Class 7 Science

Chapter 6: Physical and Chemical Changes

                                         

STAGE 0: Foundation / Basics

Changes happen around us every day. Some changes are temporary, while some changes are permanent.

Based on their nature, changes are of two types: Physical changes and Chemical changes.

Physical Change

A physical change is a change in which no new substance is formed.

Characteristics:

• Change is temporary
• Shape or size may change
• Original substance can be obtained back

Examples:

• Melting of ice
• Cutting paper
• Dissolving sugar in water

Chemical Change

A chemical change is a change in which a new substance is formed.

Characteristics:

• Change is permanent
• New substance is formed
• Original substance cannot be obtained back

Examples:

• Burning of paper
• Rusting of iron
• Cooking food

Rusting of Iron

Rusting is a chemical change in which iron reacts with oxygen and water to form rust.

Conditions required for rusting:

• Oxygen (air)
• Water (moisture)

Rusting weakens iron objects.

Prevention of Rusting

Rusting can be prevented by:

• Painting
• Oiling or greasing
• Galvanisation

STAGE 2: NCERT Textbook Questions & Answers

Q1. What is a physical change?

A physical change is a change in which no new substance is formed.

Q2. What is a chemical change?

A chemical change is a change in which a new substance is formed.

Q3. Name the conditions required for rusting.

Presence of air and moisture.

STAGE 3: Exam-Oriented Questions

Fill in the blanks:

• Rusting is a chemical change.
• Melting of ice is a physical change.

Very Short Answer:

Name one method to prevent rusting.
Answer: Painting

Quick Revision Points

• Physical change → no new substance
• Chemical change → new substance formed
• Rusting is a chemical change
• Rusting needs air and water