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NCERT Solutions for Class 9th Science Chapter 9 Atomic Foundations of Matter

Updated on 2026-09-19

About this chapter

Law of Conservation of Mass (Lavoisier, 1789): in a chemical reaction matter is neither created nor destroyed, so the total mass of the reactants equals the total mass of the products. It only looks violated in an open vessel, because a gaseous product walks off the balance pan. Law of Constant Proportions (Proust): a compound always contains the same elements in the same fixed ratio by mass, whatever its source. Pure water from a river, a borewell or the ocean is always H : O = 1 : 8 by mass. Dalton's atomic theory explains both laws with one picture — atoms are indivisible in a chemical reaction and merely rearrange, atoms of one element are identical in mass, and atoms combine in simple whole-number ratios. Atoms combine to reach a stable octet (a duplet for the K-shell). They do it in

  • Chapter opener
  • Let us investigate a physical change
  • Let us investigate a chemical change
  • Let us verify the law
  • .1 Law of Conservation of Mass
  • .2 Law of Constant Proportions
  • .3 Dalton's Atomic Theory
  • .4.1 A Molecules of elements
  • .4.1 B Molecules of compounds
  • .4.2 Bonding by electron transfer
Quick revision
TermWhat it meansWorked exampleWhere it appears
Law of Conservation of MassTotal mass of reactants = total mass of products in a chemical reaction4.0 g CaCO₃ + 2.92 g HCl → 1.76 g CO₂ + 0.72 g H₂O + 4.44 g CaCl₂; 6.92 g = 6.92 g9.1, Activity 9.2, Example 9.1
Law of Constant ProportionsA compound has a fixed composition by mass, whatever its source9 g of pure water always gives 1 g H and 8 g O — ratio 1 : 89.2, Example 9.3
PostulateA basic assumption accepted without formal proof, from which further ideas are built“Atoms are indivisible particles” — Dalton9.3 Dalton's Atomic Theory
MoleculeElectrically neutral entity of more than one atom that can exist independently and shows all the properties of the substanceH₂, Cl₂, O₂, HCl, H₂O9.4
Covalent bondBond formed by a shared pair of electrons between two atomsH—H (single), O=O (double), N≡N (triple)9.4.1
Ionic bondElectrostatic force of attraction between oppositely charged ions formed by electron transferNa → Na⁺ + e⁻; Cl + e⁻ → Cl⁻; Na⁺Cl⁻9.4.2
Cation / anionPositive ion (lost electrons) / negative ion (gained electrons)Na⁺ (11 p⁺, 10 e⁻); Cl⁻ (17 p⁺, 18 e⁻); S²⁻9.4.2, Table 9.1
Polyatomic ionAn ion made of atoms of two or more elementsOH⁻, NO₃⁻, HCO₃⁻, CO₃²⁻, SO₃₃₂⁻, NH₄⁺Table 9.1 (b)
Criss-cross methodSwap the valencies (or charge numbers) and write them as subscripts; then divide by any common factorAl³⁺, O²⁻ → Al₂O₃; Mg²⁺, O²⁻ → Mg₂O₂ = MgO9.5
Prefix naming (covalent)mono-, di-, tri-, tetra-, penta-, hexa- show how many atoms; second element ends in -ideCO carbon monoxide, PCl₃ phosphorus trichloride, SF₆ sulfur hexafluoride9.4.1 C
Crystal latticeRegular repeating 3-D arrangement of ions in an ionic solidIn NaCl each Na⁺ is surrounded by 6 Cl⁻ and each Cl⁻ by 6 Na⁺Threads of Curiosity, p. 173
Molecular massSum of the atomic masses of all atoms in one moleculeCO₂ = (12 u × 1) + (16 u × 2) = 44 u9.7
Formula unit massSum of the atomic masses of all atoms in one formula unit of an ionic compoundNa₂O = (23 u × 2) + (16 u × 1) = 62 u9.8
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Exercises

  1. Chapter opener — Think It Over Page 162
  2. Let us investigate a physical change — Activity 9.1 Page 163
  3. Let us investigate a chemical change — Activity 9.2 Page 163 – 165
  4. Let us verify the law — Activity 9.3 Page 165
  5. .1 Law of Conservation of Mass — Think as a Scientist Page 1669
  6. .1 Law of Conservation of Mass — Pause and Ponder Page 1669
  7. .2 Law of Constant Proportions — Pause and Ponder Page 1679
  8. .3 Dalton's Atomic Theory — What if … Page 1689
  9. .3 Dalton's Atomic Theory — Pause and Ponder Page 1689
  10. .4.1 A Molecules of elements — Pause and Ponder Page 1709
  11. .4.1 B Molecules of compounds — Pause and Ponder Page 1719
  12. .4.2 Bonding by electron transfer — In-text Questions Page 1729
  13. .4.2 Bonding by electron transfer — What if … Page 1749
  14. .4.2 Bonding by electron transfer — Pause and Ponder Page 1749
  15. .5 Writing Chemical Formulae — Pause and Ponder Page 1779
  16. .6 Properties of the Ionic and the Covalent Compounds — Activity 9.4 Page 177 – 1789
  17. .6 Properties of the Ionic and the Covalent Compounds — In-text Questions Page 1789
  18. .6 Properties of the Ionic and the Covalent Compounds — Pause and Ponder Page 1799
  19. .7 Molecular Mass of Covalent Compounds — Pause and Ponder Page 1799
  20. .8 Formula Unit Mass of Ionic Compounds — Pause and Ponder Page 1809
  21. End-of-chapter questions — Revise, Reflect, Refine Page 181 – 183
  22. Project work — The Journey Beyond Page 183
  23. Closing question — The Quest Continues … Page 183
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