Changes Around Us: Physical and Chemical
Explore the many changes occurring around us and learn to distinguish physical and chemical changes using observations, experiments and scientific reasoning.
Chapter Snapshot
- Changes occur continuously in living and non-living things around us.
- In a physical change, no new substance is formed.
- A physical change may alter the shape, size or state of a substance.
- In a chemical change, one or more new substances are formed.
- Formation of gas, colour change, heat or light may provide evidence of a chemical change.
- Rusting and combustion are examples of chemical changes.
- Combustion requires a combustible substance, oxygen and sufficient heat.
- Some processes, such as burning a candle, involve both physical and chemical changes.
- Changes may also be classified according to whether they can be reversed.
- A change can be desirable in one situation and undesirable in another.
What You Will Learn
- Observe and describe different changes taking place around us
- Define physical and chemical changes accurately
- Differentiate between physical and chemical changes using evidence
- Identify whether a new substance has formed during a change
- Explain the reaction of carbon dioxide with lime water
- Recognise rusting and combustion as chemical changes
- Explain the conditions required for combustion
- Define ignition temperature and combustible substance
- Explain how physical and chemical changes can occur together
- Classify changes as reversible or irreversible
- Distinguish between desirable and undesirable changes
- Apply the concepts of physical and chemical change to everyday situations
Chapter Overview
Changes are happening around us all the time.
An ice cube melts. Wet clothes dry. A fruit ripens. Wood burns. Iron develops rust. Dough is rolled into a chapati. Milk turns into curd. A candle melts while burning.
All these situations involve change, but the changes are not of the same kind.
Sometimes only the appearance, shape, size or physical state of a material changes and the material itself remains the same. In other situations, one or more completely new substances are formed.
The most important question in this chapter is therefore:
Did the substance only change its physical form, or was a new substance formed?
Detailed Explanation
1. Changes Are Everywhere
Consider some everyday examples:
- melting an ice cube;
- chopping vegetables;
- boiling water;
- making popcorn;
- cutting paper;
- drying wet clothes;
- burning wood;
- ripening a banana;
- folding clothes;
- making dough balls;
- rolling dough into chapatis.
In each case, something changes.
The change may involve:
- shape;
- size;
- state;
- colour;
- smell;
- temperature;
- texture;
- formation of gas;
- production of light;
- formation of a new material.
Observing carefully is the first step towards understanding the nature of a change.
2. Physical Changes
Imagine folding a sheet of paper into an aeroplane.
The shape has changed, but the material is still paper.
Now crush a piece of chalk.
Its size and shape change, but the chalk remains the same substance.
Similarly, ice can melt into liquid water without forming a new substance.
Examples of physical changes
- melting ice;
- freezing water;
- boiling water;
- condensation of water vapour;
- folding paper;
- cutting paper;
- crushing chalk;
- stretching a rubber band within its elastic limit;
- rolling dough;
- dissolving sugar in water.
What can change during a physical change?
A physical change may involve a change in:
- shape;
- size;
- state;
- arrangement;
- appearance.
The chemical identity of the substance remains the same.
Show answer
3. Chemical Changes
Some changes are very different.
If wood burns, we cannot say that only its shape or state has changed. New substances such as gases and ash are produced.
When milk turns into curd, substances with properties different from the original milk are formed.
A chemical change is also associated with a chemical reaction.
Possible observations during chemical changes include:
- formation of a gas;
- colour change;
- production or absorption of heat;
- production of light;
- formation of a solid from solutions;
- change in smell;
- formation of substances with new properties.
4. Carbon Dioxide and Lime Water
One useful way to identify carbon dioxide is its reaction with lime water.
When we breathe out through lime water, the solution gradually turns milky.
Why?
The air we exhale contains carbon dioxide.
Carbon dioxide reacts with lime water and forms a white substance called calcium carbonate.
Carbon Dioxide and Lime Water
[ CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O ]
where:
- (CO_2) = carbon dioxide;
- (Ca(OH)_2) = calcium hydroxide in lime water;
- (CaCO_3) = calcium carbonate;
- (H_2O) = water.
The formation of calcium carbonate makes the lime water appear milky.
The lime-water test is an important Class 7 concept.
Remember:
Carbon dioxide + lime water → milky appearance
The milkiness is caused by the formation of calcium carbonate.
5. Vinegar and Baking Soda
Another useful chemical change can be observed by mixing vinegar with baking soda.
