Where Mindful Learning Begins
Where Mindful Learning Begins
Complete NCERT Solutions for Chapter 9 of the new Class 9 Science Exploration textbook (CBSE 2026-27) — every Think It Over, Activity, Pause & Ponder, Think as a Scientist, Bridging Science and Society, Revise Reflect Refine, and Journey Beyond question on this one page, with full step-by-step working for every numerical.
Atomic Foundations of Matter puts the laws of chemical combination, atomic and molecular mass calculations, valency, and the difference between ionic and covalent bonding at the centre of the chapter. These NCERT solutions work through every numerical on formula unit mass and mole concepts step by step, alongside the ion-formation diagrams for common cations and anions — exactly the kind of content that shows up repeatedly in Class 9 Science chemistry important questions.
Atomic Foundations of Matter moves from atoms to the substances they build. It establishes the Law of Conservation of Mass and the Law of Constant Proportions through hands-on activities, uses these to justify Dalton's Atomic Theory, and then explains how atoms combine — by sharing electrons in a covalent bond or transferring them in an ionic bond — to form molecules and ionic compounds. It closes with writing chemical formulae and calculating molecular mass and formula unit mass. Every question is solved here, section by section, exactly as the textbook presents them, with full working for every numerical.
Verifying with weighing-balance activities that mass is conserved in reactions, and that compounds always combine in fixed ratios.
How atoms share electrons to form molecules like H₂, O₂ and H₂O, or transfer electrons to form ions like Na⁺ and Cl⁻.
Criss-cross method for writing chemical formulae, and calculating molecular mass and formula unit mass.
Laws of chemical combination
| Law | Statement |
|---|---|
| Law of Conservation of Mass | Mass can neither be created nor destroyed in a chemical reaction — the total mass of reactants equals the total mass of products. |
| Law of Constant (Definite) Proportions | A pure chemical compound always contains the same elements combined together in the same fixed proportion by mass, no matter how or where it's prepared. |
Key formulae
Valency is the combining capacity of an element — the number of electrons it needs to lose, gain, or share to achieve a stable (usually 8-electron) outer shell.
Ions: cations vs. anions
| Property | Cation | Anion |
|---|---|---|
| Charge | Positive | Negative |
| Formed by | Losing electrons | Gaining electrons |
| Typical elements | Metals (e.g., Na⁺, Ca²⁺) | Non-metals (e.g., Cl⁻, O²⁻) |
An atom is the smallest particle of an element that can take part in a chemical reaction; a molecule is the smallest particle of an element or compound that can normally exist on its own.
Answer: yes. According to the Law of Constant Proportions (also called the Law of Definite Proportions), a pure compound always contains the same elements in the same fixed ratio by mass, regardless of its source. Purified water from any source — rivers, borewells, oceans, or rainfall — always contains hydrogen and oxygen in the mass ratio 1:8. So all samples of pure water are chemically identical, even though the dissolved impurities may differ before purification.
O represents a single atom of oxygen — the smallest particle of the element oxygen that retains its chemical identity. It is used when referring to the element in atomic terms (e.g., in formulae like MgO, CO₂, H₂O).
O₂ represents a molecule of oxygen — the form in which oxygen exists freely in nature. It consists of two oxygen atoms covalently bonded together by a double bond. O₂ is the actual smallest unit of oxygen gas that can exist independently and shows all the properties of oxygen gas.
In short: O = one atom; O₂ = one molecule (two atoms bonded together). Similarly, H = one hydrogen atom; H₂ = hydrogen molecule.
Answer: salt (sodium chloride, NaCl) is an ionic compound. When dissolved in water, it dissociates into free-moving ions — Na⁺ and Cl⁻. These mobile ions carry electric charge through the solution, allowing it to conduct electricity.
Sugar (sucrose, C₁₂H₂₂O₁₁) is a covalent compound. When dissolved in water, it does not break into ions — it dissolves as neutral molecules. Without free-moving charged particles (ions), no electric current can flow, so sugar solution does not conduct electricity.
Electrical conductivity in a solution requires the presence of free-moving ions. Only ionic compounds (and certain covalent compounds that ionise, like acids) provide these in solution.
Observation: the reading on the digital balance before dissolving (mass of water + undissolved salt) is the same as the reading after the salt has dissolved completely.
Conclusion: the mass of the solution (salt water) equals the sum of the masses of water and salt taken. There is no change in mass during the formation of a solution. This demonstrates that mass is conserved in physical changes.
Extension (paper tearing): if a piece of paper is weighed before and after tearing into pieces, the total mass remains the same, further confirming conservation of mass in all physical changes.
