1. Laws of Chemical Combination
1.1 Law of Conservation of Mass
Key Definition
Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.
Explanation (Point-wise)
- During a chemical reaction, substances only rearrange.
- Total mass of reactants = total mass of products.
- No new matter is created, and no matter is lost.
Example
Reaction between barium chloride and sodium sulphate:
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
- Total mass before reaction = Total mass after reaction
Diagram Description
A closed flask showing reactants before reaction and products after reaction, with equal total mass indicated on both sides.
Important Points / Exam Tips
- Reaction must take place in a closed system.
- Frequently asked as definition or reasoning question.
- Supports balancing of chemical equations.
1.2 Law of Constant Proportions
Key Definition
Law of Constant Proportions states that a chemical compound always contains the same elements combined in the same proportion by mass.
Explanation (Point-wise)
- The ratio of elements in a compound is fixed.
- This ratio does not change regardless of source or method of preparation.
Example
Water (H₂O):
- Hydrogen : Oxygen = 1 : 8 by mass
Diagram Description
Molecular model of water showing two hydrogen atoms and one oxygen atom.
Important Points / Exam Tips
- Applies only to pure compounds.
- Very important 1–2 mark theory question.
- Helps in understanding chemical formulae.
2. Dalton’s Atomic Theory
Key Definition
Dalton’s Atomic Theory explains the nature of matter based on the concept of atoms.
Main Postulates (Point-wise)
- Matter is made up of tiny particles called atoms.
- Atoms are indivisible and cannot be created or destroyed.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements have different masses and properties.
- Compounds form when atoms combine in simple whole-number ratios.
- Chemical reactions involve rearrangement of atoms.
Diagram Description
Spherical atoms combining to form molecules in fixed ratios.
Important Points / Exam Tips
- Explains laws of chemical combination.
- Some postulates are modified later, but NCERT explanation is sufficient.
- Asked as list or explanation question.
3. What is an Atom?
Key Definition
An atom is the smallest particle of an element that takes part in a chemical reaction.
Explanation (Point-wise)
- Atoms are extremely small.
- Cannot be seen with naked eye.
- Represented using symbols.
Example
- Hydrogen → H
- Oxygen → O
- Carbon → C
Important Points / Exam Tips
- Atoms generally do not exist independently.
- Noble gases are an exception.
4. What is a Molecule?
Key Definition
A molecule is the smallest particle of a substance that can exist independently and shows all properties of that substance.
Types of Molecules
| Type | Explanation | Example |
|---|---|---|
| Molecules of Elements | Made of same atoms | O₂, N₂ |
| Molecules of Compounds | Made of different atoms | H₂O, CO₂ |
Diagram Description
Diatomic molecule showing two atoms joined by a bond.
Important Points / Exam Tips
- Molecules can exist independently.
- Both elements and compounds form molecules.
5. What is an Ion?
Key Definition
An ion is a charged particle formed by loss or gain of electrons.
Types of Ions
| Ion Type | Charge | Example |
|---|---|---|
| Cation | Positive | Na⁺, Ca²⁺ |
| Anion | Negative | Cl⁻, O²⁻ |
Explanation (Point-wise)
- Loss of electrons → Cation
- Gain of electrons → Anion
Important Points / Exam Tips
- Ions carry electrical charge.
- Important for writing chemical formulae.
6. Writing Chemical Formulae
Key Concept
Chemical formula represents the composition of a compound.
Steps (Point-wise)
- Write symbols of elements.
- Write valencies.
- Criss-cross valencies.
- Simplify if possible.
Example
Calcium Chloride:
- Ca²⁺ and Cl⁻
- Formula → CaCl₂
Diagram / Flowchart Description
Flowchart showing steps from symbols → valency → criss-cross → formula.
Important Points / Exam Tips
- Valency must be memorized.
- No charge is written in final formula.
- Very important for numericals.
7. Relative Atomic Mass
Key Definition
Relative atomic mass is the average mass of an atom compared to 1/12th mass of carbon-12 atom.
Explanation (Point-wise)
- Carbon-12 is the standard.
- Atomic mass has no unit.
Example
- Hydrogen = 1 u
- Oxygen = 16 u
Important Points / Exam Tips
- Written as Ar.
- Used in mole concept.
8. Mole Concept
Key Definition
A mole is the amount of substance that contains 6.022 × 10²³ particles.
Explanation (Point-wise)
- 1 mole = 6.022 × 10²³ atoms/molecules/ions
- This number is called Avogadro’s number.
Examples
- 1 mole of oxygen atoms = 6.022 × 10²³ atoms
- 1 mole of water molecules = 6.022 × 10²³ molecules
Important Points / Exam Tips
- Fundamental concept for numericals.
- Always write Avogadro’s number correctly.
9. Molar Mass
Key Definition
Molar mass is the mass of one mole of a substance.
Explanation
- Unit: g/mol
- Numerically equal to atomic or molecular mass.
Example
- Molar mass of H₂O = 2(1) + 16 = 18 g/mol
Important Points / Exam Tips
- Used in mass–mole conversions.
- Frequently asked in numericals.
10. Numericals Based on Mole Concept
Formulae to Remember
- Number of moles = Mass / Molar mass
- Number of particles = Moles × Avogadro’s number
Important Points / Exam Tips
- Units must be written properly.
- Step-wise calculation fetches full marks.
- Common in 3–5 mark questions.
Quick Revision Summary – Atoms and Molecules
- Matter follows laws of chemical combination.
- Atoms are basic building blocks of matter.
- Molecules and ions are formed from atoms.
- Chemical formulae depend on valency.
- Mole concept links mass, moles, and number of particles.
- Avogadro’s number = 6.022 × 10²³.
- Molar mass is essential for solving numericals.