The mixture begins to fizz and bubbles appear.
These bubbles contain carbon dioxide gas.
Vinegar and Baking Soda
At Class 7 level, the reaction can be represented simply as:
The formation of carbon dioxide shows that a new substance has formed.
Show answer
6. Physical Change vs Chemical Change
Physical Change vs Chemical Change
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance | Not formed | One or more new substances are formed |
| Chemical identity | Remains the same | Changes |
| Common changes | Shape, size or state | Composition and properties change |
| Reversibility | May or may not be reversible | Often difficult to reverse |
| Example | Melting ice | Burning wood |
| Another example | Cutting paper | Rusting iron |
7. Rusting — A Chemical Change
You studied rusting while learning about metals.
When iron remains exposed to suitable conditions involving air and moisture, a reddish-brown substance called rust develops.
Rust has properties different from iron.
Therefore, rusting involves the formation of a new substance and is a chemical change.
8. Combustion
Burning is another important chemical process.
When magnesium burns in oxygen, magnesium oxide is formed.
Heat and bright light are also produced.
Burning Magnesium
[ 2Mg + O_2 \rightarrow 2MgO ]
where:
- (Mg) = magnesium;
- (O_2) = oxygen;
- (MgO) = magnesium oxide.
A new substance is formed, so burning magnesium is a chemical change.
Examples include:
- wood;
- paper;
- cotton;
- kerosene;
- cooking gas;
- petrol.
9. Oxygen Is Necessary for Combustion
Consider a burning candle.
If it is covered completely with a glass tumbler, it burns for a short time and then goes out.
Why?
The oxygen available inside the covered space gradually becomes insufficient to support continued combustion.
10. Ignition Temperature
A sheet of paper can remain in air for a long time without burning.
Air contains oxygen, and paper is combustible.
Why does it not burn continuously?
Because the paper must first become sufficiently hot.
A combustible substance does not necessarily burn simply because oxygen is present.
It must also reach its ignition temperature.
11. The Fire Triangle
Three important requirements are needed for combustion:
- Fuel — a combustible substance;
- Oxygen — usually supplied by air;
- Heat — sufficient to reach the ignition temperature.
For the fire triangle, remember:
- Fuel
- Oxygen
- Heat
Heat must raise the substance to its ignition temperature.
Show answer
12. Fire Safety and Cutting Off Oxygen
If a fire can be safely controlled by cutting off its oxygen supply, covering it appropriately may help extinguish it.
For example, fire safety guidance may involve smothering certain small fires using a suitable fire blanket.
Real fires can be extremely dangerous.
Students should:
- move away from danger;
- alert an adult immediately;
- follow school or building fire procedures;
- never use water on electrical or oil fires unless trained guidance specifically requires it;
- never attempt to fight a spreading fire.
Scientific understanding should support safe behaviour, not risky experimentation.
13. Light Produced by Chemical Changes
Some chemical changes can produce light.
Fire is an obvious example, but light does not always have to be associated with intense heat.
14. Can Physical and Chemical Changes Occur Together?
Yes.
A single process may contain several changes.
A burning candle is an excellent example.
What happens to candle wax?
When a candle burns:
- solid wax near the flame melts;
- liquid wax moves through the wick;
- some wax changes into vapour;
- wax vapour burns and produces new substances.
Which are physical changes?
- melting of wax;
- solidification of melted wax;
- change of wax into vapour.
These changes involve changes in physical state.
Which is the chemical change?
Burning of wax vapour is a chemical change because new substances are formed.
Show answer
15. Can Changes Be Reversed?
Another way to classify changes is to ask whether the original object or state can be restored.
Examples include:
- melting and freezing water;
- inflating and deflating an undamaged balloon;
- folding and unfolding paper;
- rolling and unrolling a mat.
Examples include:
- burning wood;
- ripening fruit;
- making popcorn;
- curdling milk;
- cutting vegetables;
- rusting iron.
Reversible and Irreversible Changes
| Change | Can Original Form Be Restored? | Physical or Chemical? |
|---|---|---|
| Melting ice | Yes, by freezing | Physical |
| Boiling water | Yes, by condensation | Physical |
| Folding paper | Usually yes | Physical |
| Cutting paper | Original sheet cannot easily be restored | Physical |
| Rusting iron | Not by a simple reversal | Chemical |
| Burning wood | No | Chemical |
16. Are All Changes Desirable?
Changes can also be considered according to whether they are useful or harmful in a particular situation.
