NCERT Class 7 Science Chapter 6 - Physical and Chemical Changes Notes

Chapter 6 - Physical and Chemical Changes Notes

1. The changes around us can be broadly classified as - physical and chemical change.

2. Properties like shape, size, colour, state, etc., of a substance are called its physical properties.

3. A change in which a substance experiences a change in its physical properties is known as physical change.

4. Physical change is generally reversible.

5. A change in which one or more new substances are formed is called a chemical change.

6. Chemical change is generally irreversible.

7. The process of formation of red or orange coating on the surface of iron when exposed to air and moisture, consisting chiefly of ferric hydroxide and ferric oxide, is called

8. The process of rusting can be represented by the following equation:
Iron (Fe) + Oxygen (O₂, from the air) + Water (H₂O) ➝ Rust (Fe₂O₃, Iron oxide)

9. For rusting, the presence of both oxygen and water (or water vapour) is essential.

10. The process of depositing a layer of zinc metal on iron is known as galvanisation. It protects iron from rusting.

11. Crystallization is the process of separating pure crystals of a substance from its supersaturated solution on cooling.

12. Ozone layer protects us from the harmful ultraviolet radiation which come from the sun. Ozone absorbs this radiation and breaks down to oxygen. Oxygen is different from ozone. If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms. Ozone acts as a natural shield against this radiation.

13. Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.

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