ICSE Class 10 Chemistry Board Exam Question Paper 2026 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION A (40 Marks)
(Attempt all questions from this Section.)

Question 1
Choose the correct answers to the questions from the given options. [15]
(Do not copy the questions, write the correct answers only.)

(i) A non-metal which reacts with concentrated sulphuric acid to form two gases which turn lime water milky is ____________. [1 Mark]
(A) Sulphur
(B) Carbon
(C) Oxygen
(D) Nitrogen

Answer: (B) Carbon

Carbon reacts with hot concentrated sulphuric acid to form carbon dioxide and sulphur dioxide (\( \text{C} + 2\text{H}_2\text{SO}_4 \rightarrow \text{CO}_2 + 2\text{SO}_2 + 2\text{H}_2\text{O} \)), both of which turn lime water milky.

Teacher's Note:
a) Remember that non-metals like carbon and sulphur are oxidized by concentrated \(\text{H}_2\text{SO}_4\) to their respective oxides.
b) Do not confuse this with metals reacting with acids to give hydrogen gas.

(ii) Which of the following element pairs will form an ionic bond? [1 Mark]

(A) P
(B) Q
(C) R
(D) S

Answer: (C) R

Ionic bonds are formed by the complete transfer of electrons between a metal (Group 2, alkaline earth metals) and a non-metal (Group 17, halogens).

Teacher's Note:
a) Ionic compounds are typically formed between a reactive metal and a reactive non-metal.
b) Group 1 and 2 elements are both metals, whereas Group 14 and 16 elements generally form covalent bonds.

(iii) The electronic configuration of an element is 2, 8, 2. The hydroxide of this element can produce ____________ hydroxyl ions per molecule. [1 Mark]
(A) 3
(B) 2
(C) 1
(D) 0

Answer: (B) 2

The element with configuration 2, 8, 2 is Magnesium (Mg), which has a valency of 2. Its hydroxide is \(\text{Mg(OH)}_2\), which dissociates to produce 2 hydroxyl ions per molecule.

Teacher's Note:
a) The number of hydroxyl ions produced per molecule of a base corresponds to its acidity (number of replaceable \(\text{OH}^-\) ions).
b) Group 2 elements form di-acidic bases.

(iv) With respect to the electrolysis of copper (II) sulphate solution using copper electrodes, which statement is correct? [1 Mark]
(A) Copper metal is deposited at the negative electrode.
(B) Oxygen gas is produced at the positive electrode.
(C) The positive electrode increases in mass.
(D) The negative electrode decreases in mass.

Answer: (A) Copper metal is deposited at the negative electrode.

During the electrolysis of copper(II) sulphate using active copper electrodes, copper ions migrate to the cathode (negative electrode) and get discharged as copper metal.

Teacher's Note:
a) With active copper electrodes, the anode dissolves (decreases in mass) and the cathode receives copper (increases in mass).
b) No oxygen gas is evolved at the anode because copper anode itself undergoes oxidation.

(v) Identify the equation that shows the reaction of ethane with chlorine in the presence of ultraviolet light. [1 Mark]
(A) \(\text{C}_2\text{H}_6 + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_6\text{Cl}_2\)
(B) \(\text{C}_2\text{H}_6 + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_4\text{Cl}_2 + \text{H}_2\)
(C) \(\text{C}_2\text{H}_6 + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{HCl}\)
(D) \(\text{C}_2\text{H}_6 + \text{Cl}_2 \rightarrow 2\text{CH}_3\text{Cl}\)

Answer: (C) \(\text{C}_2\text{H}_6 + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{HCl}\)

Alkanes undergo substitution reactions with halogens in the presence of sunlight or ultraviolet light.

Teacher's Note:
a) Substitution involves replacing one hydrogen atom by a halogen atom, producing hydrogen chloride as a byproduct.
b) Addition reactions occur in unsaturated hydrocarbons, not alkanes.

