NCERT Solutions for Class 9 Science Chapter 9 Atomic Foundations of Matter (PDF) – 2027

The water itself is identical; the samples are not. Once each sample is purified, every one of them is the same compound H₂O, with hydrogen and oxygen combined in the fixed mass ratio 1 : 8 — that is exactly what the Law of Constant Proportions says.

So these are mixtures, and a mixture may have its components in any proportion. Distil any of them and what comes over is pure water — 9 g of it will always decompose into 1 g of hydrogen and 8 g of oxygen.

O stands for one atom of oxygen; O₂ stands for one molecule of oxygen gas, which contains two oxygen atoms joined by a double bond.

Because salt breaks into free ions in water and sugar does not. An electric current in a solution is carried by charged particles that are free to move; sugar solution has none.

Salt (NaCl) is an ionic compound: in water it splits into free Na⁺ and Cl⁻ ions, which move to the electrodes and carry the current. Sugar is a covalent compound: it dissolves as whole, neutral molecules, so the solution has no ions to carry a current.

The reading on the balance does not change. Before swirling, the pan carries water plus a heap of undissolved salt; after swirling it carries a clear salt solution — and the number on the display is the same.

So mass of solution = mass of water + mass of salt. Dissolving a solid in water is a physical change, and no mass is gained or lost in it.

A brisk effervescence — the mixture fizzes strongly as soon as the baking soda meets the vinegar, and the balance reading starts falling and then settles at a value lower than the initial one.

The reaction is

The fizzing is carbon dioxide gas being produced. In this set-up the flask is open, so the gas bubbles out of the mouth of the flask and leaves the balance pan altogether.

No. The final reading is less than the initial reading.

The final reading is lower because the carbon dioxide gas formed in the reaction escapes from the open flask, so its mass is no longer on the balance.

Because the carbon dioxide produced escapes from the open flask, and mass that has left the pan cannot be weighed.

The Law of Conservation of Mass has not been broken. The experiment was faulty, not the law — it was carried out in an open system, where a product is free to leave.

The same brisk effervescence — and this time the balloon inflates. As soon as the baking soda is tipped into the vinegar the mixture fizzes, and the carbon dioxide, having nowhere else to go, blows the balloon up.

The balance reading, meanwhile, stays where it was.

Yes. The final reading matches the initial reading (allowing for the usual uncertainty of ±1 in the last digit).

This is the experimental proof of the Law of Conservation of Mass: in a chemical reaction, matter can neither be created nor destroyed.

A milky white precipitate appears at once when the barium chloride solution is poured into the sodium sulfate solution. The clear liquid turns cloudy white.

The white solid is barium sulfate, which is insoluble in water and so settles out. The sodium chloride formed alongside it stays dissolved and is invisible.

No — the reading is unchanged. The total mass of both conical flasks together is the same after mixing as it was before.

This verifies the Law of Conservation of Mass for a reaction in which a solid is formed.

Hypothesis: if no hydrogen can escape, the total mass after the reaction equals the total mass before it.

Apparatus: 1 L plastic bottle with screw cap, small test tube, about 1 g granulated zinc, about 30 mL dilute HCl, digital balance.

Method

1. Pour the dilute acid into the bottle.
2. Stand the zinc, in the small test tube, upright inside the bottle so it does not yet touch the acid.
3. Cap the bottle tightly and weigh it: m₁.
4. Tilt the bottle so the zinc falls into the acid; hydrogen bubbles form.
5. When bubbling stops, weigh the closed bottle again: m₂.
6. Control: repeat with the cap off.

Result and conclusion: with the cap on, m₂ = m₁ (within ±1 in the last digit), so mass is conserved. With the cap off, m₂ is lower by about 0.03 g — the mass of the hydrogen that escaped.

No, the law is not violated. Nothing is left in the beaker because every product of the combustion is a gas, and in an open beaker the gases float away. The matter has moved, not disappeared.

Ethanol burns in the oxygen of the air to form carbon dioxide and water vapour. Both products escape from the open beaker, and the oxygen used also came from the air, so none of these was weighed. If the ethanol were burnt in a closed container, the total mass before and after burning would be the same.

180 g of water.

30 g of oxygen.

12 g of oxygen.

Because a compound is held together by chemical bonds in a fixed pattern of atoms, while a mixture is only a physical jumble whose parts can be present in any amount.

In water, two hydrogen atoms are bonded to one oxygen atom, always. That count cannot be changed without destroying the substance; change it and you no longer have water. So the mass ratio is locked at 2 × 1 u : 1 × 16 u = 1 : 8.

