NCERT Solutions for Class 9 Science Chapter 9 Atomic Foundations of Matter (2026-27)

Revise, Reflect, Refine (NCERT Textbook Page No. 182)

Question 1. A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.
(i) How many electrons does A tend to give or take to become stable?
(ii) What kind of job would it form?
(iii) How many electrons does B tend to give or take to become stable?
(iv) What kind of job would it form?
(v) If A and B were to combine, what kind of bond would be formed?
(vi) What would be the formula for the compound thus formed?

Answer:
(i) It tends to give one electron to achieve a stable octet (2,8).

(ii) A cation (positively charged ion, A+)

(iii) It tends to take two electrons to complete its outer shell (2, 8).

(iv) An anion (negatively charged ion B-2)

(v) When A and B combine, an ionic (electrovalent) bond is formed.

Formula: A2B. 

Question 2. An element X has six electrons in its outer shell and forms a diatomic molecule.
(i) Why would that be so?
(ii) What kind of bond would it form?
(iii) Draw the structure of the molecule it would form.
(iv) A certain other element Y has two electrons in its second shell.
Draw the structure of the molecule that X would form with Y.

Answer:

Element X has 6 electrons in its outer shell; it forms a diatomic molecule (X2).
(i) To become stable, each atom needs 2 more electrons. By sharing two pairs of electrons with another X atom, both achieve a stable octet.
(ii) It will form a double covalent bond.
(iii) The structure is represented as:

(iv) Element Y has two electrons in its second shell (valency 2), so it tends to lose two electrons to attain a stable electronic configuration, and forms Y. Element X has six electrons in its outermost shell and requires two electrons to complete its octet, so it gains two electrons and forms X2. Therefore, X and Y combine by transfer of electrons to form an ionic bond, and the compound formed is YX.

Structure of compound YX:

Question 3. You want to design a new ionic compound, where the total positive charge is 6+, and the total negative charge is 6-. Which of the following combinations gives the correct number of ions?
(i) 2Al3+ and 3 Cl–
(ii) 3 Mg2+ and 1 PO43-
(iii) 2 Fe3+ and 3 O2-
(iv) 3 Ca2+ and 2 SO2-4
Answer:
Total positive charge = 6+
Total negative charge = 6-
(i) 2Al3+ and 3d
Positive : 2 × (3+) = 6+
Negative: 3 × (1-) = 3-
(Incorrect)

(ii) 3Mg2+ and 1 PO43-
Positive : 3 × (2+) = 6+
Negative: 1 × (3-) = 3-
(Incorrect)

(iii) 2Fe3+ and 3O2-
Positive: 2 × (3+) = 6+
Negative: 3 × (2-) = 6-
(Correct)

(iv) 3 Ca2+ and 2SO2-4
Positive : 3 x (2+) = 6+
Negative: 2 x (2-) = 4-
(Incorrect)

∴ Correct option (iii).

Question 4. Choose the correct statement(s) and correct the false statement(s).
(i) Elements are made up of molecules, and compounds are made up of atoms.
(ii) The molecule of a compound is always made up of two or more atoms of the same kind.
(iii) One molecule of nitrogen gas contains three nitrogen atoms.
(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

Answer:
(i) False

Correct statement: Elements are made up of atoms or molecules containing the same kind of atoms. Compounds are made up of molecules or ions containing two or more different elements chemically combined in a fixed ratio.

(ii) False

Correct statement: A molecule of a compound is made up of two or more atoms of different elements chemically bonded together.

(iii) False

Correct statement: One molecule of nitrogen gas is N2. It contains two nitrogen atoms.

(iv) True

Water is H2O. It contains two hydrogen atoms covalently bonded with one oxygen atom.

