Chapter 9 of 13

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Updated 6 August 2026

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Atomic Foundations of Matter

CBSE · Class 9 · Science

NCERT Solutions for Atomic Foundations of Matter — CBSE Class 9 Science.

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86 Questions Solved · 8 Sections

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Think It Over

2Oxygen is sometimes represented as O and sometimes as

\mathrm{O}_2

. What is the difference between these symbols?Show solution

O represents a single oxygen atom, while **O

_2

represents an oxygen molecule** made of two oxygen atoms joined together.

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3Why does dissolved salt in water conduct electricity, but sugar does not?Show solution

Dissolved salt in water conducts electricity because salt forms ions in solution, and these ions are free to move and carry charge. Sugar does not conduct electricity because it dissolves as molecules and does not produce ions in water.

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Activity 9.1: Let us investigate a physical change

1Place a clean and dry

100~\mathrm{mL}

beaker on a digital weighing balance.Show solution

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2Set the balance reading to zero by pressing the tare or reset button.Show solution

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3Pour about

50~\mathrm{mL}

of water into the beaker.Show solution

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4Add a spatula full of common salt to the water contained in the beaker.Show solution

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5Record the reading on the weighing balance (Fig. 9.1a).Show solution

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6Swirl until the added salt dissolves and record your observations (Fig. 9.1b).Show solution

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Activity 9.2: Let us investigate a chemical change

8What do you observe?Show solution

You observe that the mass of the solution is equal to the sum of the masses of water and salt taken. So there is practically no change in mass during the formation of the solution.

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9Are the initial and the final readings same?Show solution

In this case, the initial and final readings are the same because the mass is conserved when the gas produced does not escape.

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Experimental set-up 2

1Place a clean, dry

100~\mathrm{mL}

conical flask and a medium-sized balloon on a weighing balance.Show solution

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3Pour about

20~\mathrm{mL}

of vinegar or lemon juice into the conical flask.Show solution

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4Place about

2\mathrm{g}

of baking soda (sodium hydrogencarbonate) in the balloon.Show solution

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5Fix the balloon to the mouth of the conical flask using a thread, without allowing the baking soda to mix with the vinegar.Show solution

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6Weigh the conical flask containing vinegar and the balloon containing baking soda, and record the reading (Fig. 9.3a).Show solution

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7Lift the other end of the balloon upwards, allowing the baking soda (sodium hydrogencarbonate) to fall into the vinegar (Fig. 9.3b).Show solution

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9As in experimental set-up 1, a brisk effervescence occurs, which inflates the balloon during the reaction.Show solution

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10Record the final reading (Fig. 9.3c).Show solution

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11Are the initial and the final readings same in this case?Show solution

In experimental set-up 1, the final reading is not the same as the initial reading because the gas produced in the reaction escapes, so the measured mass decreases.

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Activity 9.3: Let us verify the law — Group activity

1Place two clean and dry

100~\mathrm{mL}

conical flasks on a weighing balance, and mark them A and B.Show solution

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3Pour about

10\mathrm{mL}

1\% m / v

sodium sulfate solution into the Conical Flask marked A.Show solution

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4In the Conical Flask B, pour about

10\mathrm{mL}

1\% m / v

barium chloride solution.Show solution

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5Leave both Conical Flasks A and B on the weighing balance undisturbed, and record the total mass of both the solutions (Fig. 9.4a).Show solution

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6Transfer the solution from Conical Flask B to Conical Flask A and mix the two solutions carefully.Show solution

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8Now, place both the Conical Flasks A and B on the weighing balance again as shown in Fig. 9.4b, and note the reading.Show solution

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9Do you observe any change in the reading after mixing the solutions?Show solution

No change is observed in the reading after mixing the solutions because the reaction is carried out in a way that no matter escapes, so the total mass remains the same.

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Think as a Scientist

1Design and perform an experiment to test the hypothesis that mass is conserved during the chemical reaction. You may use a set-up different from the one shown in Activity 9.2.Show solution

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2You 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 set-up different from the one shown in Activity 9.2.Show solution

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Pause and Ponder

1A 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.Show solution

No, this does not mean the Law of Conservation of Mass is violated. In an open beaker, the ethanol burns and its products such as carbon dioxide and water vapour escape into the air. Since the products are not all collected and weighed, no residue is left in the beaker. The total mass would be conserved if the system were closed and all products were accounted for.

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

By 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

So, the mass of water formed = 180 g.

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3A compound consists of

40\%

sulfur and

60\%

oxygen by mass. In a sample of the same compound containing

20\mathrm{g}

of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?Show solution

The compound has 40% sulfur and 60% oxygen by mass.

So the mass ratio of sulfur : oxygen =

40:60 = 2:3

.

If sulfur =

20\,g

, then oxygen needed is

\frac{3}{2} \times 20 = 30\,g

So, 30 g of oxygen must be present.