Yes, their results justify the law — because 8 : 2 is the same ratio as 4 : 1.

Nothing around us would have a reliable identity. Fixed ratios are what make a substance a substance; drop them and chemistry as a science collapses.

• “Water” could be H₂O in one sample and H₃O in another, each with different properties, so no substance would have fixed properties.
• The Law of Constant Proportions would fail; chemical formulae and molecular masses would have no fixed meaning.
• Medicines, fertilisers and other materials could not be made reliably, because every batch would be different.

Option (iii) — A is true, but R is false.

A is true: H : O = 2 g : 16 g = 1 : 8, the fixed mass ratio in water, and 2 g + 16 g = 18 g by the Law of Conservation of Mass.

R is false: Dalton’s postulate says atoms combine in a simple whole-number ratio of their numbers (2 H atoms : 1 O atom in water), not of their masses. Mass ratios are often not whole numbers — in NaCl, Na : Cl = 23 : 35.5.

Nitrogen (atomic number 7) has the configuration 2, 5. With five valence electrons it needs three more to reach an octet. Neither atom can take three from the other, so the two atoms share three pairs — a triple bond.

Fluorine has atomic number 9, so its electronic configuration is 2, 7 — seven electrons in the valence (L) shell, one short of an octet.

The shared pair attracts both nuclei at the same time, and it is that double attraction that holds the two atoms together as one molecule, F₂.

(i) Carbon dioxide, CO₂

(ii) Hydrogen sulfide, H₂S

(iii) Ammonia, NH₃

Because neon’s valence shell is already full.

Atoms lose, gain or share electrons only to complete their octet. Neon already has a complete octet, so it is stable and has no tendency to lose, gain or share electrons. That is why neon exists as single atoms and does not form compounds.

Lithium, beryllium, sodium, magnesium and aluminium — any four of these.

No — a sodium atom that has lost one electron is no longer neutral. It carries a charge of +1 and is called the sodium cation, Na⁺.

It would turn chemistry from inference into observation — but “seeing” an atom is not like seeing a marble, and that is where the difficulty starts.

How it would help: scientists could directly see how atoms are arranged in molecules and crystals, watch bonds form and break during reactions, spot a single misplaced atom in a material, and design new materials and medicines atom by atom.

Challenges: atoms are thousands of times smaller than the wavelength of light, so no ordinary microscope can show them; to “see” an atom something must be bounced off it, which disturbs it; and the instruments needed are very complex and costly.

Oxygen forms the oxide anion, O²⁻ — a negatively charged ion carrying two units of negative charge.

Completed sentence: “However, chlorine can take only one electron to become the chloride ion, Cl⁻. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride.”

Potassium (Z = 19): 2, 8, 8, 1. One electron in the outermost shell — losing it leaves the stable 2, 8, 8 arrangement of argon.

Calcium (Z = 20): 2, 8, 8, 2. Two electrons in the outermost shell — losing both leaves 2, 8, 8.

Potassium chloride, KCl. The one electron potassium gives away is taken by one chlorine atom (2, 8, 7 → 2, 8, 8).

Calcium chloride, CaCl₂. Calcium must dispose of two electrons, and each chlorine can accept only one — so two chlorine atoms are needed.

Sodium is 2, 8, 1 — one electron to give. Sulfur is 2, 8, 6 — two electrons short of an octet. Since each sodium can supply only one electron, two sodium atoms are needed for one sulfur atom.

(i) Carbon dioxide   (ii) Nitrogen dioxide   (iii) Sulfur hexafluoride   (iv) Phosphorus trichloride

(i) NaHCO₃   (ii) SO₂   (iii) FeCl₃   (iv) Cu₂O

(i) Fe³⁺ and OH⁻ → Fe(OH)₃

(ii) K⁺ and CO₃²⁻ → K₂CO₃

These are the observations you should record in Table 9.2.

The compounds fall into three groups.

• Group 1 — sodium chloride, copper sulfate (ionic): soluble in water, insoluble in kerosene and petrol; do not conduct in the solid state but conduct in water.
• Group 2 — camphor, naphthalene (covalent): insoluble in water, soluble in kerosene and petrol; do not conduct.
• Group 3 — sugar (covalent): soluble in water, but the solution does not conduct.

Because they contain no charged particles that are free to move — neither ions nor free electrons.

