood | New substances such as ash and gases are formed. |
| 8 | Drying wet clothes | Water changes into water vapour. |
| 9 | Making small balls of dough | The shape of dough changes. |
| 10 | Rolling small balls of dough into chapatis | The shape and size of dough change. |
| 11 | Any other | Observe and record your own example. |
🔵 5.1 A Substance May Change in Appearance but Remain the Same!
Activity 5.2: Let us create and discuss
A. Creating some objects with paper
Take a few sheets of paper and fold them to create new objects.
B. Playing with a balloon
Take a balloon and inflate it. Now, loosen your grip and let the air escape out.
Take another balloon; inflate it and grip the opening tightly. Now, prick it with a pin.
C. Crushing a piece of chalk
Crush a small piece of chalk into powder.
💡 What is a Physical Change?
A physical change is a change in which the physical properties such as shape, size or state of a substance change, but no new substance is formed.
Examples:
- Melting of ice
- Freezing of water
- Cutting paper
- Folding clothes
- Chopping vegetables
- Rolling dough
- Evaporation of water
🔵 Physical Change
- No new substance is formed.
- Only physical properties change.
- Shape, size or state may change.
- Many physical changes can be reversed.
🟣 Chemical Change
- One or more new substances are formed.
- A chemical reaction occurs.
- New properties appear.
- Many chemical changes cannot be reversed easily.
🟣 5.2 A Substance May Change in Appearance and Not Remain the Same!
Activity 5.3: Let us explore
Take two glass tumblers or small transparent bottles. Mark them A and B. Fill tumbler A with tap water and tumbler B with lime water.
Blow air into each tumbler using separate straws. Do not suck the water or lime water.
🧪 Chemical Change
A chemical change is a change in which one or more new substances are formed. It involves a chemical reaction.
In the lime water experiment, carbon dioxide reacts with calcium hydroxide and produces calcium carbonate and water.
Chemical Equation
Calcium hydroxide + Carbon dioxide
↓
Calcium carbonate + Water
(Lime water) (Insoluble white substance)
The formation of calcium carbonate makes lime water appear milky. Therefore, turning lime water milky is a useful test for carbon dioxide.
Activity 5.4: Let us experiment
Take a teaspoonful of vinegar or lemon juice in a test tube. Add a pinch of baking soda.
🟤 5.3 Some Other Processes Involving Chemical Changes
5.3.1 Rusting
When iron reacts with oxygen and moisture over time, a new brown-coloured substance called rust is formed.
🔥 5.3.2 Combustion
When magnesium ribbon burns, a new substance called magnesium oxide is formed. Heat and light are also produced.
Reaction
Magnesium + Oxygen → Magnesium oxide + Heat + Light
🔥 What is Combustion?
A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called combustion.
Substances that undergo combustion are called combustible substances.
Examples: wood, paper, cotton and kerosene.
Activity 5.5: Let us investigate
Place two identical candles on two separate petri dishes and light them. Cover one candle with a glass tumbler.
🧯 Science and Society
✨ Fascinating Fact: Fireflies
Some insects called fireflies produce light through a chemical change. Light production in living organisms without heat is called bioluminescence.
🔥 Is the Presence of Air Enough for Combustion?
Paper can remain in air for a long time without catching fire. Therefore, something else is needed to start combustion.
Activity 5.6: Let us investigate
Hold a piece of paper with tongs and bring a lighted matchstick to it.
A magnifying glass can focus sunlight onto paper.
🌡️ Ignition Temperature
The minimum temperature at which a substance catches fire is called its ignition temperature.
🔥 Fire Triangle
Three things are needed for combustion:
Combustible substance + Oxygen + Heat
🕯️ 5.4 Can Physical and Chemical Changes Occur in the Same Process?
