Class 7 Science Curiosity Solutions English Medium

NCERT Solutions Class 7 Science Curiosity English Medium

Chapter 5. Changes Around Us–Physical and Chemical

Topic: Important Notes

Chapter 5: Changes Around Us – Physical and Chemical

Introduction: In our daily life, we observe many changes happening around us. Some changes are temporary, while others are permanent. For example, melting of ice, cooking food, burning paper, and rusting of iron are all different types of changes.

To understand these changes properly, scientists classify them into two main types — physical changes and chemical changes. This classification helps us understand the nature and effect of changes.

This chapter covers the following topics:

• Types of changes
• Physical changes and their properties
• Chemical changes and their properties
• Reversible and irreversible changes
• Examples from daily life
• Difference between physical and chemical changes

What is a Change? A change is any alteration that takes place in an object or substance. It may involve a change in shape, size, state, or composition.

Example: When ice melts into water, it changes its state from solid to liquid.

Physical Changes: A physical change is a change in which only the physical properties of a substance are altered, and no new substance is formed.

• Change in shape, size, or state
• No new substance is formed
• Usually reversible

Example: Melting of ice, tearing paper, dissolving sugar in water.

Chemical Changes: A chemical change is a change in which a new substance is formed with different properties.

• New substance is formed
• Usually irreversible
• Often involves energy change

Example: Burning of paper, rusting of iron, cooking food.

Key Point: In a physical change, the substance remains the same, while in a chemical change, a new substance is formed.

Reversible Changes: A reversible change is a change that can be undone and the original substance can be obtained again. In such changes, no new substance is formed.

Example: When ice melts into water and then freezes again, it returns to its original form. This shows that the change is reversible.

• Original form can be restored
• No new substance is formed
• Mostly physical changes

Irreversible Changes: An irreversible change is a change that cannot be undone. Once the change takes place, the original substance cannot be obtained back.

Example: When paper is burnt, it turns into ash and cannot be converted back into paper.

• Original form cannot be restored
• New substance is formed
• Mostly chemical changes

How to Identify Physical and Chemical Changes: Sometimes it becomes difficult to identify whether a change is physical or chemical. Some important points help us understand this.

Indicators of Physical Change:

• No new substance is formed
• Change is temporary
• Can be reversed easily
• Only physical properties change

Indicators of Chemical Change:

• New substance is formed
• Change is permanent
• Cannot be reversed easily
• Often accompanied by heat, light, gas, or colour change

Example: When milk turns into curd, a new substance is formed, so it is a chemical change.

Changes in Daily Life: We observe both types of changes in our surroundings.

• Cutting vegetables – Physical change
• Cooking food – Chemical change
• Boiling water – Physical change
• Burning fuel – Chemical change

Understanding these changes helps us use materials properly and safely.

Key Point: Reversible changes are usually physical, while irreversible changes are mostly chemical.

Detailed Comparison between Physical and Chemical Changes: To clearly understand the difference, it is important to compare both types of changes side by side.

• Physical Change: No new substance is formed, while Chemical Change forms a new substance.
• Physical Change: Usually reversible, while Chemical Change is mostly irreversible.
• Physical Change: Only physical properties change, while Chemical Change changes chemical composition.
• Physical Change: No energy change or very little, while Chemical Change often involves heat, light, or gas.

Real-Life Importance of Changes: Understanding changes helps us in many practical situations.

• Helps in cooking food safely
• Helps in preserving materials
• Helps in preventing unwanted changes like rusting
• Helps in industrial processes

Example: Food preservation methods like refrigeration slow down chemical changes, preventing spoilage.

Common Mistakes (Exam Traps):

• All reversible changes are physical — ❌ (mostly true but not always)
• All chemical changes involve heat — ❌ (not necessary in all cases)
• Dissolving always means chemical change — ❌ (usually physical)

Overall Understanding: Changes around us can be physical or chemical. Knowing their differences helps us understand nature and use materials effectively. Both types of changes are important and have different applications in our daily life.

Key Point: The formation of a new substance is the most important factor in identifying a chemical change.

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