Camphor and naphthalene are covalent compounds made of electrically neutral molecules. They have no ions and no free electrons, and they do not dissolve in water to give ions. Without charged particles that can move, no current can flow.

Ionic compounds conduct in the molten state; covalent compounds do not.

Reasoning for the ionic case. In a solid ionic compound the Na⁺ and Cl⁻ ions are already charged, but they are locked in fixed positions by strong electrostatic forces. Heat the solid past its melting point and those positions break down: the ions are still charged, but now they can move. Charge that can move is a current, so the compound conducts.

Reasoning for the covalent case. Melting a covalent solid separates the molecules from one another, but the covalent bonds inside each molecule survive. The liquid is a crowd of neutral molecules, and neutral particles cannot carry a current.

An ionic bond. That pair of observations is the standard fingerprint of an ionic compound.

• No conduction as a solid — the ions are there, but they are clamped in fixed positions in the crystal lattice by strong electrostatic forces, so no charge can move.
• Conduction in water — water pulls the lattice apart and sets the ions free. Once mobile, the cations drift towards the negative electrode and the anions towards the positive one, and that drift is the current.

First identify the metal. Its valence shell is the M shell and it holds two electrons, so the configuration is 2, 8, 2 — that is Z = 12, magnesium.

(i) Formula — MO (magnesium oxide, MgO). M loses 2 electrons to form M²⁺ and O gains 2 electrons to form O²⁻; the charges balance in a 1 : 1 ratio.

(ii) Type of bond — ionic. A metal with two valence electrons meets a non-metal that is two electrons short. Electrons are transferred, not shared, giving M²⁺ and O²⁻, which are then held together by electrostatic attraction.

(iii) Its aqueous solution conducts electricity, but only weakly. The small amount that dissolves gives free ions, which carry the current; since the compound is only slightly soluble, there are few ions, so the bulb glows dimly.

63 u.

16 u.

74.5 u.

58 u.

A has one electron in its third shell, so its first two shells are full: A = 2, 8, 1 (sodium). B has six electrons in its second shell: B = 2, 6 (oxygen).

(i) A gives away 1 electron (it then has 2, 8, a complete octet).

(ii) A forms a cation with charge +1, A⁺ (11 protons, 10 electrons).

(iii) B takes 2 electrons (it then has 2, 8).

(iv) B forms an anion with charge −2, B²⁻.

(v) An ionic bond — electrons are transferred from A to B, and the oppositely charged ions attract each other.

(vi) A₂B (sodium oxide, Na₂O): two A⁺ ions balance one B²⁻ ion, since 2(+1) + (−2) = 0.

X has six electrons in its outermost shell, so it is two electrons short of an octet. Taking the smallest such element, X = oxygen (2, 6).

(i) X is two electrons short of an octet. Two X atoms each share two electrons with each other, so both complete their octets — hence X forms a diatomic molecule, X₂.

(ii) A covalent bond — specifically a double bond, since two pairs of electrons are shared.

(iii) Structure of X₂

(iv) Y (2, 2 — beryllium) has 2 electrons to give and X needs 2. So Y transfers both electrons to X, forming Y²⁺ and X²⁻, which combine 1 : 1 to give YX (BeO).

Option (iii) — 2 Fe³⁺ and 3 O²⁻.

Only (iii) has both totals equal to 6, so only (iii) gives a neutral compound. The formula is Fe₂O₃ — ferric oxide, the red-brown compound in rust and in haematite ore.

Only statement (iv) is correct.

(i) False. Correct: Elements are made up of atoms of only one kind (existing singly, like He, or as molecules, like O₂); compounds are made up of atoms of two or more different elements.

(ii) False. Correct: The molecule of a compound is always made up of two or more atoms of different kinds (e.g. HCl, H₂O).

(iii) False. Correct: One molecule of nitrogen gas contains two nitrogen atoms (N₂, joined by a triple bond).

(iv) True. In H₂O each of the two hydrogen atoms shares one pair of electrons with the oxygen atom.

Write the cation first, put the charge numbers beneath, criss-cross them as subscripts, and divide by any common factor.

Option (ii).

A chloride ion is a chlorine atom that has gained one electron, so the diagram must show:

Count the dots on each of the four diagrams printed in the book:

Atomic masses: H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, P = 31 u.

(i) Ammonium nitrate, NH₄NO₃ → 80 u

(ii) Phosphoric acid, H₃PO₄ → 98 u

(iii) Sodium hydrogencarbonate, NaHCO₃ → 84 u

Work out the ion each element forms, then balance the charges.