Examples include:
- cooking food;
- ripening fruits;
- milk changing into curd;
- germination of seeds;
- decomposition of kitchen waste to make compost.
Examples include:
- rusting of machinery;
- spoilage of stored food;
- uncontrolled burning;
- damage caused by erosion.
Context matters
A change may be undesirable in one situation but useful in another.
Food decomposition in a refrigerator is unwanted.
However, controlled decomposition of biodegradable waste in a compost pit is useful because it produces compost.
17. Changes and the Environment
Human activities can also cause long-term changes in the environment.
Burning large quantities of fuels can increase the amount of carbon dioxide released into the atmosphere.
Some paints and other materials can release substances into air while drying.
Therefore, studying changes is not limited to laboratory reactions.
It also helps us understand:
- pollution;
- waste management;
- composting;
- combustion;
- material deterioration;
- environmental protection.
Organising the Chapter
Concept Map
Important Equations
Formula / Key Relations
Carbon Dioxide with Lime Water
[ CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O ]
Carbon dioxide reacts with lime water to form calcium carbonate, producing a milky appearance.
Burning Magnesium
[ 2Mg + O_2 \rightarrow 2MgO ]
Magnesium reacts with oxygen to form magnesium oxide.
Exam Focus
Important Exam Areas
- definition of physical change;
- definition of chemical change;
- new substance formation as evidence of chemical change;
- examples of physical and chemical changes;
- physical changes that are not easily reversible;
- carbon dioxide and lime-water test;
- vinegar and baking soda reaction;
- rusting as a chemical change;
- combustion;
- combustible substances;
- role of oxygen in combustion;
- ignition temperature;
- three requirements of the fire triangle;
- candle burning as both physical and chemical change;
- reversible and irreversible changes;
- desirable and undesirable changes;
- application of these ideas to everyday situations.
Key Terms
Key Terms
Quick Revision
Quick Revision
- Changes occur continuously around us.
- A physical change does not produce a new substance.
- Shape, size or state may change during a physical change.
- Melting, freezing and boiling of water are physical changes.
- Cutting paper is physical even though it is not easily reversed.
- A chemical change produces one or more new substances.
- Gas formation, heat, light or colour change may provide clues of chemical change.
- Carbon dioxide turns lime water milky by forming calcium carbonate.
- Vinegar and baking soda react to produce carbon dioxide.
- Rusting is a chemical change.
- Combustion is a chemical process involving reaction with oxygen and release of heat and/or light.
- A combustible substance can undergo combustion.
- Ignition temperature is the minimum temperature at which a substance catches fire.
- Combustion requires fuel + oxygen + sufficient heat.
- Candle burning involves both physical and chemical changes.
- Reversibility is different from physical-versus-chemical classification.
- Some physical changes are irreversible.
- Desirable and undesirable changes depend on context.
- Compost formation can be useful even though decomposition of stored food is undesirable.
- The best test for a chemical change is evidence that a new substance has formed.
Practice Questions
A. Multiple Choice Questions
Which of the following is a physical change?
A. Rusting iron B. Burning wood C. Melting ice D. Curdling milk
View Solution
Correct Answer: C. Melting ice
Melting changes water from solid to liquid, but no new substance is formed.
The most important characteristic of a chemical change is:
A. change in shape B. change in size C. formation of a new substance D. change in position
View Solution
Correct Answer: C. formation of a new substance
A chemical change produces one or more substances with properties different from the original substances.
Which gas turns lime water milky?
A. Oxygen B. Carbon dioxide C. Nitrogen D. Hydrogen
View Solution
Correct Answer: B. Carbon dioxide
Carbon dioxide reacts with lime water to form calcium carbonate.
Which statement about physical changes is correct?
A. All physical changes are reversible. B. A new substance is always formed. C. No new substance is formed. D. Physical changes always produce heat.
View Solution
Correct Answer: C. No new substance is formed.
Some physical changes are reversible and some are not easily reversible, but they do not form a new substance.
Which of the following is required for combustion?
A. Fuel only B. Oxygen only C. Heat only D. Fuel, oxygen and sufficient heat
View Solution
Correct Answer: D. Fuel, oxygen and sufficient heat
These three requirements form the fire triangle.