Question 5. Write the chemical formulae for the following compounds.
(i) Aluminium nitrate
(ii) Calcium oxide
(iii) Ferric oxide

Answer:
(i) Aluminium nitrate: Al(NO3)3 

(ii) Calcium oxide: CaO 

(iii) Ferric oxide: Fe2O3 

Question 6.
Write the formulae of the compounds formed from the following pairs of ions.
(i) Ca2+ and Br–

(ii) Al3+ and CO32-

(iii) K+ and SO42-

(iv) NH4+ and Cl–

Answer:

(i) CaBr2 (Calcium bromide)

(ii) Al2(CO3)3 (Aluminium carbonate)

(iii) K2SO4 (Potassium sulphate)

(iv) NH4Cl (Ammonium chloride) 

Question 7. Which of the following, in Fig. 9.18, correctly represents Cl ion (Atomic number of chlorine = 17)?

Answer:
Atomic number of chlorine = 17

So, a neutral chlorine atom has 17 electrons.

Electronic configuration of the chlorine atom:

2, 8, 7

A chlorine atom gains one electron to complete its octet and becomes a chloride ion.

Cl + e- → Cl-

Electronic configuration of the chloride ion:

2, 8, 8

Therefore, the correct figure is the one showing 18 electrons arranged as 2, 8, 8. Hence, option (ii) is correct.

Question 8. Determine the formula unit mass of the following substances.
(i) Ammonium nitrate (NH4NO3 ), used as a nitrogen fertiliser, which is essential for plant growth.
(ii) Phosphoric acid (H3PO4 ), used to make phosphate fertiliser and detergents.
(iii) Sodium hydrogencarbonate (NaHCO3 ), used to relieve acidity and help in digestion.

Answer:
Given atomic masses:

H = 1 u
N = 14 u
O = 16 u
P = 31 u
Na = 23 u
C = 12 u

(i) Ammonium nitrate, NH4NO3 

Atoms present:

N = 2
H = 4
O = 3

Formula unit mass = (2 × 14) + (4 × 1) + (3 × 16)

= 28 + 4 + 48
= 80 u

Therefore, the formula unit mass of NH4NO3 = 80 u.

(ii) Phosphoric acid, H3PO4 

Atoms present:

H = 3
P = 1
O = 4

Formula unit mass = (3 × 1) + (1 × 31) + (4 × 16)

= 3 + 31 + 64
= 98 u

Therefore, the formula unit mass of H3PO4 = 98 u.

(iii) Sodium hydrogencarbonate, NaHCO3 

Atoms present:

Na = 1
H = 1
C = 1
O = 3

Formula unit mass = (1 × 23) + (1 × 1) + (1 × 12) + (3 × 16)

= 23 + 1 + 12 + 48
= 84 u

Therefore, the formula unit mass of NaHCO3 = 84 u.

Question 9. Write the formulae for the compounds formed by the reaction of:
(i) Magnesium and nitrogen
(ii) Lithium and nitrogen
(iii) Sodium and sulfur
(iv) Aluminium and oxygen

Answer:

Question 10. Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. LiNO3 is given as an example.
Table 9.3

Answer:

Question 11. 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Verify whether the law of conservation of mass is valid.

Answer:
Given reaction:

Sodium carbonate + Acetic acid → Carbon dioxide + Water + Sodium acetate

Mass of reactants:

Mass of sodium carbonate = 5.3 g
Mass of acetic acid = 6.0 g

Total mass of reactants = 5.3 + 6.0
= 11.3 g

Mass of products:

Mass of carbon dioxide = 2.2 g
Mass of water = 0.9 g
Mass of sodium acetate = 8.2 g

Total mass of products = 2.2 + 0.9 + 8.2
= 11.3 g

Since the total mass of reactants is equal to the total mass of products, the law of conservation of mass is valid.

Question 12. If a species has 11 protons, 12 neutrons and 10 electrons, then
(i) What is its atomic number and mass number?
(ii) Is it neutral, a cation or an anion? Explain.
(iii) Write its electronic configuration.
(iv) Name the species.

Answer:
Given:

Number of protons = 11
Number of neutrons = 12
Number of electrons = 10

(i) Atomic number and mass number:

Atomic number = Number of protons

Atomic number = 11

Mass number = Number of protons + Number of neutrons

Mass number = 11 + 12
Mass number = 23

(ii) Nature of the species:

The species has 11 protons and 10 electrons.