Names: (i) magnesium nitride, (ii) lithium nitride, (iii) sodium sulfide, (iv) aluminium oxide.

Criss-cross the charge numbers in every cell, then simplify. Brackets go round a polyatomic ion whenever its subscript is 2 or more.

The law is obeyed — both sides come to 11.3 g.

Hence the Law of Conservation of Mass is valid for this reaction.

(i) Atomic number Z = 11, mass number A = 23.

(ii) It is a cation, carrying a charge of +1.

(iii) Electronic configuration: 2, 8. The species has only 10 electrons, so the K shell takes 2 and the L shell the remaining 8 — the same arrangement as neon. (The neutral atom would have been 2, 8, 1.)

(iv) It is the sodium ion, Na⁺ — more precisely ²³₁₁Na⁺.

A is 2, 8, 5 — five valence electrons, three short of an octet (Z = 15, phosphorus). B is 2, 8, 7 — seven valence electrons, one short (Z = 17, chlorine).

(i) B is more reactive.

(ii) Covalent bonds. A and B are both non-metals that need to gain electrons (A needs 3, B needs 1); neither will give electrons away, so they share electron pairs.

(iii) Formula: AB₃ (PCl₃, phosphorus trichloride) — A shares one electron pair with each of three B atoms, so all four atoms complete their octets.

Option (iii) — A is true, but R is false.

A is true. Copper sulfate is an ionic compound. In the solid its Cu²⁺ and SO₄²⁻ ions are locked in fixed lattice positions, so no charge can move and it does not conduct. Melt it and the lattice breaks down; the ions become mobile and the melt conducts.

R is false — it has the two states exactly the wrong way round.

Use two rules: neutrons = mass number − protons, and electrons = protons − charge (a positive charge means electrons are missing, a negative charge means extra electrons are present).

Aim: to show that water from any source has hydrogen and oxygen in the same ratio.

Principle: an electric current splits water into hydrogen (at the negative electrode) and oxygen (at the positive electrode). If the composition is always the same, the volume of hydrogen is always twice that of oxygen.

Method

1. Collect water from 3–4 sources (tap, rain, well, river). Filter and distil each sample.
2. Add the same few drops of dilute sulfuric acid to each sample so that it conducts.
3. Pass current from a 9 V battery through each sample for the same time, collecting the gas from each carbon electrode in an inverted graduated test tube.
4. Read the two volumes and test the gases (hydrogen burns with a pop; oxygen relights a glowing splint).

Conclusion: for every source, hydrogen : oxygen = 2 : 1 by volume, so all the samples have the same composition, H₂O.

Take sodium, magnesium and chlorine — two metals that lose electrons and one non-metal that gains one.

What the graph shows. Sodium and magnesium drop to 10 electrons; chlorine climbs to 18. The numbers move in opposite directions, yet all three land on the same kind of arrangement — a complete outermost shell. Na⁺ and Mg²⁺ both become 2, 8 (like neon) and Cl⁻ becomes 2, 8, 8 (like argon).

Sample design — “Ion Match”

Cards: make 3 cards of each ion — cations Na⁺, K⁺, NH₄⁺, Ca²⁺, Mg²⁺, Cu²⁺, Al³⁺, Fe³⁺ and anions Cl⁻, OH⁻, NO₃⁻, O²⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻ (15 ions × 3 = 45 cards). Colour cations green and anions red.

1. Deal 7 cards to each of 2–4 players. Put the rest face down as the pile and turn one card face up.
2. On your turn, take the top card of the pile or the open card, then discard one unwanted card face up.
3. Lay down cards that form a compound only if the total positive and negative charges cancel. Name it and write its formula (e.g. Ca²⁺ + 2 Cl⁻ → CaCl₂).
4. You may ask another player for an ion you need; if they have it, they must give it and draw a new card.
5. Score 1 point for each card used in a compound. The round ends when a player has no cards left.

The link is to PhET’s Build a Molecule simulation from the University of Colorado Boulder:

Use it to build molecules such as H₂, O₂, H₂O, CO₂, NH₃ and CH₄, and see each molecule’s formula, name and 3-D shape.

For every chemical change you will meet, the answer is no — mass is conserved, provided the system is closed. But the honest, deeper answer has a small and beautiful exception.

Cases that seem to break the law are open systems. When a gas escapes (wood burning, vinegar and baking soda in an open flask) the mass seems to fall; when a gas from the air is taken up (magnesium burning, iron rusting) it seems to rise. If the system is closed, the total mass stays the same.