NCERT Solutions Class 9 Science Ch 9 Matter 2026-27

Pure water is chemically identical everywhere. Water is a compound with a fixed composition: H₂O, made of 2 hydrogen atoms and 1 oxygen atom. So pure water (such as distilled water) is the same wherever it comes from.

But the water we get from different sources (river, sea, rain, tap) is not pure. It contains dissolved salts, gases, minerals and other impurities in different amounts, so these samples are not chemically identical.

• Salt (NaCl) is an ionic compound. In water it breaks into free ions, NaCl → Na⁺ + Cl⁻. These moving ions carry charge, so the solution conducts electricity.
• Sugar is a covalent compound. In water it stays as neutral molecules and forms no ions. With no charged particles, it cannot conduct electricity.

No, the law is not violated. The law says that mass is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.

Ethanol burns with the oxygen of the air and forms carbon dioxide and water vapour. These products are gases and escape into the air from the open beaker, so no residue is seen. If we collected all the products, we would find that their mass equals the mass of the ethanol plus the oxygen used.

1. Law of conservation of massMass of reactants = mass of products
2. Mass of water20 g + 160 g = 180 g

180 g of water is formed.

1. Mass ratioS : O = 40 : 60 = 2 : 3
2. Calculation2 parts of sulfur = 20 g, so 1 part = 10 g.Oxygen = 3 × 10 = 30 g

30 g of oxygen must be present.

1. One part3 parts of carbon = 9 g, so 1 part = 3 g
2. Oxygen (4 parts)4 × 3 = 12 g

12 g of oxygen will combine with 9 g of carbon.

In a compound, the elements combine chemically in a fixed ratio by mass, so every sample of the compound has the same composition (for example, water always has hydrogen and oxygen in the mass ratio 1 : 8).

In a mixture, the components are only mixed physically and can be present in any proportion (for example, sugar and sand can be mixed in any amounts). So the law does not hold for mixtures.

Yes. Both ratios simplify to 4 : 1. The same compound contains the same elements in a fixed ratio by mass, so the results justify the Law of Constant Proportions.

Substances would not have a definite composition. Different samples of the same substance (such as water or carbon dioxide) would contain different amounts of their elements, so the Law of Constant Proportions would not hold.

The properties of substances, such as melting point, boiling point and density, would change from sample to sample. Substances would be unreliable and unpredictable, and chemistry could not be studied in a systematic way.

Correct option: (a)

Both are true and R explains A. Atoms of hydrogen and oxygen combine in a fixed simple ratio (2 : 1), which gives the fixed mass ratio of 1 : 8 (2 g : 16 g). So water always has the same composition.

Each nitrogen atom needs 3 more electrons to complete its octet. So two nitrogen atoms share three pairs of electrons and form a triple covalent bond.

• Each nitrogen atom has 3 shared pairs (the triple bond) and 1 lone pair.
• Each atom now has 8 electrons around it (an octet).

The electronic configuration of fluorine is 2, 7. Each fluorine atom has 7 valence electrons and needs 1 more electron to complete its octet.

Two fluorine atoms share one pair of electrons and form a single covalent bond, so each atom gets a stable octet.

(i) Carbon dioxide (CO₂): carbon has 4 valence electrons and each oxygen has 6. Carbon shares 2 electrons with each oxygen, forming two double covalent bonds. The molecule is linear (O=C=O) and every atom has a complete octet.

(ii) Hydrogen sulfide (H₂S): sulfur has 6 valence electrons and each hydrogen has 1. Sulfur shares 1 electron with each hydrogen, forming two single bonds and keeping two lone pairs. The shape is bent (V-shaped).

(iii) Ammonia (NH₃): nitrogen has 5 valence electrons and each hydrogen has 1. Nitrogen shares 1 electron with each of three hydrogen atoms, forming three single bonds and keeping one lone pair. The shape is trigonal pyramidal.

The electronic configuration of neon is 2, 8. Its outermost shell is already completely filled (an octet), which makes it very stable. So neon does not need to gain, lose or share electrons, and it does not usually form chemical bonds.

Help: scientists could see how atoms are arranged and combine and how reactions take place at the atomic level. This would help in understanding bonding and molecular structure and in designing new materials and medicines more accurately.

Challenges: atoms are extremely small and in constant motion, so it would need very powerful and costly technology, and it would still be hard to measure their motion and behaviour precisely.

Oxygen has atomic number 8 and electronic configuration 2, 6. It has 6 valence electrons and needs 2 more to complete its octet. So it gains 2 electrons and forms a negative ion, the oxide ion (O²⁻).