The minimum temperature at which a substance catches fire is called:
A. boiling point B. melting point C. ignition temperature D. freezing point
View Solution
Correct Answer: C. ignition temperature
A combustible material begins burning only after reaching its ignition temperature.
Which process involves both physical and chemical changes?
A. Melting ice B. Burning a candle C. Folding paper D. Crushing chalk
View Solution
Correct Answer: B. Burning a candle
Melting and evaporation of wax are physical changes, while combustion of wax vapour is a chemical change.
Which change is chemical?
A. Cutting vegetables B. Boiling water C. Rolling a mat D. Ripening a fruit
View Solution
Correct Answer: D. Ripening a fruit
Ripening involves chemical changes that produce substances with different properties, colours, tastes and smells.
B. Very Short Answer Questions
-
Define a physical change.
-
Define a chemical change.
-
Give two examples of physical changes.
-
Give two examples of chemical changes.
-
What happens when carbon dioxide is passed through lime water?
-
What is combustion?
-
What is ignition temperature?
-
Name the three requirements for combustion.
-
Give one example of a reversible change.
-
Give one example of an irreversible chemical change.
Answers
-
A physical change is one in which physical properties may change but no new substance is formed.
-
A chemical change is one in which one or more new substances are formed.
-
Melting ice and cutting paper.
-
Rusting iron and burning wood.
-
Lime water turns milky due to the formation of calcium carbonate.
-
Combustion is a chemical process in which a substance reacts with oxygen and produces heat and/or light.
-
Ignition temperature is the minimum temperature at which a substance catches fire.
-
Fuel, oxygen and sufficient heat.
-
Melting ice followed by refreezing.
-
Burning wood.
C. Short Answer Questions
Why is crushing a piece of chalk a physical change even though the original chalk piece cannot easily be restored?
View Solution
Crushing chalk changes only its shape and size.
No new substance is formed, so it is a physical change.
Reversibility is not the deciding factor for distinguishing physical and chemical changes.
Why does lime water turn milky when carbon dioxide is passed through it?
View Solution
Carbon dioxide reacts with calcium hydroxide present in lime water and forms calcium carbonate.
[ CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O ]
Calcium carbonate makes the solution appear milky.
Explain why rusting is classified as a chemical change.
View Solution
Rusting produces a new substance called rust on the surface of iron.
Because the properties of rust differ from those of iron and a new substance is formed, rusting is a chemical change.
Why does a candle covered with a glass tumbler stop burning after some time?
View Solution
Combustion requires oxygen.
A covered candle has only a limited supply of air. As the available oxygen becomes insufficient, combustion stops and the flame goes out.
Explain the difference between a reversible change and a physical change.
View Solution
A physical change is classified according to whether a new substance is formed. In a physical change, no new substance is formed.
A reversible change is classified according to whether the original form or state can be restored.
The two ideas are different.
For example, cutting paper is a physical change but is not easily reversible.
D. Long Answer Questions
Compare physical and chemical changes with suitable examples.
View Solution
Physical Change
In a physical change:
- no new substance is formed;
- shape, size or state may change;
- the chemical identity remains the same.
Examples:
- melting ice;
- boiling water;
- cutting paper;
- crushing chalk.
Chemical Change
In a chemical change:
- one or more new substances are formed;
- the new substances have properties different from the original substances.
Examples:
- rusting iron;
- burning wood;
- curdling milk;
- ripening fruit.
The most reliable distinction is therefore whether a new substance has formed.
Explain the three conditions required for combustion.
View Solution
Combustion requires three conditions:
1. Fuel
A combustible substance must be present.
Examples include wood, paper and kerosene.
2. Oxygen
Oxygen, usually supplied by air, supports combustion.
3. Sufficient Heat
The fuel must receive enough heat to reach its ignition temperature.
These three conditions form the fire triangle.
Removing one of them can stop combustion.
Explain why burning a candle involves both physical and chemical changes.
View Solution
When a candle burns, several processes occur.
Physical changes
Wax near the flame melts.
Some liquid wax later solidifies again.
Wax also changes into vapour.
These are physical changes because no new substance is formed during the change of state.
Chemical change
Wax vapour burns in oxygen.
New substances are produced during combustion.
Therefore, burning a candle involves both physical and chemical changes.
Application and HOTS Questions
A student tears a sheet of paper and says, "It cannot be changed back into the original sheet, so it must be a chemical change." Explain the error.
