📘 Chapter Overview
Changes are happening all around us every day. Ice melts into water, paper is cut into pieces, food is cooked, iron rusts and fruits ripen. These changes may look very different, but they can be studied scientifically.
Some changes only affect the physical form or state of a substance, while others result in the formation of one or more new substances.
In this chapter, we learn how to identify and distinguish physical changes and chemical changes.
📑 Table of Contents
1. What Is a Change?
A change occurs when something becomes different from its original condition.
Changes may happen naturally or may be caused by human activities.
🌍 Examples of Changes Around Us
- Melting of ice
- Boiling of water
- Cutting of paper
- Ripening of fruits
- Cooking food
- Rusting of iron
- Burning of wood
Scientists classify changes in different ways depending on what happens to the substance during the change.
2. Physical Changes
A physical change is a change in which the physical properties of a substance change, but no new substance is formed.
Examples
🧊 Melting Ice
Ice changes from solid water to liquid water. The substance remains water.
📄 Cutting Paper
Cutting changes the size and shape of paper, but the paper remains the same substance.
💧 Boiling Water
Water changes from liquid to vapour without becoming a different substance.
🪨 Breaking Stone
The stone changes in size and shape, but no new substance is formed.
3. Reversible Changes
A change that can be reversed to obtain the original substance or condition is called a reversible change.
Examples
- Melting of ice followed by freezing
- Inflating and deflating a balloon
- Folding and unfolding paper
- Melting and solidifying wax under suitable conditions
4. Chemical Changes
A chemical change is a change in which one or more new substances are formed.
The new substances usually have properties different from those of the original substances.
Examples
- Rusting of iron
- Burning of paper
- Cooking food
- Digestion of food
- Ripening of fruits
- Formation of curd from milk
5. Signs of a Chemical Change
Several observations can provide clues that a chemical change may have taken place.
🎨 Change in Colour
A noticeable and permanent colour change can indicate the formation of a new substance.
💨 Formation of Gas
The production of gas during a reaction can be a sign of a chemical change.
🌡️ Change in Temperature
Some chemical reactions release or absorb heat, causing the temperature to change.
🫧 Formation of a Solid
Sometimes a new solid substance forms when two substances react. This solid may be called a precipitate.
👃 Change in Smell
A new smell can sometimes indicate that new substances have formed.
6. Irreversible Changes
An irreversible change is a change that cannot easily be reversed to obtain the original substance or condition.
Examples
- Burning of paper
- Cooking an egg
- Rusting of iron
- Curdling of milk
- Burning of wood
7. Physical Change vs Chemical Change
| Basis | Physical Change | Chemical Change |
|---|---|---|
| New substance | Generally not formed | One or more new substances are formed |
| Composition | Chemical composition remains essentially unchanged | Chemical composition changes |
| Properties | Physical properties may change | New substances have different properties |
| Examples | Melting ice, cutting paper | Rusting, burning, cooking |
8. Changes in Everyday Life
We observe both physical and chemical changes every day.
🧊 Physical Change
Ice melts into water. The state changes, but the substance remains water.
🔥 Chemical Change
Wood burns and produces new substances such as gases and ash.
✂️ Physical Change
Cutting cloth changes its size and shape without producing a new substance.
🍳 Chemical Change
Cooking an egg causes chemical changes and produces substances with different properties.
9. Rusting of Iron
Rusting is a chemical change in which iron reacts with oxygen in the presence of moisture and forms rust.
Rust is a new substance with properties different from those of iron.
Conditions Required for Rusting
- Iron
- Oxygen from air
- Moisture or water
How Can Rusting Be Prevented?
- Painting
- Oiling or greasing
- Galvanisation
- Using suitable protective coatings
10. Cooking as a Chemical Change
Cooking changes the chemical composition and properties of many food substances.
When food is cooked, new substances may form, and the original food cannot generally be returned to exactly its original state.
Examples
- Cooking rice
- Boiling an egg
- Baking bread
- Cooking vegetables
⚡ Quick Revision
📝 Important Questions
A physical change is a change in which the physical properties of a substance change without forming a new substance.
A chemical change is a change in which one or more new substances are formed.
Melting of ice and cutting of paper are examples of physical changes.
Rusting of iron and burning of wood are examples of chemical changes.
A reversible change is a change that can be reversed to obtain the original condition or substance.
An irreversible change is a change that cannot easily be reversed to obtain the original condition.
Rusting is a chemical change because iron reacts with oxygen and moisture to form a new substance called rust.
Colour change, gas formation, temperature change, formation of a solid and change in smell can provide clues of a chemical change.
Ice changes from solid water to liquid water, but no new substance is formed.
Cooking produces substances with new properties and changes the chemical composition of many food materials.
Rusting requires iron, oxygen and moisture.
In a physical change no new substance is generally formed, whereas a chemical change results in the formation of one or more new substances.
🎯 Exam Tips
- Learn the definitions of physical change and chemical change.
- Remember at least three examples of each type of change.
- Learn the difference between reversible and irreversible changes.
- Remember the main signs that may indicate a chemical change.
- Learn why rusting of iron is a chemical change.
- For classification questions, ask: “Has a new substance been formed?”
❓ Frequently Asked Questions
What is Chapter 5 of Class 7 Science?
Chapter 5 is titled “Changes Around Us: Physical and Chemical”.
What is a physical change?
A physical change changes the physical properties of a substance without forming a new substance.
What is a chemical change?
A chemical change results in the formation of one or more new substances.
Is melting of ice a physical change?
Yes. Melting ice changes its state from solid to liquid, but it remains water.
Is rusting a physical or chemical change?
Rusting is a chemical change because a new substance called rust is formed.
Is cooking food a chemical change?
Cooking is generally a chemical change because new substances with different properties are formed.
What is the easiest way to identify a chemical change?
Ask whether a new substance has been formed. Formation of a new substance is a key indication of a chemical change.