Answers: Cl⁻; one (Mg²⁺); two (Cl⁻)

One Mg²⁺ ion combines with two Cl⁻ ions to give MgCl₂.

• Potassium chloride: K loses 1 electron to form K⁺ and Cl gains it to form Cl⁻. One K⁺ and one Cl⁻ give KCl.
• Calcium chloride: Ca loses 2 electrons to form Ca²⁺, and each of two Cl atoms gains 1 electron to form 2 Cl⁻. One Ca²⁺ and two Cl⁻ give CaCl₂.

• Sodium (2, 8, 1) loses 1 electron to form Na⁺ (2, 8).
• Sulfur (2, 8, 6) needs 2 electrons to complete its octet, so it gains 2 electrons to form S²⁻ (2, 8, 8).
• One sulfur atom needs 2 electrons, so two sodium atoms each give 1 electron.

• (i) One Fe³⁺ needs 3 OH⁻ to balance the charge. Formula: Fe(OH)₃
• (ii) One CO₃²⁻ needs 2 K⁺ to balance the charge. Formula: K₂CO₃

The compound has an ionic bond. In the solid state the ions are fixed in a crystal lattice and cannot move, so it does not conduct electricity. When dissolved in water, the compound splits into free ions that can move and carry current, so the solution conducts electricity.

• (i) Formula: M loses 2 electrons to form M²⁺, and oxygen gains 2 electrons to form O²⁻. The charges balance in a 1 : 1 ratio, so the formula is MO.
• (ii) Type of bond: electrons are transferred from the metal to the non-metal, so it is an ionic bond.
• (iii) Conductivity: the compound gives M²⁺ and O²⁻ ions in water, and these ions move freely, so the solution conducts electricity (weakly, as only a little of it dissolves).

Molecular mass of HNO₃ = 63 u

Molecular mass of CH₄ = 16 u

Formula unit mass of KCl = 74.5 u

Formula unit mass of Mg(OH)₂ = 58 u

Mass of reactants = mass of products, so the Law of Conservation of Mass is obeyed.

1. For 1 g of carbon44 ÷ 12 g of carbon dioxide
2. For 2.4 g of carbon(44 ÷ 12) × 2.4 = 8.8 g

8.8 g of carbon dioxide will be produced.

71 g of chlorine is needed.

Molecular mass of H₂O = 18 u

Molecular mass of CO₂ = 44 u

Formula unit mass of Na₂O = 62 u

Formula unit mass of Ca(NO₃)₂ = 164 u

(i) X needs 2 more electrons to complete its octet. Two atoms of X share electrons with each other, so each atom completes its octet and forms a diatomic molecule (X₂).

(ii) Electrons are shared, so it is a covalent bond, and it is a double bond (two pairs are shared).

(iii) Structure:

(iv) Y has 2 electrons in its second shell, so it loses these 2 electrons to form Y²⁺. X gains 2 electrons to form X²⁻. One Y atom combines with one X atom.

Correct option: (a) 2Fe³⁺ and 3O²⁻

2 × (+3) = +6 and 3 × (−2) = −6, so the total positive and negative charges are equal. In (b) the charges are +6 and −4, in (c) +6 and −3, and in (d) +6 and −3, so they are not balanced.

1. Neutral chlorine atom17 protons and 17 electrons; configuration 2, 8, 7.
2. Cl⁻ ionThe negative sign shows that it has gained 1 electron.Electrons = 17 + 1 = 18
3. Electronic configuration2 electrons in the K shell, 8 in the L shell and 8 in the M shell: 2, 8, 8 (a full octet).

Diagram (ii), which shows 18 electrons, represents the Cl⁻ ion.

Mass of reactants = mass of products, so the law of conservation of mass is valid.

• (i) B is more reactive, because it needs only 1 electron to complete its octet, while A needs 3.
• (ii) A needs 3 electrons and B needs 1. Both are non-metals and neither can easily give up electrons, so they share electrons and form covalent bonds.
• (iii) One A atom shares one electron with each of three B atoms, so the formula is AB₃.

Correct option: (c)

• A is true: in the molten state the ions are free to move, so it conducts; in the solid state the ions are fixed.
• R is false: it reverses the facts. In the solid state the ions are fixed in the lattice, and in the molten state they move freely.

No. In every chemical reaction mass is conserved; it is neither created nor destroyed. Sometimes the mass seems to change because the reaction takes place in an open system, where gases can escape or enter. In a closed system, the total mass of the reactants is always equal to the total mass of the products.