View Solution
The student is using reversibility as the only test.
Tearing paper changes its shape and size but does not form a new substance.
Therefore, tearing paper is a physical change, even though restoring the original sheet is difficult.
Two colourless liquids are mixed and bubbles appear. Can you immediately conclude that a chemical reaction has occurred? Explain.
View Solution
Bubble formation may indicate that a gas has formed, which can be evidence of a chemical reaction.
However, scientific conclusions should be based on careful observation.
We should determine whether the bubbles result from formation of a new gas or merely from trapped or released air.
Evidence of a new substance provides the strongest conclusion.
Why does removing oxygen help stop some fires?
View Solution
Oxygen is one of the three requirements for combustion.
Without sufficient oxygen, the combustion reaction cannot continue.
Therefore, safely cutting off the oxygen supply can extinguish some fires.
A student says, "Every desirable change is reversible." Give an example to show why this statement is incorrect.
View Solution
Cooking food is usually a desirable change, but it cannot normally be reversed to obtain all the original raw ingredients.
Similarly, changing milk into curd may be useful but is not easily reversed.
Therefore, desirability and reversibility are different ways of describing changes.
A banana changes from green to yellow, becomes softer and develops a sweeter smell. Is this merely a physical change because its colour and texture changed?
View Solution
No.
During ripening, chemical changes occur inside the fruit and new substances are produced.
The changes in colour, smell, taste and texture are observations associated with these chemical processes.
Therefore, fruit ripening is a chemical change.
Assertion–Reason
Assertion: Cutting paper is a physical change.
Reason: No new substance is formed when paper is cut.
Choose the correct option:
A. Both Assertion and Reason are true, and Reason correctly explains Assertion. B. Both are true, but Reason does not explain Assertion. C. Assertion is true, but Reason is false. D. Assertion is false, but Reason is true.
View Solution
Correct Answer: A
Cutting changes the size or shape of paper without forming a new substance.
Assertion: All physical changes are reversible.
Reason: No new substance is formed during a physical change.
Choose the correct option:
A. Both statements are true. B. Assertion is true, but Reason is false. C. Assertion is false, but Reason is true. D. Both statements are false.
View Solution
Correct Answer: C
The Reason is true: no new substance is formed during a physical change.
However, the Assertion is false because some physical changes, such as cutting paper or crushing chalk, cannot easily be reversed.
Assertion: Burning magnesium is a chemical change.
Reason: Magnesium oxide, a new substance, is formed.
Choose the correct option:
A. Both Assertion and Reason are true, and Reason correctly explains Assertion. B. Both are true, but Reason does not explain Assertion. C. Assertion is true, but Reason is false. D. Both statements are false.
View Solution
Correct Answer: A
Burning magnesium forms magnesium oxide, so the process is a chemical change.
Apply Your Learning
Observe changes taking place in a kitchen or classroom.
Choose six safe examples and complete a table like this:
| Change | Physical or Chemical? | Reversible? | Evidence | | ------------- | --------------------- | ----------- | ---------------------------------- | | Ice melting | Physical | Yes | No new substance | | Cutting fruit | Physical | No | Only size and shape change | | Cooking food | Chemical | Usually no | New substances and properties form |
Then answer:
- Which changes formed new substances?
- Which physical changes could not easily be reversed?
- Did any process contain more than one type of change?
- Which changes were desirable?
- Could any desirable change become undesirable in another situation?
Common Mistakes to Avoid
Final Chapter Summary
Chapter Summary
- Changes around us may affect shape, size, state, colour, smell or other properties.
- A physical change does not form a new substance.
- Melting, freezing, cutting and crushing can be physical changes.
- Physical changes are not necessarily reversible.
- A chemical change forms one or more new substances.
- Carbon dioxide turns lime water milky because calcium carbonate forms.
- Vinegar and baking soda react to produce carbon dioxide.
- Rusting is a chemical change because rust is a new substance.
- Combustion is a chemical reaction involving oxygen and production of heat and/or light.
- Combustion requires fuel, oxygen and sufficient heat.
- Ignition temperature is the minimum temperature required for a substance to catch fire.
- Burning a candle involves both physical and chemical changes.
- Changes may separately be classified as reversible or irreversible.
- Changes may also be desirable or undesirable depending on the situation.
- New substance formation is the most important criterion for identifying a chemical change.
- Careful observation and evidence are essential for scientifically classifying changes.