Circle of Learning • New NCERT textbook "Exploration" (2026-27) • Pause and Ponder + Revise, Reflect, Refine (all questions) + extra important questions
Pause and Ponder (In-text Questions)
A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left. Does this mean the Law of Conservation of Mass is violated? Explain.
Answer
No. Ethanol reacts with oxygen from the air to form carbon dioxide and water vapour, which are gases and escape from the open beaker. The mass has not disappeared; it has left as gases. If the reaction were done in a closed container, the total mass of reactants (ethanol + oxygen) would equal the total mass of products (CO2 + water).
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
Mass of water = 20 + 160 = 180 g
A compound consists of 40% sulfur and 60% oxygen by mass. In a sample containing 20 g of sulfur, what mass of oxygen must be present?
Answer
S : O = 40 : 60 = 2 : 3. For 20 g of sulfur, oxygen = 20 × 60/40 = 30 g
Carbon monoxide contains carbon and oxygen in the mass ratio of 3 : 4. How much oxygen will combine with 9 g of carbon to form CO?
Answer
Oxygen = 9 × 4/3 = 12 g
The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.
Answer
In a compound, elements are chemically combined in a fixed ratio by mass, so the composition is always the same. In a mixture, the components are only physically mixed and can be present in any proportion (e.g. a little or a lot of sugar in water). So mixtures have no fixed composition.
Students X and Y prepared an oxide of copper by combining copper and oxygen in the ratios 4 : 1 and 8 : 2 respectively. Do their results justify the Law of Constant Proportions?
Answer
Yes. 8 : 2 simplifies to 4 : 1. Both samples have copper and oxygen in the same ratio by mass, as the law requires for the same compound.
Assertion (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.
Answer
(iii) A is true, but R is false.
A is correct – it follows the law of conservation of mass (2 + 16 = 18 g) and the fixed ratio 1 : 8. R is not stated correctly: Dalton said atoms combine in a simple whole-number ratio of numbers of atoms (e.g. 2 H atoms : 1 O atom in water), not a ratio by mass. The mass ratio (1 : 8) results from the different masses of the atoms.
Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N2).
Answer
Each nitrogen atom (2, 5) needs 3 more electrons. The two atoms share three pairs of electrons, forming a triple bond: N≡N. In the electron-dot diagram, draw two N atoms side by side with three shared pairs between them and one lone pair on each atom.
The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F2).
Answer
Fluorine has configuration 2, 7 – it needs one more electron to complete its octet. Two fluorine atoms each share one electron, forming one shared pair (a single covalent bond). Both atoms now have 8 electrons in their outer shells. The molecule is F–F.
Show the formation of: (i) Carbon dioxide (CO2) (ii) Hydrogen sulfide (H2S) (iii) Ammonia (NH3)
Answer
(i) CO2: Carbon (2, 4) needs 4 electrons; each oxygen (2, 6) needs 2. Carbon shares two pairs with each oxygen, forming two double bonds: O=C=O. All three atoms complete their octets.
(ii) H2S: Sulfur (2, 8, 6) needs 2 electrons; each hydrogen needs 1. Sulfur shares one pair with each of two hydrogen atoms: H–S–H. Sulfur gets an octet and each hydrogen gets a duplet.
(iii) NH3: Nitrogen (2, 5) needs 3 electrons. It shares one pair with each of three hydrogen atoms, forming three single bonds; nitrogen keeps one lone pair. Nitrogen gets an octet and each hydrogen a duplet.
(Draw these with shells and dots/crosses as shown for H2O in Fig. 9.10.)
Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.
Answer
Neon has the configuration 2, 8 – its outermost shell already has a complete octet, which is very stable. It has no need to gain, lose or share electrons, so it does not form bonds. That is why neon is an inert (noble) gas with valency zero.
Answer
What kind of ion will oxygen (O) form?
Oxygen (2, 6) gains 2 electrons to complete its octet, forming an anion, O2− (oxide ion).
Fill in the blanks: 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
Chlorine can take only one electron to become Cl−. Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride (MgCl2).
Show the formation of cations of potassium (K) and calcium (Ca), and the formation of their chlorides using diagrams.
Answer
Potassium: K (2, 8, 8, 1)
⇒ K+ (2, 8, 8) + e−
K+ + Cl−
⇒ KCl (the electron lost by K is gained by Cl: 2, 8, 7
⇒ 2, 8, 8)
Calcium: Ca (2, 8, 8, 2)
⇒ Ca2+ (2, 8, 8) + 2e−
Ca2+ + 2Cl−
⇒ CaCl2 (each of two chlorine atoms takes one electron from Ca)
Diagram: draw shells of K and Cl with an arrow showing one electron moving from K's outer shell to Cl's outer shell; for Ca, draw two Cl atoms, one on each side, each receiving one electron.