Since there is one more proton than electrons, the species has a +1 charge.

Therefore, it is a cation.

(iii) Electronic configuration:

The species has 10 electrons.

Electronic configuration = 2, 8

(iv) Name of the species:

The element with atomic number 11 is sodium.

Since it has lost one electron and has a +1 charge, the species is sodium ion.

Name = Sodium ion
Symbol = Na+

Question 13. Two elements, A and B, have the following configurations-
A: 2,8,5
B: 2,8,7
(i) Which element is more reactive?
(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.
(iii) Predict the formula of the compound they would form.

Answer:
A has an electronic configuration 2, 8, 5. So, it has 5 valence electrons.

B has an electronic configuration 2, 8, 7. So, it has 7 valence electrons.

(i) Element B is more reactive because it needs only one electron to complete its octet. Elements with seven valence electrons are usually highly reactive.

(ii) A and B will form covalent bonds because both are non-metals and both need electrons to complete their octets. Instead of transferring electrons, they share electrons.

A needs three more electrons to complete its octet.
B needs one more electron to complete its octet.

So, one atom of A shares electrons with three atoms of B.

(iii) Formula of the compound:

A combines with three atoms of B.

Formula = AB3

Question 14. Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state.
Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state, they can move freely.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.

Answer:
The correct option is (iii) A is true, but R is false.

Assertion (A) is true because copper sulfate is an ionic compound. In the solid state, its ions are fixed in position and cannot move freely. So, solid copper sulfate does not conduct electricity.

In the molten state, the ions become free to move and can carry an electric current. Hence, molten copper sulfate conducts electricity.

Reason (R) is false because it states the opposite. In the molten state, ions are free to move, while in the solid state, ions are fixed in the lattice.

Question 15. The species 27Al, 80Br– and 201Hg2+  have 13,35 and 80 protons, respectively. How many electrons and neutrons do they have?

Answer:

Explanation:

For a neutral atom, the number of electrons = number of protons.

For a negative ion, electrons are added.

For Br-, one extra electron is present, so electrons = 35 + 1 = 36.

For a positive ion, electrons are removed.

For 1Hg2+, two electrons are lost, so electrons = 80 - 2 = 78.

Think it Over (NCERT Textbook Page No. 162)

Question 1. Water can be obtained from various sources. Are all these samples of water chemically identical?

Answer:
Pure samples of water from any source are chemically identical.

Water always contains hydrogen and oxygen combined in a fixed ratio. One molecule of water has two hydrogen atoms and one oxygen atom.

Formula of water = H2O

However, water obtained from rivers, wells, lakes, rain, or taps may contain dissolved salts, gases, dust, or other impurities. Such samples may not be identical in composition.

After purification, all pure water samples are chemically identical because their composition remains H2O.

Question 2. Oxygen is sometimes represented as O and sometimes as O2 . What is the difference between these symbols?

Answer:
O represents one atom of oxygen.

O2 represents one molecule of oxygen gas. It contains two oxygen atoms chemically bonded together.

Difference:

O = oxygen atom
O2 = oxygen molecule

An oxygen atom usually does not exist independently under normal conditions, but oxygen gas exists as O2 molecules.

Question 3. Why does dissolved salt in water conduct electricity, but sugar does not?

Answer:
Salt is an ionic compound. When salt dissolves in water, it dissociates into ions.

For example:

NaCl → Na+ + Cl-

These ions are free to move in the solution and carry electric current. Therefore, salt solution conducts electricity.

Sugar is a covalent compound. When sugar dissolves in water, it does not form ions. It remains as neutral molecules.

Since sugar solution does not contain free-moving ions, it does not conduct electricity.

Think as a Scientist (NCERT Textbook Page No. 166)

Question 1. You are given a chemical reaction in which zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.
Zinc + Hydrochloric acid (dilute) → Zinc chloride + Hydrogen
Design and perform an experiment to test the hypothesis that mass is conserved during the chemical reaction. You may use a setup different from the one shown in Activity 9.2.

