Science Class 7 - Physical And-Chemical-Changes Notes

Physical and Chemical Changes

Welcome to the chapter on Physical and Chemical Changes for Class 7. In this chapter, you will learn what physical and chemical changes are, how to identify them, and see examples from everyday life. By the end of this chapter, you will be able to tell the difference between physical and chemical changes and explain why they happen!

Introduction

Changes happen all around us. Some changes only affect how something looks, while others create new substances. These are called physical and chemical changes.

Physical Changes

A physical change is when the shape, size, or state of a substance changes, but no new substance is formed. Physical changes are usually reversible.

• Melting ice to water
• Tearing paper
• Dissolving sugar in water
• Breaking glass

Chemical Changes

A chemical change is when a new substance is formed and the change cannot be easily reversed. Chemical changes often involve color change, heat, light, or gas.

• Burning paper
• Rusting of iron
• Cooking food
• Souring of milk

Differences Between Physical and Chemical Changes

Physical Change	Chemical Change
No new substance is formed	New substance is formed
Usually reversible	Usually irreversible
Change in shape, size, or state	Change in chemical properties

Indicators of Chemical Change

• Change in color
• Formation of gas or bubbles
• Change in temperature
• Formation of a precipitate

Fun Activity: Observe Changes!

Try melting ice and burning a piece of paper (with adult help). Which is a physical change and which is a chemical change?

Summary

• Physical changes do not make new substances and are often reversible.
• Chemical changes make new substances and are usually irreversible.
• We see both types of changes in daily life.

Practice Questions

1. Is melting butter a physical or chemical change?
2. What happens when iron rusts?
3. Give two examples of chemical changes.
4. Can you reverse the burning of paper?
5. What is formed when milk turns sour?

Challenge Yourself

• List three physical changes and three chemical changes you see at home.
• Explain why cooking an egg is a chemical change.

Did You Know?

• Photosynthesis in plants is a chemical change!
• Freezing water is a physical change, but burning wood is a chemical change.

Glossary

• Physical Change: Change in appearance, no new substance formed.
• Chemical Change: Change where a new substance is formed.
• Irreversible: Cannot be changed back.
• Precipitate: Solid formed in a liquid during a chemical change.

Answers to Practice Questions

1. Physical change
2. A new substance (rust) is formed
3. Burning wood, rusting iron
4. No, it is irreversible
5. Curd (a new substance)

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Notice changes around you and try to tell if they are physical or chemical!