General Science Notes Chapter 13 Class 7 Maharashtra Board

Notes For All Chapters – General Science Class 7

Changes – Physical and Chemical

1. Introduction

• Changes happen everywhere around us. Some changes are temporary, while others are permanent.
• Changes can affect the shape, size, color, or state of a substance.

2. Types of Changes

There are two main types of changes:

(A) Physical Changes

✔ A physical change is when only the appearance or state changes, but no new substance is formed.
✔ These changes are usually reversible.

Examples of Physical Changes:

• Melting of ice → Ice turns to water but remains H₂O.
• Breaking of glass → The shape changes, but it is still glass.
• Folding paper → The paper remains the same but changes shape.

(B) Chemical Changes

✔ A chemical change creates a new substance with different properties.
✔ These changes are usually irreversible.

Examples of Chemical Changes:

• Burning of paper → It turns into ash and smoke, which cannot be reversed.
• Rusting of iron → A new reddish-brown substance (rust) forms.
• Cooking food → The food undergoes changes in taste and texture.

3. Reversible and Irreversible Changes

4. Natural and Man-Made Changes

✔ Natural Changes → Happen on their own, without human effort.
✔ Man-Made Changes → Occur due to human activities.

Examples:

5. Fast and Slow Changes

✔ Fast Changes → Occur quickly in a short time.
✔ Slow Changes → Take a long time to happen.

Examples:

6. Rusting of Iron (Corrosion)

✔ Rusting is a chemical reaction between iron, water, and air, forming rust.
✔ It makes iron weak and brittle.

How to Prevent Rusting?

✔ Painting or oiling metal objects.
✔ Galvanization → Coating iron with zinc to prevent rust.

7. Importance of Changes in Daily Life

✔ Physical and chemical changes are important for survival.
✔ Some changes are useful, while others can be harmful.