(vi) A hydrocarbon X undergoes the reaction under suitable conditions as shown below:

[Figure: Structural equation showing alkane X (\(\text{C}_4\text{H}_{10}\)) reacting to form alkene Y (\(\text{C}_3\text{H}_6\)) and alkane Z (\(\text{CH}_4\), methane)]

Which compound(s) is/are unsaturated? [1 Mark]
(A) X only
(B) Y only
(C) X and Z
(D) Y and Z

Answer: (B) Y only

Compound Y contains a carbon-carbon double bond (\(\text{C}=\text{C}\)), making it an alkene, which is an unsaturated hydrocarbon.

Teacher's Note:
a) Alkanes (like X and Z) contain only single covalent bonds and are saturated.
b) Alkenes and alkynes contain multiple bonds and are unsaturated.

(vii) Rita added dilute hydrochloric acid to four metals. She recorded her observations in the table given below. While noting her observations she made some errors. [1 Mark]

Which observation(s) made by Rita is/are correct?
(A) 1 only
(B) 3 only
(C) Both 1 and 4
(D) Both 2 and 4

Answer: (D) Both 2 and 4

Metals placed above hydrogen in the reactivity series (such as iron and zinc) react with dilute acids to evolve hydrogen gas. Copper is below hydrogen and does not react, while magnesium reacts vigorously to evolve gas.

Teacher's Note:
a) Metals below hydrogen cannot displace hydrogen from dilute acids.
b) Magnesium definitely gives off hydrogen gas with dilute \(\text{HCl}\), so observation 3 is incorrect.

(viii) An atom of X forms an ion according to the equation \(\text{X} \rightarrow \text{X}^{+2}\). The atomic number of the atom X is: [1 Mark]
(A) 16
(B) 10
(C) 12
(D) 14

Answer: (C) 12

Formation of \(\text{X}^{+2}\) involves the loss of 2 electrons. Elements that lose 2 electrons to form stable divalent cations belong to Group 2, such as Magnesium with atomic number 12.

Teacher's Note:
a) Metals lose electrons to form positive ions (cations).
b) An atomic number of 12 corresponds to electronic configuration 2, 8, 2, which readily loses 2 electrons.

(ix) Salts can be prepared by the action of dilute acid on:
1. bases
2. carbonates
3. metals
4. sulphites
The method which cannot be used for the preparation of copper salts is: [1 Mark]
(A) 1
(B) 2
(C) 3
(D) 4

Answer: (C) 3

Copper is less reactive than hydrogen and cannot displace hydrogen from dilute acids; hence, copper salts cannot be prepared by the action of dilute acids on copper metal.

Teacher's Note:
a) Copper salts are prepared using copper oxide, copper carbonate, or by reacting copper with concentrated oxidizing acids.
b) Direct displacement using dilute acids only works for metals above hydrogen in the reactivity series.

(x) The compound that has the highest melting point amongst the following is: [1 Mark]
(A) Methane
(B) Sodium chloride
(C) Ammonia
(D) Ethanol

Answer: (B) Sodium chloride

Sodium chloride is an electrovalent (ionic) compound held together by strong electrostatic forces of attraction, requiring a very high amount of thermal energy to break the crystal lattice.

Teacher's Note:
a) Ionic compounds generally have high melting and boiling points compared to covalent compounds.
b) Methane, ammonia, and ethanol are covalent compounds with weak intermolecular forces.

(xi) Assertion (A): Dilute Sulphuric acid is a stronger electrolyte than concentrated Sulphuric acid.
Reason (R): Dilute Sulphuric acid has a higher concentration of mobile ions. [1 Mark]
(A) (A) is true but (R) is false.
(B) (A) is false but (R) is true.
(C) Both (A) and (R) are true and (R) is the correct explanation of (A).
(D) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Answer: (C) Both (A) and (R) are true and (R) is the correct explanation of (A).

Concentrated sulphuric acid is almost entirely unionized molecules and is a poor conductor, whereas dilute sulphuric acid undergoes complete ionization, providing a high concentration of ions to conduct electricity effectively.

Teacher's Note:
a) Electrolytic conduction depends on the presence of mobile ions.
b) Pure covalent or molecular acids in concentrated form do not furnish free ions.