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4Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with

9\mathrm{g}

of carbon to form carbon monoxide?Show solution

In carbon monoxide, carbon : oxygen =

3:4

.

If carbon =

9\,g

, then oxygen required is

\frac{4}{3} \times 9 = 12\,g

So, 12 g of oxygen will combine with

9\,g

of carbon.

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

A compound always has its elements combined in a fixed ratio by mass, no matter how it is formed. A mixture does not have a fixed composition; its components can be present in any proportion. Therefore, the Law of Definite Proportions holds for compounds but not for mixtures.

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6Students 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.Show solution

Yes, their results justify the Law of Constant Proportions. In both cases, the ratio of copper to oxygen is the same:

-

4:1

8:2 = 4:1

So copper and oxygen are combining in a fixed ratio by mass, which supports the law.

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7Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g 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.Show solution

Both statements are true. Water has mass

2\,g + 16\,g = 18\,g

, so Assertion (A) is correct. Dalton's Atomic Theory says atoms combine in simple whole number ratios to form compounds, which explains why hydrogen and oxygen combine to form water in a fixed proportion. Hence, R correctly explains A.

Correct option: (i)

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8Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule

(\mathrm{N}_2)

.Show solution

Nitrogen has 5 valence electrons, so each nitrogen atom needs 3 more electrons to complete its octet. Two nitrogen atoms therefore share three pairs of electrons, forming a triple bond.

Structure: N\u2261N

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9The atomic number of fluorine is 9. Explain the formation of the fluorine molecule

(\mathrm{F}_2)

.Show solution

Fluorine has atomic number 9, so its electronic configuration is 2, 7. Each fluorine atom needs one electron to complete its octet. Therefore, two fluorine atoms share one electron each and form the molecule **F

_2

with a single covalent bond.

Structure: F—F**

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10Show the formation of the following molecules:

(i) Carbon dioxide

(\mathrm{CO}_{2})

(ii) Hydrogen sulfide

(\mathrm{H}_2\mathrm{S})

(iii) Ammonia

(\mathrm{NH}_3)

Show solution

- **Carbon dioxide (CO

_2

): Carbon has valency 4 and oxygen has valency 2, so one carbon atom shares electrons with two oxygen atoms. Structure: O=C=O
- Hydrogen sulfide (H

_2

S): Sulfur needs 2 electrons; each hydrogen shares 1 electron. Structure: H—S—H
- Ammonia (NH

_3

): Nitrogen needs 3 electrons; three hydrogen atoms each share 1 electron with nitrogen. Structure:

H

|

H—N—H**

These are covalent molecules formed by sharing of electrons.

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

Neon has electronic configuration 2, 8. Its valence shell is already complete with an octet, so it is stable. Therefore, neon neither gains, loses, nor shares electrons.

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12What kind of ion will oxygen (O) form?Show solution

Oxygen has 6 valence electrons and needs 2 more to complete its octet. So it forms a divalent anion, the oxide ion, written as **O

^{2-}

**.

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13Fill in the blanks.

Among magnesium and chlorine, magnesium atom can give two electrons to become

\mathrm{Mg}^{2+}

. However, chlorine can take only one electron to become ________. Now, ________ ion of magnesium and ________ ions of chlorine combine to give magnesium chloride.Show solution

Chlorine can take one electron to become **Cl

^-

. Then, one Mg

^{2+}

ion and two** chloride ions combine to give magnesium chloride.

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14Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.Show solution

For potassium:
- Potassium atom has 1 valence electron.
- It loses 1 electron to form **K

^+

.
- With chlorine, the compound formed is KCl.

For calcium:
- Calcium atom has 2 valence electrons.
- It loses 2 electrons to form Ca

^{2+}

.
- With chlorine, the compound formed is CaCl

_2

.

These are formed by electron transfer and ionic bonding**.

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15Illustrate how sodium sulfide

(\mathrm{Na}_2\mathrm{S})

is formed.Show solution

Sodium has 1 valence electron and sulfur has 6 valence electrons.

- Two sodium atoms each lose one electron to form **2 Na

^+

.
- Sulfur gains those two electrons to form S

^{2-}

.
- The oppositely charged ions combine to form Na

_2

S.

So sodium sulfide is formed by transfer of electrons and an ionic bond**.

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16Name the following:

(i)

\mathrm{CO}_{2}

(ii)

\mathrm{NO}_2

(iii)

\mathrm{SF}_6

(iv)

\mathrm{PCl}_3

17Write the formula for the following:

(i) Sodium hydrogencarbonate
(ii) Sulfur dioxide
(iii) Ferric chloride
(iv) Cuprous oxide

18Write the formulae for the compounds formed from the following pairs of ions:

(i)

\mathrm{Fe}^{3+}

and

\mathrm{OH}^{-}

(ii)

\mathrm{K}^{+}

and

\mathrm{CO}_{3}^{2-}

19What 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?