Answer:
Aim: To verify the law of conservation of mass during the reaction between zinc and dilute hydrochloric acid.

Materials required:

• Conical flask

• Dilute hydrochloric acid

• Zinc granules

• Balloon or gas syringe

• Rubber cork

• Weighing balance

Procedure:

1. Take a clean and dry conical flask.

2. Add a small amount of dilute hydrochloric acid to the flask.

3. Put a few zinc granules in a small folded paper or in a small container kept inside the flask, making sure they do not react immediately.

4. Fix a balloon tightly over the mouth of the flask so that no gas can escape.

5. Measure the mass of the entire setup using a weighing balance and record it as the initial mass.

6. Now tilt the flask so that the zinc granules come in contact with dilute hydrochloric acid.

7. Allow the reaction to take place completely.

8. Hydrogen gas will be produced and collected inside the balloon.

9. After the reaction stops, measure the mass of the entire closed setup again and record it as the final mass.

Observation:

Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.

Effervescence is observed due to the formation of hydrogen gas.

The balloon inflates because hydrogen gas is collected inside it.

The initial mass and final mass of the closed setup remain the same.

Conclusion:

The total mass before the reaction is equal to the total mass after the reaction.

Therefore, mass is conserved during the chemical reaction.

This verifies the law of conservation of mass.

Pause and Ponder (NCERT Textbook Page No. 166)

Question 1. A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.

Answer:
No, the law of conservation of mass is not violated.

When ethanol burns, it reacts with oxygen present in the air and forms carbon dioxide and water vapour.

Ethanol + Oxygen → Carbon dioxide + Water vapour

In an open beaker, carbon dioxide and water vapour escape into the surroundings. Since these gaseous products are not collected or measured, it may appear that the mass has disappeared.

Actually, the mass is conserved. The total mass of ethanol and oxygen used is equal to the total mass of carbon dioxide and water vapour formed.

Therefore, the law of conservation of mass is valid, but the open setup does not allow all products to be measured.

Question 2. When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed according to the Law of Conservation of Mass?

Answer:
According to the law of conservation of mass:

Total mass of reactants = Total mass of products

Mass of hydrogen = 20 g
Mass of oxygen = 160 g

Total mass of reactants = 20 + 160
= 180 g

Since hydrogen and oxygen react completely, the mass of water formed will be equal to the total mass of reactants.

Therefore, mass of water formed = 180 g.

Pause and Ponder (NCERT Textbook Page No. 167)

Question 3. A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?

Answer:
The compound contains sulfur and oxygen in the ratio:

Sulfur : Oxygen = 40 : 60

Simplifying,

Sulfur : Oxygen = 2 : 3

This means 2 parts of sulfur combine with 3 parts of oxygen.

If sulfur = 20 g, then oxygen required is:

Oxygen = 20 × 3/2
= 30 g

Therefore, 30 g of oxygen must be present.

Question 4. Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?

Answer:
In carbon monoxide, the mass ratio of carbon to oxygen is:

Carbon : Oxygen = 3 : 4

This means 3 g of carbon combines with 4 g of oxygen.

For 9 g of carbon:

Oxygen required = 9 × 4/3
= 12 g

Therefore, 12 g of oxygen will combine with 9 g of carbon to form carbon monoxide.

Question 5. The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.

Answer:
The law of definite proportions states that a chemical compound always contains the same elements combined in a fixed ratio by mass.

This law holds for compounds because elements are chemically combined in fixed proportions. For example, water always contains hydrogen and oxygen in the fixed mass ratio of 1 : 8.

However, mixtures are formed by physical mixing of substances. Their composition can vary. For example, salt and sand can be mixed in any ratio.

Since mixtures do not have a fixed composition, the law of definite proportions does not apply to mixtures.

Question 6. Students X and Y both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.

Answer:
Yes, their results justify the law of constant proportions.