(xii) What volume of carbon dioxide is produced at STP when 5 litres of propane is burnt completely according to the equation given below?
\(\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}\) [1 Mark]
(A) 10 litres
(B) 15 litres
(C) 20 litres
(D) 25 litres

Answer: (B) 15 litres

According to Gay-Lussac's Law of Combining Volumes, 1 volume of propane gives 3 volumes of carbon dioxide. Therefore, 5 litres of propane gives \(5 \times 3 = 15\) litres of carbon dioxide.

Teacher's Note:
a) Use stoichiometric coefficients directly as volume ratios for gases at constant temperature and pressure.
b) \(1\text{ volume of }\text{C}_3\text{H}_8 \equiv 3\text{ volumes of }\text{CO}_2\).

(xiii) An unsaturated hydrocarbon with three atoms of carbon and six atoms of hydrogen is: [1 Mark]
(A) propyne
(B) propane
(C) propene
(D) propanol

Answer: (C) propene

Propene belongs to the alkene homologous series with general formula \(\text{C}_n\text{H}_{2n}\), hence for \(n = 3\), the formula is \(\text{C}_3\text{H}_6\).

Teacher's Note:
a) Propane is a saturated alkane (\(\text{C}_3\text{H}_8\)).
b) Propyne is an alkyne with formula \(\text{C}_3\text{H}_4\).

(xiv) Assertion (A): In the electrolysis of acidified water, the volume of hydrogen liberated is twice the volume of oxygen formed.
Reason (R): Water has hydrogen and oxygen in the ratio of 1:2 by volume. [1 Mark]
(A) (A) is true but (R) is false.
(B) (A) is false but (R) is true.
(C) Both (A) and (R) are true and (R) is the correct explanation of (A).
(D) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Answer: (A) (A) is true but (R) is false.

Water consists of hydrogen and oxygen in the ratio of 2:1 by volume (or 2:1 by atoms in formula \(\text{H}_2\text{O}\)), making the assertion true and reason false.

Teacher's Note:
a) Chemical formula \(\text{H}_2\text{O}\) indicates two volumes of hydrogen combine with one volume of oxygen.
b) Always check the compositional ratio carefully.

(xv) In the electrolysis of molten aluminium oxide for extraction of aluminium, the following reactions take place:
1. \(\text{Al}^{+3} + 3\text{e}^- \rightarrow \text{Al}\)
2. \(2\text{O}^{-2} \rightarrow \text{O}_2 + 4\text{e}^-\)
3. \(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\)
4. \(\text{Al}_2\text{O}_3 \rightleftharpoons 2\text{Al}^{+3} + 3\text{O}^{-2}\)
The reaction that takes place at the negative electrode is: [1 Mark]
(A) 1
(B) 2
(C) 3
(D) 4

Answer: (A) 1

Reduction takes place at the negative electrode (cathode) where positively charged aluminium ions gain electrons to form liquid aluminium metal.

Teacher's Note:
a) Cathode attracts cations (\(\text{Al}^{3+}\)) for reduction.
b) Anode reactions involve oxidation of oxide ions or burning of carbon anodes.

Question 2

(i) Given below is the industrial process for the manufacture of ammonia gas. Study the schematic diagram to answer the following questions. [5 Marks]

[Figure: Schematic diagram of Haber's Process showing nitrogen and hydrogen entering a compressor (C), passing through a catalytic chamber, a condenser for liquid ammonia separation, and a recycler for unreacted gases.]

(a) Name the process.
(b) Which catalyst is used in the above process?
(c) In the above diagrammatic setup, how is ammonia gas separated from the unreacted gases to obtain liquid ammonia?
(d) Which two properties of ammonia gas can be demonstrated by the Fountain Experiment?

Answer:
(a) Haber's Process.
(b) Finely divided Iron (Fe) promoted by Molybdenum (Mo).
(c) By passing the mixture through a liquefier/condenser where ammonia is cooled and liquefied, leaving unreacted nitrogen and hydrogen to be recycled.

(d) Extremely high solubility of ammonia in water and its basic (alkaline) nature.