Dissolving is a physical change — no new substance is formed. The salt merely disperses into water. Since no atoms are created or destroyed, mass is conserved.
Observation: a brisk effervescence (fizzing) is observed as carbon dioxide gas is rapidly produced. The final reading on the balance is less than the initial reading.
Reason for the difference: the carbon dioxide gas produced during the reaction escapes into the surrounding air. Since CO₂ is no longer on the balance, the measured mass decreases. The mass appears to have been lost, but in reality it has not — it has simply left the system as a gas.
Conclusion: this set-up does not properly verify conservation of mass because the system is not closed — gas escapes.
Observation: again, brisk effervescence occurs, inflating the balloon as CO₂ is produced. This time, the final reading equals the initial reading.
Conclusion: when the system is closed (no gas can escape), the total mass before the reaction equals the total mass after the reaction. This demonstrates the Law of Conservation of Mass: matter is neither created nor destroyed in a chemical reaction, only rearranged.
The balloon traps the CO₂ gas, keeping the entire mass — reactants and all products — on the balance. Only a closed system gives a true test of mass conservation.
Observation: when the two solutions are mixed, a white precipitate of barium sulfate (BaSO₄) is immediately formed, and the solution becomes cloudy. The reaction is:
The reading on the balance after mixing is the same as the reading before mixing. No gas is produced in this reaction, so no mass escapes. This is an open system, but since all products remain in the flask, mass is conserved.
Conclusion: the total mass of products (barium sulfate + sodium chloride in solution) equals the total mass of reactants (sodium sulfate solution + barium chloride solution). The Law of Conservation of Mass is verified.
Both conical flasks are kept on the balance during transfer to avoid error from solution sticking to the flask walls.
| Compound | Water | Kerosene/Petrol | Conducts (solid) | Conducts (water) |
|---|---|---|---|---|
| Camphor (covalent) | Insoluble | Soluble | No | No |
| Sodium chloride (ionic) | Soluble | Insoluble | No | Yes |
| Copper sulfate (ionic) | Soluble | Insoluble | No | Yes |
| Sugar (covalent) | Soluble | Insoluble | No | No |
| Naphthalene (covalent) | Insoluble | Soluble | No | No |
Grouping
Why camphor and naphthalene do not conduct even dissolved: these covalent compounds dissolve without forming ions, so no charged particles exist to carry current.
Prediction for molten state: ionic compounds (like NaCl) will conduct electricity in the molten state because melting breaks the crystal lattice and releases free-moving ions. Covalent compounds will generally not conduct in the molten state since no ions are formed.
Answer: no, the Law of Conservation of Mass is not violated. When ethanol burns, it reacts with oxygen from the air to form carbon dioxide (CO₂) and water vapour (H₂O), both of which are gases that escape into the atmosphere. The products are invisible and disperse into the air, so no residue is seen in the beaker. However, if the masses of all products (CO₂ and H₂O vapour) were collected and measured, their total mass would equal the mass of ethanol burnt plus the mass of oxygen used. The reaction takes place in an open system, so the products escape — but mass is still conserved.
The ratio of sulfur to oxygen by mass = 40:60 = 2:3.
In a compound, elements combine chemically in a fixed ratio by mass (determined by their atomic masses and the formula), regardless of how the compound was prepared or from where it was obtained. For example, water always has H:O = 1:8 by mass.
In a mixture, components are simply mixed together physically, not chemically bonded. They can be present in any proportion — for example, air is a mixture of nitrogen, oxygen, etc., and their ratios vary, and salt water can have any amount of salt. There is no fixed ratio, so the Law of Definite Proportions does not apply to mixtures.
Answer: yes. Student X has ratio Cu:O = 4:1. Student Y has ratio Cu:O = 8:2 = 4:1 (dividing both by 2). Both ratios are identical. Both students produced the same copper oxide with the same fixed mass ratio, confirming the Law of Constant Proportions.
Answer: (ii) Both A and R are true, but R is not the correct explanation of A.
The Assertion (A) is true — 2 g H + 16 g O = 18 g H₂O (verified by the Law of Conservation of Mass and the Law of Definite Proportions). The Reason (R) is also true — Dalton stated that atoms combine in simple whole number ratios; in water, 2 atoms of H combine with 1 atom of O (2:1 atom ratio). However, R does not correctly explain A. The assertion is about mass ratios, while the reason talks about atom number ratios. The correct explanation for the mass ratio (2:16) comes from the atomic masses of hydrogen (1 u each) and oxygen (16 u) and the formula H₂O, not directly from the statement that atoms combine in whole number ratios.
