Subject: General Science
Book: NCERT Science Class 9
Atoms are the basic units of matter.
- **Laws of Chemical Combination**:
- Law of Conservation of Mass.
- Law of Constant Proportion.
- **Concept of Molecules**:
- Molecules of elements: O₂, N₂.
- Molecules of compounds: H₂O, CO₂.
- **Chemical Formulae**:
- Represent the composition of a compound (H₂O for water).
- **Mole Concept**:
- 1 mole = 6.022 × 10²³ particles.
**Possible Exam Questions:**
1. State the laws of chemical combination.
2. Differentiate between atoms and molecules.
3. What is the significance of a chemical formula?
4. How is the mole concept used in chemistry?
5. Explain Avogadro’s number with an example.
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