Teacher's Note:
a) Mention both the catalyst (iron) and promoter (molybdenum) for complete credit in part (b).
b) The fountain experiment strikingly demonstrates the high solubility of ammonia and the alkaline nature of its aqueous solution.

(ii) Identify the following terms / compounds: [5 Marks]
(a) A compound which reacts with water to give acetylene gas.
(b) The gas evolved when sodium propanoate is heated with soda lime.
(c) A reddish-brown precipitate formed when ferric chloride solution reacts with an alkali.
(d) A pair of electrons present in an atom which is not shared with any other atom during bond formation.
(e) The relative molecular mass of a substance expressed in grams.

Answer:
(a) Calcium carbide (\(\text{CaC}_2\)).
(b) Ethane (\(\text{C}_2\text{H}_6\)).
(c) Ferric hydroxide (\(\text{Fe(OH)}_3\)).
(d) Lone pair of electrons.
(e) Gram molecular mass (or Gram mole).

Teacher's Note:
a) Decarboxylation of sodium propanoate removes carbon dioxide as sodium carbonate, yielding ethane gas.
b) Ensure proper chemical formulas and precise chemical terminology are used.

(iii) Match Column A with Column B: [5 Marks]

Answer:
(a) - 4 (Redox)
(b) - 3 (Reduction)
(c) - 5 (Electrolytic dissociation)
(d) - 1 (Oxidation)
(e) - 2 (Ionization)

Teacher's Note:
a) Reaction (a) involves both reduction of copper oxide and oxidation of carbon, hence it is a redox reaction.
b) Polar covalent compounds like \(\text{HCl}\) undergo ionization in water, whereas ionic compounds like \(\text{PbBr}_2\) undergo electrolytic dissociation.

(iv) Complete the following sentences by choosing the correct option from the brackets: [5 Marks]
(a) The oxide that dissolves in caustic potash (\(\text{KOH}\)) is ____________. \([\text{ZnO} / \text{CuO}]\)
(b) The reaction that takes place at the anode during the electrolysis of molten lead bromide is ____________. \([2\text{Br}^- - 2\text{e}^- \rightarrow 2\text{Br} / 2\text{Br}^- - 2\text{e}^- \rightarrow \text{Br}_2]\)
(c) The volume occupied by 8 grams of oxygen gas at STP is ____________ litres. \([5.6 / 8.96]\) \([Atomic weight of \text{O} = 16]\)
(d) ____________ does not give hydrogen gas when it reacts with cold and very dilute nitric acid. \([\text{Cu} / \text{Mn}]\)
(e) ____________ is a polar covalent compound. \([\text{HCl} / \text{CCl}_4]\)

Answer:
(a) \(\text{ZnO}\)
(b) \(2\text{Br}^- - 2\text{e}^- \rightarrow \text{Br}_2\)
(c) \(5.6\)
(d) \(\text{Cu}\)
(e) \(\text{HCl}\)

Teacher's Note:
a) For part (c), moles of \(\text{O}_2 = 8 / 32 = 0.25\text{ moles}\). Volume at \(\text{STP} = 0.25 \times 22.4 = 5.6\text{ litres}\).
b) Zinc and magnesium react with very dilute \(\text{HNO}_3\) to give hydrogen, while copper does not react with dilute \(\text{HNO}_3\) to give hydrogen.

(v) (a) Give the IUPAC name for the following organic compounds: [5 Marks]
1. \(\text{HO}-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3\)
2. \(\text{H}_3\text{C}-\text{CH}_2-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3\)
(b) Draw the structural diagram for each of the following organic compounds:
1. butanal
2. pent-2-yne
3. The isomer of n-butane

Answer:
(a) 1. Butan-2-ol
2. 3-methylpentane
(b) 1. Butanal: \(\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CHO}\) (drawn with full single/double covalent bonds showing carbon, hydrogen, and oxygen atoms).
2. Pent-2-yne: \(\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_2-\text{CH}_3\).
3. Isomer of n-butane: 2-methylpropane (\(\text{(CH}_3)_3\text{CH}\)).

Teacher's Note:
a) IUPAC naming must follow longest chain and lowest locant rules strictly.
b) Structural diagrams must explicitly show every covalent bond between atoms.