Nitrogen atom (atomic number 7) has electronic configuration 2, 5. It has 5 valence electrons and needs 3 more electrons to complete its octet. Each nitrogen atom shares 3 electrons with the other nitrogen atom, forming 3 shared pairs (3 bonding pairs). This results in a triple bond.
This is a very strong bond, which is why nitrogen gas (N₂) is very stable and unreactive under normal conditions.
Fluorine (atomic number 9) has electronic configuration 2, 7. It has 7 valence electrons and needs 1 more electron to complete its octet. Each fluorine atom shares 1 electron with the other fluorine atom, forming 1 shared pair — a single covalent bond.
The fluorine molecule (F₂) is held together by a single covalent bond, with each F atom achieving a stable octet configuration.
(i) Carbon dioxide (CO₂): carbon (atomic number 6) has 4 valence electrons and needs 4 more; oxygen (atomic number 8) has 6 valence electrons and needs 2 more. One carbon atom shares 2 electrons each with two oxygen atoms → two double bonds formed. Representation: O=C=O
(ii) Hydrogen sulfide (H₂S): sulfur (atomic number 16) has 6 valence electrons and needs 2 more; hydrogen needs 1 more. Two hydrogen atoms each share 1 electron with the sulfur atom → two single bonds. Representation: H—S—H (with 2 lone pairs on S).
(iii) Ammonia (NH₃): nitrogen has 5 valence electrons and needs 3 more; hydrogen needs 1 more. Three hydrogen atoms each share 1 electron with the nitrogen atom → three single bonds, with 1 lone pair on N. Formula: NH₃.
Answer: neon has atomic number 10 and electronic configuration 2, 8. Its outermost shell (L shell) already contains 8 electrons — a complete octet. Neon is already in a stable electronic configuration and has no tendency to lose, gain, or share electrons, because doing so would disturb its stable arrangement. Therefore, neon does not form chemical bonds and exists as a monoatomic gas. It belongs to the noble gas group (Group 18), all of which are chemically inert for the same reason.
Oxygen (atomic number 8) has electronic configuration 2, 6. It has 6 valence electrons and needs 2 more to complete its octet. Oxygen gains 2 electrons from another atom, acquiring 2 units of negative charge. It forms the oxide anion: O²⁻.
Chlorine can take only one electron to become Cl⁻ (chloride anion). Now, 1 ion of magnesium (Mg²⁺) and 2 ions of chlorine (Cl⁻) combine to give magnesium chloride (MgCl₂) — two Cl⁻ ions are needed to balance the 2+ charge of one Mg²⁺ ion, making the compound neutral.
Potassium (K, atomic number 19): electronic configuration 2, 8, 8, 1. It has 1 valence electron. K loses 1 electron to form K⁺ (potassium cation). K⁺ + Cl⁻ → KCl (potassium chloride).
Calcium (Ca, atomic number 20): electronic configuration 2, 8, 8, 2. It has 2 valence electrons. Ca loses 2 electrons to form Ca²⁺ (calcium cation). Ca²⁺ + 2Cl⁻ → CaCl₂ (calcium chloride) — two chloride ions are needed to balance the 2+ charge.
Sodium (Na, atomic number 11): configuration 2, 8, 1. Has 1 valence electron, loses 1 to form Na⁺.
Sulfur (S, atomic number 16): configuration 2, 8, 6. Has 6 valence electrons, needs 2 to complete octet, gains 2 to form S²⁻.
Since each Na atom provides only 1 electron and S needs 2 electrons, 2 Na atoms each donate 1 electron to 1 S atom:
The 2+ charge from 2 Na⁺ ions balances the 2− charge of S²⁻. Formula: Na₂S.
(i) Fe³⁺ and OH⁻: valencies are 3 and 1. Criss-cross: Fe(OH)₃ (ferric hydroxide). Brackets needed as OH is polyatomic.
(ii) K⁺ and CO₃²⁻: valencies are 1 and 2. Criss-cross: K₂CO₃ (potassium carbonate). No brackets needed.
Answer: the compound has an ionic bond. In the solid state, the ions (cations and anions) are held in fixed positions in a crystal lattice by strong electrostatic forces, so they cannot move and the solid does not conduct electricity. When the ionic compound is dissolved in water, the lattice breaks down and the ions become free to move in solution — these mobile ions carry electric charge, allowing the solution to conduct electricity. Examples: sodium chloride (NaCl), copper sulfate (CuSO₄), potassium nitrate (KNO₃).
Metal M has 2 valence electrons in its M (third) shell, so its electronic configuration is 2, 8, 2 — this is magnesium (Mg, atomic number 12).