Activity 5.7: Think, pair, and share
🕯️ Burning Candle – Both Types of Changes
| Change | Type |
|---|---|
| Melting of wax | Physical change |
| Flowing of wax | Physical change |
| Evaporation of wax | Physical change |
| Solidification of wax | Physical change |
| Burning of wax vapour | Chemical change |
Thus, the burning of a candle involves both physical and chemical changes.
👨🔬 Know a Scientist – Michael Faraday
Michael Faraday studied many areas of science and gave a famous series of lectures called Chemical History of a Candle. He used a candle to explain scientific ideas such as melting, vapourisation and combustion.
🔄 5.5 Are Changes Permanent?
Activity 5.8: Let us think
| Change | Can the original state be brought back? |
|---|---|
| Melting ice cubes | Yes |
| Chopping vegetables | No |
| Boiling water | Yes, by condensation |
| Making popcorn from corn | No |
🔄 Reversible Changes
Changes in which the original substance or object can be obtained again.
Examples:
- Melting ice
- Freezing water
- Evaporation of water
- Condensation of water vapour
🚫 Irreversible Changes
Changes in which the original substance or object cannot be obtained again easily.
Examples:
- Making popcorn
- Chopping vegetables
- Cooking food
- Rusting iron
👍 5.6 Are All Changes Desirable?
Desirable Changes
Many useful changes happen in our daily life.
- Milk changing into curd
- Ripening of fruits
- Cutting fruits
- Cooking food
- Making compost from food waste
Undesirable Changes
- Rusting of iron
- Decay of food during storage
- Pollution caused by excessive fuel consumption
A change that is undesirable in one situation may be useful in another. For example, decomposition of food waste can help make compost.
🌍 Environmental Impact
Burning large amounts of fuels in cars, trains and aeroplanes increases the amount of carbon dioxide in the atmosphere.
Drying paint can also release substances into the atmosphere through evaporation, contributing to air pollution.
🏔️ 5.7 Some Slow Natural Changes
5.7.1 Weathering of Rocks
Large rocks can slowly break into smaller pieces. These smaller materials may collect as sediments.
Causes of Weathering
- Changes in temperature
- Growing roots of trees
- Freezing of water inside cracks
- Water
- Chemicals present in water
🌊 5.7.2 Erosion
Erosion is the process in which rock pieces, soil and sediments are broken down and moved from one place to another by natural forces such as wind and flowing water.
📌 In a Nutshell
- A physical change changes physical properties and forms no new substance.
- A chemical change forms one or more new substances.
- Combustion, cooking and rusting are examples of chemical changes.
- Combustible substances undergo combustion.
- Heat and/or light may be produced during combustion.
- The lowest temperature at which a substance catches fire is called its ignition temperature.
- Some changes can be reversed and some cannot.
- Some changes are desirable and some are undesirable.
- Weathering involves physical and chemical changes in rocks.
- Erosion caused by flowing water and wind is mainly a physical change.
📝 Let Us Enhance Our Learning
(i) The state of the substance may or may not change.
(ii) A substance with different properties is formed.
(iii) No new substance is formed.
(iv) The substance undergoes a chemical reaction.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv)
A physical change may change the state, but no new substance is formed.
(i) Stitching cloth to a shirt
(ii) Twisting of straight string
(iii) Making idlis from a batter
(iv) Dissolving sugar in water
(v) Drawing water from a well
(vi) Ripening of fruits
(vii) Boiling water in an open pan
(viii) Rolling up a mat
(ix) Grinding wheat grains to flour
(x) Forming of soil from rocks
| Change | Reversible? |
|---|---|
| Stitching cloth to a shirt | No |
| Twisting of straight string | Yes, generally |
| Making idlis from a batter | No |
| Dissolving sugar in water | Yes, by evaporation |
| Drawing water from a well | Yes |
| Ripening of fruits | No |
| Boiling water in an open pan | Yes, by condensation |
| Rolling up a mat | Yes |
| Grinding wheat grains to flour | No |
| Forming of soil from rocks | No |
(i) Melting of wax is necessary for burning a candle. (True/False)
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
(iii) The process of converting leaves into compost is a chemical change. (True/False)
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
(i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ________, and this is a ________ change.