Student X obtained the ratio:

Copper : Oxygen = 4 : 1

Student Y obtained the ratio:

Copper : Oxygen = 8 : 2

Simplifying 8 : 2,

8 : 2 = 4 : 1

Both students obtained the same mass ratio of copper and oxygen.

Therefore, the compound contains copper and oxygen in a fixed ratio by mass. This confirms the law of constant proportions.

Pause and Ponder (NCERT Textbook Page No. 168)

Question 7. Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18g of water.
Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.

Answer:
The correct option is (ii) Both A and R are true, but R is not the correct explanation of A.

Assertion (A) is true because:

2 g hydrogen + 16 g oxygen = 18 g water

This follows the law of conservation of mass.

Reason (R) is also true because Dalton’s atomic theory states that atoms combine in simple whole-number ratios to form compounds.

However, the reason does not correctly explain the assertion. The assertion is mainly based on the conservation of mass, not on the whole-number ratio of atoms.

Therefore, both A and R are true, but R is not the correct explanation of A.

Pause and Ponder (NCERT Textbook Page No. 170)

Question 8. Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N2).

Answer:
Nitrogen has 5 valence electrons.

To complete its octet, each nitrogen atom needs 3 more electrons.

So, two nitrogen atoms share three pairs of electrons. This forms a triple covalent bond.

Structure of N2:

Each nitrogen atom has one lone pair of electrons and shares three pairs of electrons with the other nitrogen atom.

Thus, both nitrogen atoms complete their octets.

Question 9. The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F2 ).

Answer:
Atomic number of fluorine = 9

Electronic configuration of fluorine = 2, 7

Fluorine has 7 valence electrons and needs one more electron to complete its octet.

When two fluorine atoms come together, each atom shares one electron with the other. This forms one shared pair of electrons between the two atoms.

Thus, a single covalent bond is formed.

Structure of F2 :

Each fluorine atom gets a stable octet by sharing one pair of electrons.

Pause and Ponder (NCERT Textbook Page No. 171)

Question 10. Show the formation of the following molecules:
(i) Carbon dioxide (CO2)
(ii) Hydrogen sulfide (H2S)
(iii) Ammonia (NH3 )

Answer:
(i) Carbon dioxide (CO2)

Carbon has 4 valence electrons. Oxygen has 6 valence electrons.

Carbon needs 4 more electrons to complete its octet. Each oxygen atom needs 2 more electrons.

So, carbon shares two pairs of electrons with each oxygen atom. This forms two double covalent bonds.

Structure:

(ii) Hydrogen sulfide (H2S)

Sulfur has 6 valence electrons. Each hydrogen atom has 1 electron.

Sulfur needs 2 more electrons to complete its octet. Each hydrogen needs one more electron to complete its duplet.

So, sulfur shares one electron with each hydrogen atom.

Structure:

Sulfur forms two single covalent bonds with two hydrogen atoms.

(iii) Ammonia (NH3 )

Nitrogen has 5 valence electrons. Each hydrogen atom has 1 electron.

Nitrogen needs 3 more electrons to complete its octet. Three hydrogen atoms each share one electron with nitrogen.

Structure:

Nitrogen forms three single covalent bonds with three hydrogen atoms and has one lone pair of electrons.

Question 11. Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.

Answer:
Atomic number of neon = 10

Electronic configuration of neon = 2, 8

Neon already has 8 electrons in its outermost shell. This is a complete octet and is highly stable.

Since neon is already stable, it does not need to gain, lose, or share electrons.

Therefore, neon neither transfers nor shares its valence electrons. This is why neon is chemically inert.

Pause and Ponder (NCERT Textbook Page No. 174)

Question 12. What kind of ion will oxygen (O) form?

Answer:
Oxygen has an atomic number of 8.

Electronic configuration of oxygen = 2, 6

Oxygen has 6 valence electrons and needs 2 more electrons to complete its octet.

So, oxygen gains two electrons and forms a negatively charged ion.