20Metal 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.

21Find the molecular mass of nitric acid (

\mathrm{HNO}_3

22Find the molecular mass of methane (

\mathrm{CH}_4

23Find the formula unit mass of potassium chloride (KCl).

24Find the formula unit mass of magnesium hydroxide,

\mathrm{Mg(OH)_2}

1A 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 ion would it form?
(iii) How many electrons does B tend to give or take to become stable?
(iv) What kind of ion 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?

2An 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.

3You 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)

2\mathrm{Al}^{3+}

and

3\mathrm{Cl}^{-}

(ii)

3\mathrm{Mg}^{2+}

and

1\mathrm{PO}_4^{3-}

(iii)

2\mathrm{Fe}^{3+}

and

3\mathrm{O}^{2-}

(iv)

3\mathrm{Ca}^{2+}

and

2\mathrm{SO}_4^{2-}

4Choose 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.

5Write the chemical formulae for the following compounds.

(i) Aluminium nitrate
(ii) Calcium oxide
(iii) Ferric oxide

6Write the formulae of the compounds formed from the following pairs of ions.

(i)

\mathrm{Ca^{2 + }}

and

\mathrm{Br^{-}}

(ii)

\mathrm{Al}^{3+}

and

\mathrm{CO}_{3}^{2-}

(iii)

\mathrm{K}^+

and

\mathrm{SO}_4^{2-}

(iv)

\mathrm{NH}_4^+

and

\mathrm{Cl}^-

7Which of the following, in Fig. 9.18, correctly represents

\mathrm{Cl^-}

ion (Atomic number of chlorine = 17).

8Determine the formula unit mass of the following substances.

(i) Ammonium nitrate

\mathrm{(NH_4NO_3)}

, used as a nitrogen fertiliser, which is essential for plant growth.
(ii) Phosphoric acid

\mathrm{(H_3PO_4)}

, used to make phosphate fertiliser and detergents.
(iii) Sodium hydrogencarbonate

\mathrm{(NaHCO_3)}

, used to relieve acidity and helps in digestion.

9Write 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

10Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top.

\mathrm{LiNO}_3

is given as an example.

5.3\mathrm{g}

of sodium carbonate and

6.0\mathrm{g}

of acetic acid react to produce

2.2\mathrm{g}

of carbon dioxide,

0.9\mathrm{g}

of water, and

8.2\mathrm{g}

of sodium acetate. Verify whether the law of conservation of mass is valid.

12If 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.

13Two 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.

14Assertion (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.

15The species

^{27}\mathrm{Al}

^{80}\mathrm{Br}^{-}

and

^{201}\mathrm{Hg}^{2+}

have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?

10Show the formation of the following molecules:

13Fill in the blanks.

16Name the following:

17Write the formula for the following:

18Write the formulae for the compounds formed from the following pairs of ions:

20Metal 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:

24Find the formula unit mass of magnesium hydroxide,

\mathrm{Mg(OH)_2}

Revise, Reflect, Refine

1A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.

2An element X has six electrons in its outer shell and forms a diatomic molecule.

3You 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?

4Choose the correct statement(s) and correct the false statement(s).

5Write the chemical formulae for the following compounds.

6Write the formulae of the compounds formed from the following pairs of ions.

8Determine the formula unit mass of the following substances.

9Write the formulae for the compounds formed by the reaction of:

12If a species has 11 protons, 12 neutrons and 10 electrons then

13Two elements, A and B, have the following configurations—

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Frequently Asked Questions

What are the important topics in Atomic Foundations of Matter for CBSE Class 9 Science?

Key topics in Atomic Foundations of Matter include Chapter 9 Gravitation — Complete Concept Map, Gravitation Chapter — Complete Concept Map, Chapter 9 Gravitation — Concept Overview. These are the concepts CBSE Class 9 examiners draw on most — study them first, then practise related questions.

How to score full marks in Atomic Foundations of Matter — CBSE Class 9 Science?

Understand the core concepts first, then work through the 83 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

Where can I get free NCERT Solutions for Atomic Foundations of Matter Class 9 Science?

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Atomic Foundations of Matter

Studying Atomic Foundations of Matter? Get the full interactive chapter.

Think It Over

Activity 9.1: Let us investigate a physical change

Activity 9.2: Let us investigate a chemical change

Experimental set-up 2

Activity 9.3: Let us verify the law — Group activity

Think as a Scientist

Pause and Ponder

Revise, Reflect, Refine

Frequently Asked Questions

Sources & Official References

More resources for Atomic Foundations of Matter

Practice Quiz

Important Questions

Revision Notes

Formula Sheet

Chapter Summary

Concept Maps

Study Plan

Flashcards

Syllabus

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