(ii) Folding a handkerchief is a _________________ change and can be________.
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called _________, and this is a __________ change.
(iv) Magnesium, when burnt in air, produces a substance called __________. The substance formed is _________ in nature. Burning of magnesium is a _________ change.
Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change).
Chopping vegetables – Physical change
Collecting waste – Physical change
Decomposition into compost – Chemical change
Germination and growth – Chemical change
Suitable title: “Prithvi’s Eco-Friendly Garden”
Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.
Tearing of paper, Melting of ice, Folding of clothes
B – Chemical changes:
Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar
C – Both physical and chemical changes:
Burning of a candle
(a) Vinegar and baking soda
(b) Vinegar and common salt
(c) Lemon juice and vinegar
(d) Lemon juice and baking soda
Lime water turns milky in (a) and (d).Vinegar + baking soda and lemon juice + baking soda produce carbon dioxide gas. Carbon dioxide turns lime water milky.
🔎 Exploratory Projects
🍋 Project 1: Invisible Lemon Message
Write a message on paper using lemon juice and let it dry. The message becomes invisible. Carefully warm the paper. The letters become dark brown.
Safety: Perform this activity under adult supervision.
🏔️ Project 2: Landslides and Rock Erosion
Discuss how landslides and rock erosion can be reduced.
- Plant more trees.
- Protect vegetation on slopes.
- Improve drainage of rainwater.
- Avoid unnecessary cutting of hills.
- Use suitable slope protection methods.
🍳 Project 3: Changes in the Kitchen
Observe kitchen activities and record changes that can be reversed. Identify whether they are physical or chemical changes.
| Kitchen Change | Type | Can it be reversed? |
|---|---|---|
| Melting butter | Physical | Yes |
| Boiling water | Physical | Yes |
| Cooking vegetables | Chemical | No |
| Making curd | Chemical | No |
🍞 Project 4: How Does Yeast Work?
Take a small bottle, sugar, fresh yeast, water and a balloon. Prepare a sugar solution, add yeast and cover the bottle with the balloon. Leave it for about an hour.
🦎 Project 5: Chameleons
Chameleons can change colour to blend with their surroundings and also in response to certain situations.
⭐ Quick Revision for Students
🔵 Physical
No new substance.
Example: melting ice.
🟣 Chemical
New substance formed.
Example: rusting.
🔥 Combustion
Substance reacts with oxygen and gives heat/light.
🌡️ Ignition
Minimum temperature at which a substance catches fire.
🔄 Reversible
Original form can be obtained again.
🚫 Irreversible
Original form cannot be obtained easily.
📖 Important Keywords
| Term | Easy Meaning |
|---|---|
| Physical Change | Change in physical properties without formation of a new substance. |
| Chemical Change | Change in which a new substance is formed. |
| Chemical Reaction | Process through which new substances are formed. |
| Combustion | Reaction with oxygen that produces heat and/or light. |
| Combustible Substance | A substance that can undergo combustion. |
| Ignition Temperature | Minimum temperature at which a substance catches fire. |
| Weathering | Physical and chemical breakdown of rocks. |
| Erosion | Breaking and movement of soil, rocks and sediments by natural forces. |
| Bioluminescence | Production of light by living organisms through a chemical process. |
🧠 Easy Memory Tricks
Physical Change = Same Substance
P = Physical = Properties change
Remember: No New Substance!
Chemical Change = New Substance
C = Chemical = Composition changes
Remember: New Substance is formed!
🔥 Fire Triangle
Fuel + Oxygen + Heat = Combustion
Prepare for your exams with this easy and student-friendly Class 7 Science Chapter 5 worksheet. Practice the questions carefully to revise important concepts, strengthen your understanding, and improve your Science skills.
Class 7 Science Chapter 5 Worksheet
Practice worksheet for Class 7 Science Chapter 5 with important questions, extra practice and answer key.
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