O +  2e– → O2- 

Therefore, oxygen forms an oxide ion, O2-, which is an anion.

Question 13. Fill in the blanks.
Among magnesium and chlorine, magnesium atom can give two electrons to become Mg2+. However, chlorine can take only one electron to become …………………… . Now, ……………………… ion of magnesium and …………………… ions of chlorine combine to give magnesium chloride.

Answer:
Among magnesium and chlorine, magnesium atom can give two electrons to become Mg2+. However, chlorine can take only one electron to become Çl–. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride.

Question 14. Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.
Answer:
Formation of cation of potassium atom:
The electronic configuration of potassium (K) is 2, 8, 8, 1. It loses one electron to become K+ (cation).

Formation of the cation of calcium atom: The electronic configuration of calcium (Ca) is 2, 8, 8, 2. It loses two electrons to become Ca2+ (cation).

Formation of an anion of a chlorine atom: The electronic configuration of chlorine (Cl) is 2, 8, 7. It gains one electron to become Cl– (anion).

Formation of potassium chloride (KCl):
In the formation of potassium chloride, the electron lost by a potassium atom is gained by a chlorine atom. The oppositely charged ions (K and Cl–) are held together by electrostatic forces of attraction, forming an ionic bond

Formation of calcium chloride (CaCl2):
In the formation of calcium chloride, the two electrons lost by a calcium atom are gained by two chlorine atoms. The ions (Ca2+ and 2Cl–) are held together by electrostatic forces of attraction, forming an ionic bond.

Question 15. illustrate how sodium sulfide (Na2S) is formed.
Answer:
Sodium (Na) has the electronic configuration 2, 8, 1 and tends to lose one electron to form Na+ (cation). While sulfur (S) has the electronic configuration 2, 8, 6 and requires two electrons to complete its octet, So it gains two electrons to form S2- (anion). Two sodium atoms each donate one electron to one sulfur atom.
2Na → 2Na+ + 2e–
S+2e– → S2-
The oppositely charged ions (Na+ and S2- are held together by electrostatic forces of attraction, forming an ionic bond. Thus, Sodium sulfide (Na2S) is formed by the transfer of electrons from sodium to sulfur.
2Na+ + S2- → Na2S

Pause and Ponder (NCERT Textbook Page No. 177)

Question 16. Name the following:
(i) CO2
(ii) NO2
(iii) SF6
(iv) PCl3
Answer:
(i) Carbon dioxide
(ii) Nitrogen dioxide
(iii) Sulfur hexafluoride
(iv) Phosphorus trichloride

Question 17. Write the formula for the following:
(i) Sodium hydrogencarbonate
(ii) Sulfur dioxide
(iii) Ferric chloride
(iv) Cuprous oxide
Answer:
(i) Sodium hydrogencarbonate – NaHCO3
(ii) Sulfur dioxide – SO2
(iii) Ferric chloride – FeCl3
(iv) Cuprous oxide – Cu2O

Question 18. Write the formulae for the compounds formed from the following pairs of ions:
(i) Fe3+ and OH–
(ii) K– and CO32-
Answer:
(i) Fe3+ and OH–

Formula: Fe(OH)3

(ii) K– and CO32-

Formula: K2CO3.

Pause and Ponder (NCERT Textbook Page No. 179)

Question 19. What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state, but conducts electricity when dissolved in water?
Answer:
Such compounds contain ionic bonds.

• In the solid state, the ions are held in a fixed lattice structure by strong electrostatic forces and are not free to move, so they do not conduct electricity.

• When dissolved in water, the lattice breaks down, and the ions become free to move, allowing the solution to conduct electricity.

Question 20. Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water.
Predict its:
(i) formula
(ii) type of bond
(iii) electrical conductivity of its aqueous solution.
Answer:
Metal M has the electronic configuration 2, 8, 2, so it is Magnesium (Mg).

(ii) Type of bond: Ionic bond, formed by the transfer of electrons from the magnesium (metal) to oxygen (non-metal).