(i) Formula: Mg²⁺ (valency 2) + O²⁻ (valency 2); criss-cross gives MgO (magnesium oxide) — the two valencies are equal, so the formula simplifies to MgO.
(ii) Type of bond: ionic bond. Magnesium loses 2 electrons to form Mg²⁺; oxygen gains 2 electrons to form O²⁻. The electrostatic attraction between these oppositely charged ions forms an ionic bond.
(iii) Electrical conductivity of aqueous solution: MgO is slightly soluble in water (forms Mg(OH)₂, which partially ionises). The aqueous solution will show slight electrical conductivity due to the small number of Mg²⁺ and OH⁻ ions present.
Hypothesis: mass of reactants = mass of products in the reaction Zn + HCl(dil.) → ZnCl₂ + H₂↑
Challenge: H₂ gas is produced and could escape if the system is open.
Experimental Design (Closed System)
Expected result: initial mass = final mass. The H₂ gas is trapped in the balloon, so no mass escapes. This confirms the Law of Conservation of Mass.
If using a fully sealed system (no balloon), the pressure builds up. The balloon design is safer and more practical for this experiment.
Mass of reactants = Mass of products = 6.92 g. Conclusion: the Law of Conservation of Mass is obeyed.
1 g of carbon produces 44/12 g of CO₂.
Nuclear energy is released when the nuclei of atoms either split (nuclear fission) or combine (nuclear fusion) to form new elements. The mass of the products is slightly less than the mass of the reactants, and this mass difference (called mass defect) is converted into an enormous amount of energy, as described by Einstein's equation \(E = mc^2\).
In nuclear power plants, fission of uranium-235 nuclei releases heat energy. This heat produces steam, which drives turbines connected to generators, producing electricity. Nuclear power is a cleaner alternative to fossil fuels as it does not produce CO₂ or other greenhouse gases during operation.
Applications of Nuclear Energy
Raja Ramanna is often called the Father of the Indian Nuclear Programme. He made significant contributions in developing India's nuclear energy programme and promoting its peaceful use for national development.
Element A: configuration 2, 8, 1 (third shell has 1 electron) — this is sodium (Na, atomic number 11).
(i) A tends to give 1 electron (easier to lose 1 than gain 7 to complete the octet). (ii) A forms a cation: A⁺ (Na⁺).
Element B: second shell has 6 electrons, so configuration is 2, 6 (oxygen, atomic number 8).
(iii) B tends to take 2 electrons (needs 2 more to complete its octet of 8). (iv) B forms an anion: B²⁻ (O²⁻).
(v) Bond type: ionic bond. A loses 1 electron; B gains 2 electrons. Since each A provides 1 electron and B needs 2, two A atoms combine with one B atom.
(vi) Formula: A⁺ valency 1; B²⁻ valency 2. Criss-cross: A₂B. If A = Na and B = O, the compound is Na₂O (sodium oxide).
Element X has 6 valence electrons → needs 2 more for octet → this is oxygen (O, atomic number 8).
(i) Oxygen forms a diatomic molecule (O₂) because each oxygen atom needs 2 more electrons to achieve a stable octet; by sharing 2 electrons with another oxygen atom, both achieve stability.
(ii) Bond type: covalent bond (a double bond, since 2 electron pairs are shared between the two O atoms).
(iii) Structure of X₂ (O₂): O=O (two atoms joined by a double bond, with 2 lone pairs on each O atom).
(iv) Element Y, with configuration 2, 1 (one valence electron), is hydrogen. Two H atoms (Y) each share 1 electron with O (X), forming 2 single covalent bonds: H—O—H (water).
Answer: (iii) 2 Fe³⁺ and 3 O²⁻
Cl atom (atomic number 17) has electronic configuration 2, 8, 7 (17 electrons). The Cl⁻ ion has gained 1 electron, so it has 18 electrons total: configuration 2, 8, 8 — all shells now complete.
The correct diagram must show 3 shells, with 2 electrons in the first (K), 8 electrons in the second (L), and 8 electrons in the third (M) shell, carrying a negative charge.
Cl⁻ has the same electron configuration as argon (2, 8, 8) but with 17 protons (unlike Ar which has 18), so it carries a 1− charge.
| NO₃⁻ | SO₄²⁻ | PO₄³⁻ | |
|---|---|---|---|
| NH₄⁺ | NH₄NO₃ | (NH₄)₂SO₄ | (NH₄)₃PO₄ |
| Li⁺ | LiNO₃ | Li₂SO₄ | Li₃PO₄ |
| Al³⁺ | Al(NO₃)₃ | Al₂(SO₄)₃ | AlPO₄ |
| Cu²⁺ | Cu(NO₃)₂ | CuSO₄ | Cu₃(PO₄)₂ |
Mass of reactants = Mass of products = 11.3 g. Conclusion: the Law of Conservation of Mass is valid for this reaction.