Illustrate how sodium sulfide (Na2S) is formed.
Answer
Sodium (2, 8, 1) loses 1 electron to form Na+. Sulfur (2, 8, 6) needs 2 electrons. So two sodium atoms each give one electron to one sulfur atom:
2Na
⇒ 2Na+ + 2e−; S + 2e−
⇒ S2− (2, 8, 8)
2Na+ + S2− → Na2S
Answer
Name: (i) CO2 (ii) NO2 (iii) SF6 (iv) PCl3
(i) Carbon dioxide (ii) Nitrogen dioxide (iii) Sulfur hexafluoride (iv) Phosphorus trichloride
Write the formula for: (i) Sodium hydrogencarbonate (ii) Sulfur dioxide (iii) Ferric chloride (iv) Cuprous oxide
Answer
(i) NaHCO3 (ii) SO2 (iii) FeCl3 (iv) Cu2O
Write the formulae for compounds formed from: (i) Fe3+ and OH− (ii) K+ and CO32−
Answer
(i) Fe(OH)3 (ii) K2CO3
What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state but conducts when dissolved in water?
Answer
An ionic bond. In the solid, ions are fixed in the crystal lattice and cannot move; in water, the ions become free to move and carry current.
Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound slightly soluble in water. Predict its (i) formula (ii) type of bond (iii) electrical conductivity of its aqueous solution.
Answer
M has configuration 2, 8, 2 – it is magnesium, valency 2.
(i) MO (MgO)
(ii) Ionic bond (M2+ and O2−)
(iii) Its aqueous solution conducts electricity (weakly, as only a small amount dissolves) because it contains free ions.
Find the molecular mass of nitric acid (HNO3).
Answer
1 + 14 + 3 × 16 = 63 u
Find the molecular mass of methane (CH4).
Answer
12 + 4 × 1 = 16 u
Find the formula unit mass of potassium chloride (KCl).
Answer
39 + 35.5 = 74.5 u
Find the formula unit mass of magnesium hydroxide, Mg(OH)2.
Answer
24 + 2 × (16 + 1) = 24 + 34 = 58 u
Revise, Reflect, Refine (Textbook Exercise)
Element A has one electron in its third shell. Element B has six electrons in its second shell. (i) How many electrons does A give or take? (ii) What ion would it form? (iii) How many electrons does B give or take? (iv) What ion would it form? (v) What bond would form? (vi) Formula of the compound?
Answer
A = 2, 8, 1 (sodium); B = 2, 6 (oxygen).
(i) A tends to give (lose) 1 electron.
(ii) A forms a cation, A+ (Na+).
(iii) B tends to take (gain) 2 electrons.
(iv) B forms an anion, B2− (O2−).
(v) An ionic bond (electron transfer).
(vi) A2B (Na2O).
An element X has six electrons in its outer shell and forms a diatomic molecule. (i) Why? (ii) What kind of bond? (iii) Draw the structure. (iv) Element Y has two electrons in its second shell. Draw the structure of the molecule X would form with Y.
(i) X needs 2 more electrons to complete its octet. Two X atoms can each share 2 electrons with the other, so both become stable by forming a diatomic molecule X2 (like O2).
(ii) A covalent double bond (two shared pairs).
(iii) X=X – two atoms with two shared pairs between them and two lone pairs on each atom.
Answer
(iv) Y has configuration 2, 2 (beryllium), so it has 2 valence electrons and valency 2. X (valency 2) and Y (valency 2) combine in a 1 : 1 ratio to form YX (e.g. BeO). Structure: Y's two valence electrons pair with X's two unpaired electrons – drawn as Y=X with two shared pairs, X completing its octet.
You want to design an ionic compound with total positive charge 6+ and total negative charge 6−. Which combination is correct?
Answer
(iii) 2 Fe3+ and 3 O2−
⇒ (+6) and (−6). (Fe2O3)
Check others: (i) +6 and −3; (ii) +6 and −3; (iv) +6 and −4 – not balanced.
Choose the correct statement(s) and correct the false ones.
Answer
(i) Elements are made up of molecules and compounds are made up of atoms. – False. Elements are made of only one kind of atom (which may exist as single atoms or as molecules, e.g. O2); compounds are made of molecules or formula units containing atoms of two or more different elements.
(ii) The molecule of a compound is always made up of two or more atoms of the same kind. – False. A molecule of a compound contains atoms of different kinds (e.g. H2O).
(iii) One molecule of nitrogen gas contains three nitrogen atoms. – False. It contains two nitrogen atoms (N2).
(iv) Water is made of two hydrogen atoms covalently bonded with one oxygen atom. – True.