(iii) Electrical conductivity: The aqueous solution conducts electricity because ions are present in the solution. When MgO reacts with water, it forms magnesium hydroxide Mg(OH)2, which releases ions into the solution, allowing it to conduct electricity.

Question 21. Find the molecular mass of nitric acid (HNO3). Atomic mass H = 1 u; N= 14 u; O = 16 u.
Answer:
Nitric acid (HNO3)
Molecular mass
= (1 × mass of H) + (1 × mass of N) + (3 × mass of O)
=(1 × 1 u)+(1 × 14u)+(3 × 16u)
1 u + 14 u + 48 u = 63 u

Question 22. Find the molecular mass of methane (CH4).
Atomic mass C = 12 u; H = 1 u.
Answer:
Methane (CH4)
Molecular mass = (1 × mass of C) + (4 × mass of H)
= (1 × 12 u) + ( 4 × 1 u) = 12 u + 4 u = 16 u

Pause and Ponder (NCERT Textbook Page No. 180)

Question 23. Find the formula unit mass of potassium chloride (KCl). Atomic mass K = 39 u; Cl = 35.5 u.
Answer:
Formula unit mass of KCl
= (1 × mass of K) + (1 × mass of Cl)
=(1 × 39u)+(1 × 35.5u)
= 39 u + 35.5 u= 74.5 u

Question 24. Find the formula unit mass of magnesium hydroxide, Mg(OH)2. Atomic mass Mg = 24 u; 0 16 u; H = 1 u.
Answer:
Formula unit mass of Mg(OH)2
= (1 × mass of Mg) + (2 × mass of O) + (2 × mass of H)
= (1 × 24u) + (2 × 16u) + (2 × 1 u)
= 24 u + 32 u + 2 u = 58 u

What if… (NCERT Textbook Page No. 174)

Question 1. Could we see atoms directly? How would it help scientists, and what challenges would it cause?
Answer:
If we could see atoms directly, it would revolutionise science, but also bring significant technical and practical challenges. How would it help scientists? Better understanding of matter: Scientists could directly observe how atoms are arranged in substances and how they combine.

Clear view of chemical reactions:

• Reactions could be observed step-by-step at the atomic level, improving understanding of how new substances form.

• Advanced technology development: It would help in designing new materials, medicines, and nanotechnology with greater precision.

• Verification of theories: Atomic models and theories could be confirmed more accurately.

Challenges it would cause:

• Technical limitations: Atoms are extremely small, so observing them directly requires highly advanced instruments.

• Disturbance of atoms: Observing atoms might alter their position or behaviour, making accurate study difficult.

• High cost and complexity: The required equipment would be expensive and difficult to operate.

• Data overload: Handling and interpreting such detailed information could be challenging.

Class 9 Science Chapter 9 Question Answer (Activities)

Activity 9.1: Let Us Investigate A Physical Change (NCERT Textbook Page No. 163)

Aim: To verify that mass is conserved during a physical change (dissolving salt in water).

Observation: The mass of the solution after dissolving the salt remains equal to the total mass of salt and water taken initially.

Conclusion: During the formation of a solution, the total mass remains constant. Hence, mass is conserved during a physical change.

Activity 9.2: Let Us Investigate A Chemical Change (NCERT Textbook Page No. 163)

Experimental set-up – 1
Aim: To investigate whether mass is conserved during a chemical change in an open setup (reaction between vinegar and baking soda).

Vinegar + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + Other substances

Observation:

• When baking soda is added to vinegar, brisk effervescence is observed due to the formation of carbon dioxide gas.

• The final reading on the weighing balance is less than the initial reading.

• The initial and final masses are not the same.

Conclusion:
The mass appears to decrease because the gas produced escapes from the system.

Experimental set-up 2
Aim: To verify the law of conservation of mass during a chemical reaction in a closed setup (reaction between vinegar and baking soda).

Observation:

• When baking soda reacts with vinegar, brisk effervescence is again observed.

• The carbon dioxide gas produced inflates the balloon and does not escape.

