CHAPTER- 5 PHYSICAL AND CHEMICAL CHANGES
NOTES
1. In nature, two types of changes are widely observed: physical and chemical.
- Physical Change- The properties of a substance such as shape, size, color, and state are known as its physical properties. A change in which the physical properties of a substance are altered is generally reversible. No new substance is formed during such a change.
Example- Making a paper airplane from a piece of paper.
- Chemical Change- A change in which one or more new substances are formed is called a chemical change. Chemical changes are also referred to as chemical reactions.
2.In a chemical change, one or more new substances are formed. In addition to the new products, the following events occur during a chemical change-
- Release or absorption of heat, light, or any other form of radiation.
- Generation of sound.
- Change in smell or the formation of a new smell.
- Formation of a gas.
- Change in color.
3. Rusting of iron- Rust forms on iron due to the presence of oxygen and water vapor.
Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Formation of Iron Oxide (Fe₂O₃)
- For rusting, the presence of both oxygen and water (water vapor) is essential. To protect iron objects from rust, a regular layer of paint or grease should be applied on all items. Another method is to coat the iron with a layer of metals like chromium or zinc.
- Galvanization- The process of coating iron with a layer of zinc is called galvanization.
4. Stainless Steel- It is made by mixing carbon with metals like chromium, nickel, and manganese. It does not rust.
5. Crystallization- The process of obtaining pure crystals from a solution of a substance is called crystallization. It is a physical change.