Write the chemical formulae: (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide
Answer
(i) Al(NO3)3 (ii) CaO (iii) Fe2O3
Write formulae for compounds from: (i) Ca2+ and Br− (ii) Al3+ and CO32− (iii) K+ and SO42− (iv) NH4+ and Cl−
Answer
(i) CaBr2 (ii) Al2(CO3)3 (iii) K2SO4 (iv) NH4Cl
Which of the diagrams in Fig. 9.18 correctly represents the Cl− ion (atomic number of chlorine = 17)?
Answer
(ii). Cl− has 17 + 1 = 18 electrons arranged as 2, 8, 8. Diagram (ii) shows 2 electrons in the K shell, 8 in the L shell and 8 in the M shell. (Diagram (iv) shows 2, 8, 7 – a neutral chlorine atom; the other two have wrong numbers of electrons in their shells.)
Answer
Determine the formula unit mass of: (i) NH4NO3 (ii) H3PO4 (iii) NaHCO3
(Atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, P = 31 u)
(i) NH4NO3 = 14 + 4 + 14 + 48 = 80 u
(ii) H3PO4 = 3 + 31 + 64 = 98 u
(iii) NaHCO3 = 23 + 1 + 12 + 48 = 84 u
Write the formulae for compounds formed by: (i) Magnesium and nitrogen (ii) Lithium and nitrogen (iii) Sodium and sulfur (iv) Aluminium and oxygen
(i) Mg2+, N3− → Mg3N2
(ii) Li+, N3− → Li3N
(iii) Na+, S2− → Na2S
(iv) Al3+, O2− → Al2O3
Answer
Complete Table 9.3.
Answer
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 |
3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g CO2, 0.9 g water and 8.2 g sodium acetate. Verify the law of conservation of mass.
Answer
Mass of reactants = 5.3 + 6.0 = 11.3 g
Mass of products = 2.2 + 0.9 + 8.2 = 11.3 g
Both are equal, so the law of conservation of mass is valid.
A species has 11 protons, 12 neutrons and 10 electrons. (i) Atomic number and mass number? (ii) Neutral, cation or anion? (iii) Electronic configuration. (iv) Name the species.
(i) Atomic number = 11; mass number = 11 + 12 = 23
(ii) Cation – it has one more proton than electrons, so it carries a charge of +1.
(iii) 2, 8
(iv) Sodium ion, Na+
Answer
A: 2, 8, 5 and B: 2, 8, 7. (i) Which is more reactive? (ii) Will they form ionic or covalent bonds? (iii) Predict the formula.
Answer
A is phosphorus (15) and B is chlorine (17).
(i) B (chlorine) is more reactive – it needs only one electron to complete its octet, while A needs three.
(ii) Both are non-metals that need to gain electrons, so neither can easily give electrons to the other. They complete their octets by sharing electrons, forming covalent bonds. A shares one electron with each of three B atoms.
(iii) AB3 (PCl3)
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 the molten state, while in the solid state they can move freely.
Answer
(iii) A is true, but R is false. It is the other way round – ions are fixed in the solid lattice and become free to move in the molten state.
The species 27Al, 80Br− and 201Hg2+ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?
Answer
Species | Electrons | Neutrons |
27Al (neutral atom) | 13 | 27 − 13 = 14 |
80Br− (gained 1 electron) | 35 + 1 = 36 | 80 − 35 = 45 |
201Hg2+ (lost 2 electrons) | 80 − 2 = 78 | 201 − 80 = 121 |
Extra Important Questions
Multiple Choice Questions (1 mark)
1. The law of constant proportions was given by
(a) Lavoisier (b) Proust (c) Dalton (d) Thomson
Answer: (b) Proust (Lavoisier gave the law of conservation of mass)
Which of these is a covalent compound?
Answer
(a) NaCl (b) MgO (c) CH4 (d) CaCl2
Answer: (c) CH4
3. The valency of the sulfate ion is
(a) 1 (b) 2 (c) 3 (d) 4
Answer: (b) 2
4. Ionic compounds generally have
(a) low melting points (b) high melting points (c) no charge on particles (d) shared electrons
Answer: (b) high melting points
Short Answer Questions
Differentiate between ionic and covalent compounds (any three points).
Answer
Ionic compounds are formed by transfer of electrons, usually between metals and non-metals; they have high melting and boiling points and conduct electricity in molten or dissolved state. Covalent compounds are formed by sharing electrons, usually between non-metals; they generally have low melting and boiling points and do not conduct electricity.
What is a polyatomic ion? Give two examples.
Answer
A group of atoms carrying an overall charge and acting as a single unit, e.g. ammonium (NH4+), sulfate (SO42−), nitrate (NO3−).
What is the difference between molecular mass and formula unit mass?
Answer
Molecular mass is the sum of atomic masses of all atoms in a molecule of a covalent substance. Formula unit mass is the sum of atomic masses of all atoms in a formula unit of an ionic compound, since ionic compounds do not form discrete molecules.