• The final reading on the weighing balance remains equal to the initial reading.

• The initial and final masses are the same in this case.

Conclusion: The total mass before and after the reaction remains constant because the system is closed and no substance escapes. This activity shows that mass is conserved in a chemical reaction when it takes place in a closed system.

Activity 9.3: Let Us Verify The Law – Group Activity (NCERT Textbook Page No. 165)

Aim: To verify the law of conservation of mass by carrying out a chemical reaction between sodium sulfate and barium chloride solutions.

Observation: Initially, the total mass of both conical flasks (A and B) containing their respective solution is recorded. When the solutions are mixed, a white precipitate of barium sulfate is formed. After the reaction, the total mass of the system (both flasks together) is measured again. It is observed that there is no change in the total mass before and after the reaction.

Conclusion: The experiment shows that mass remains constant during a chemical reaction. Hence, it verifies the law of conservation of mass, which states that matter can neither be created nor destroyed in a chemical reaction.

Activity 9.4: Let Us Experiment (NCERT Textbook Page No. 177-178)

Aim: To study and compare the solubility and electrical conductivity of different ionic and covalent compounds.

Observation: Table 9.2: Solubility and Electrical Conductivity Observations 

• Sodium chloride, copper sulfate and calcium chloride dissolve in water but do not dissolve in kerosene and petrol.

• Camphor and naphthalene do not dissolve in water but dissolve in kerosene and petrol.

• Sugar dissolves in water but does not dissolve in kerosene and petrol.

• In the solid state, none of the compounds conduct electricity.

• Aqueous solutions of sodium chloride, copper sulfate, and calcium chloride conduct electricity, as indicated by the glowing bulb.

• Aqueous solutions of sugar do not conduct electricity as they do not provides ions in the solution.

• Camphor and naphthalene do not conduct electricity in any form.

Table 9.2: Solubility and Electrical Conductivity Observations

Conclusion:

• Ionic compounds are generally soluble in water and insoluble in organic solvents like kerosene and petrol.

• Covalent compounds are generally insoluble in water but soluble in organic solvents.

• Ionic compounds do not conduct electricity in the solid state because ions are fixed in position by strong forces.

• Ionic compounds conduct electricity in aqueous solution because they produce free-moving ions.

• Covalent compounds do not conduct electricity because they do not form ions in solution.

Class 9 Science Chapter 9 Atomic Foundations of Matter Solutions

Vedantu provides NCERT Solutions for Class 9 Science Chapter 9, Atomic Foundations of Matter, from the Exploration textbook for the 2026-27 academic session. This chapter helps students understand how atoms form the basis of matter and how elements combine to form molecules, ions, and compounds. It explains important concepts such as laws of chemical combination, atoms and molecules, covalent bonds, ionic bonds, chemical formulae, molecular mass, formula unit mass, and the properties of ionic and covalent compounds.

The solutions include clear answers for exercise questions, in-text questions, assertion-reason questions, formula-based questions, table-completion tasks, activity-based questions, and numerical problems. Students can use these solutions to learn how to write chemical formulae, balance ionic charges, calculate molecular mass, understand the law of conservation of mass, and compare ionic and covalent compounds. The downloadable FREE PDF also helps students revise the full chapter offline before class tests and exams.

CBSE Class 9 Science Chapter 9 Study Materials

Students can use the Chapter 9 study materials below to revise important Chemistry concepts, practise extra questions, and strengthen their understanding of atoms, molecules, ions, bonds, formulae, and chemical laws.

Explore More NCERT Solutions for Class 9 Science Chapters

The chapter-wise NCERT Solutions for Class 9 Science help students understand concepts from different areas of science in a simple and organised way. These resources provide clear explanations, textbook answers, activity-based solutions, diagrams, examples, and revision support for each chapter.

Related Study Material for Class 9 Science

The following Class 9 Science study materials support concept learning, practice, revision, and exam preparation. Students can use them along with the Exploration textbook solutions for better understanding and regular study.