Laws of Chemical Combination

• Law of conservation of mass (Lavoisier) — mass neither created nor destroyed in chemical reaction.
• Law of constant proportions (Proust) — elements in compound always in same mass ratio.
• Dalton's atomic theory — atoms indivisible (partially updated later); explain laws.
• Balancing equations reflects conservation of mass and atoms.

1. Conservation of mass — total mass of reactants = total mass of products (closed system).
2. Constant proportions — e.g. water always 1:8 H:O by mass (2:16 = 1:8).
3. Dalton's postulates — atoms of same element identical; compounds from fixed atom ratios.
4. Limitations — atoms divisible (subatomic particles); isotopes exist.
5. Diagram (text) — closed flask before/after reaction: same total mass on balance.
6. NCERT Ch. 3 — magnesium burning in air (compare mass carefully).
7. Real world — stoichiometry in cooking recipes; industrial chemical yields.
8. Balancing — adjust coefficients, never subscripts.

Verifying conservation of mass: Zn + H₂SO₄ → ZnSO₄ + H₂ (conceptual)

Step 1 — Reactants: Zn (65 u) + H₂SO₄ (98 u) → total 163 u (simplified atomic masses).
Step 2 — Products: ZnSO₄ (161 u) + H₂ (2 u) → total 163 u.
Step 3 — Same number of each atom type on both sides.
Step 4 — In lab: weigh flask before/after in closed system — mass unchanged.
Conclusion: atoms rearrange; none created or destroyed.

• Changing subscripts to balance equations (only coefficients allowed).
• Misconception: mass not conserved if gas escapes open container (appears lost).
• Confusing law of multiple proportions with constant proportions (Class 9: constant only).
• Thinking atoms can be created in ordinary chemical reactions.
• Ignoring isotopes when discussing Dalton's identical atoms.

• State the law of conservation of mass.
• State the law of constant proportions with water example.
• Why must chemical equations be balanced?
• Name the scientist associated with each law.

Open the Practice tab for graded questions on Laws of Chemical Combination.