(i) Atomic number = protons = 11. Mass number = protons + neutrons = 11 + 12 = 23.
(ii) It is a cation. Protons (11) > Electrons (10) → net positive charge of 1+ → it is Na⁺ (sodium ion).
(iii) Electronic configuration of Na⁺ (10 electrons): K = 2, L = 8. Configuration: 2, 8.
(iv) The species is the sodium ion (Na⁺) — the ion formed when a sodium atom loses 1 valence electron. Its electron configuration (2, 8) is the same as that of neon (a noble gas).
Element A: configuration 2,8,5 → 5 valence electrons; atomic number = 15 → phosphorus (P). Needs 3 more electrons.
Element B: configuration 2,8,7 → 7 valence electrons; atomic number = 17 → chlorine (Cl). Needs 1 more electron.
(i) Reactivity: element B (chlorine) is more reactive — it needs only 1 electron to complete its octet, making it a highly reactive non-metal (halogen). Element A (phosphorus) needs 3 electrons, which is relatively harder.
(ii) Bond type: both A and B are non-metals, and non-metals generally form covalent bonds by sharing electrons. A will share 3 electrons with 3 B atoms, forming 3 single covalent bonds.
(iii) Formula: A needs 3 electrons; B provides 1 electron each. Three B atoms share one electron each with one A atom → AB₃ — in terms of P and Cl: PCl₃ (phosphorus trichloride).
Answer: (iii) A is true, but R is false.
Explanation of A: the Assertion is true. CuSO₄ (an ionic compound) does not conduct electricity in the solid state (ions are fixed in the crystal lattice) but does conduct in the molten state (ions become free to move when the lattice melts).
Explanation of R: the Reason is false — it has the explanation exactly backwards. In the solid state, ions are fixed (locked in the lattice) and cannot move — so solid does not conduct. In the molten (melted) state, the lattice breaks down and ions become free to move — so it conducts electricity. The reason incorrectly states the opposite of the truth.
²⁷Al (aluminium atom, neutral): protons = 13, so electrons = 13. Neutrons = mass number − protons = 27 − 13 = 14.
⁸⁰Br⁻ (bromide ion, charge 1−): protons = 35; gained 1 electron, so electrons = 36. Neutrons = 80 − 35 = 45.
²⁰¹Hg²⁺ (mercury ion, charge 2+): protons = 80; lost 2 electrons, so electrons = 78. Neutrons = 201 − 80 = 121.
Hypothesis: all samples of pure water contain hydrogen and oxygen in a fixed mass ratio of 1:8 (or volume ratio of 2:1 for H₂:O₂), regardless of source.
Method — Electrolysis of Water
Expected result: for all three samples, the volume ratio of H₂:O₂ = 2:1 (or mass ratio H:O = 1:8). This confirms that water from all sources has the same fixed composition, verifying the Law of Constant Proportions.
Example elements: Sodium (Na), Chlorine (Cl), Magnesium (Mg).
Bar graph description: X-axis shows species (atom/ion for each element); Y-axis shows number of electrons. Sodium bars: 11 and 10. Chlorine bars: 17 and 18. Magnesium bars: 12 and 10.
For all ordinary chemical reactions (the type studied in this chapter), the Law of Conservation of Mass holds exactly. No chemical change violates this law.
However, in nuclear reactions (not ordinary chemical changes), the situation is different. In nuclear fission and fusion, a tiny fraction of mass is converted directly into energy according to Einstein's famous equation \(E = mc^2\). The total mass of products is very slightly less than the total mass of reactants. This mass difference (called mass defect) appears as an enormous release of energy (nuclear energy).
This means that at the nuclear level, mass and energy are interconvertible. The more fundamental conservation law is the conservation of mass-energy (not mass alone). For all practical purposes in chemistry, however, the energy changes in chemical reactions are so small that the corresponding mass changes are immeasurably tiny and can be completely ignored. The Law of Conservation of Mass holds to extremely high precision for all chemical reactions.
In everyday chemistry, mass is always conserved. Only at the nuclear scale (involving the strong nuclear force and atomic nuclei) does the conversion of mass to energy become significant.
Mass is conserved and elements combine in fixed ratios in every chemical reaction — and these two experimentally-verified laws are exactly why Dalton's picture of atoms simply rearranging, never being created or destroyed, works, and why atoms combine by sharing electrons (covalent bonds) or transferring them (ionic bonds) in the same fixed whole-number ratios every single time.
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