SECTION B (40 Marks)
(Attempt any four questions from this Section.)

Question 3

(i) Alex was given a solution of an unknown salt Y for analysis. He performed the following tests and recorded his observations: [2 Marks]
- To a part of the solution Y, he added silver nitrate solution and obtained a white precipitate which was soluble in ammonium hydroxide solution.
- To the remaining solution he added a few drops of sodium hydroxide solution and obtained a pale blue precipitate.
Based on the observations made by Alex, identify:
(a) the anion and
(b) the cation present in salt Y.

Answer:
(a) Anion: Chloride ion (\(\text{Cl}^-\))
(b) Cation: Copper(II) ion (\(\text{Cu}^{2+}\))

Teacher's Note:
a) Silver chloride is a white precipitate soluble in ammonium hydroxide.
b) Copper(II) hydroxide is a pale blue precipitate.

(ii) Study the reaction given below and answer the questions that follow: [2 Marks]
\(\text{NaNO}_3 + \text{H}_2\text{SO}_4 (\text{conc.}) \xrightarrow{\lt 200^{\circ}\text{C}} \text{NaHSO}_4 + \text{HNO}_3\)

(a) Give one reason for maintaining the temperature below \(200^{\circ}\text{C}\).
(b) Why is concentrated Sulphuric acid used in the above reaction?

Answer:
(a) To prevent damage to the glass apparatus, save fuel, and avoid the formation of hard-to-remove normal sodium sulphate ($\text{Na}_2\text{SO}_4$).
(b) Concentrated sulphuric acid is a non-volatile acid which displaces the more volatile nitric acid from its salt.

Teacher's Note:
a) High temperatures cause corrosion of glassware and lead to formation of sodium sulphate.
b) Non-volatile acid displacement principle is key here.

(iii) Element A belongs to group 15 and period 2 of the Periodic Table. [3 Marks]
(a) Identify the element A.
(b) Write the formula of the compound formed when element A combines with hydrogen.
(c) Draw the dot (\(\bullet\)) and cross (\(\times\)) structure of the compound formed in (b) above.

Answer:
(a) Nitrogen (\(\text{N}\))
(b) Ammonia (\(\text{NH}_3\))
(c) Electron dot structure of \(\text{NH}_3\) showing three single covalent bonds between nitrogen and three hydrogen atoms, with one lone pair on nitrogen.

Teacher's Note:
a) Group 15, period 2 has atomic number 7 (Nitrogen).
b) Show shared electron pairs clearly using dots and crosses.

(iv) Complete and balance the following equations: [3 Marks]
(a) \(\text{C}_2\text{H}_4\text{Br}_2 + \text{KOH} (\text{alcoholic}) \rightarrow\)
(b) \(\text{NH}_3 (\text{excess}) + \text{Cl}_2 \rightarrow\)
(c) \(\text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\text{conc. }\text{H}_2\text{SO}_4} \)

Answer:
(a) \(\text{C}_2\text{H}_4\text{Br}_2 + 2\text{KOH} (\text{alc}) \rightarrow \text{C}_2\text{H}_2 + 2\text{KBr} + 2\text{H}_2\text{O}\)
(b) \(8\text{NH}_3 + 3\text{Cl}_2 \rightarrow \text{N}_2 + 6\text{NH}_4\text{Cl}\)
(c) \(\text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\text{conc. }\text{H}_2\text{SO}_4} 12\text{C} + 11\text{H}_2\text{O}\)

Teacher's Note:
a) Alcoholic KOH with vicinal dihalide produces alkynes (acetylene).
b) Excess ammonia with chlorine gives nitrogen gas and ammonium chloride.

Question 4

(i) Differentiate between the following pairs based on the criteria given in the bracket: [2 Marks]
(a) Acetic acid and Sulphuric acid (number of replaceable hydrogen ion per molecule)
(b) Electrolyte and Metallic conductor (the particles conducting electricity)

Answer:

Teacher's Note:
a) Acetic acid is a weak monobasic acid having only one ionizable hydrogen atom in its carboxyl group.
b) Metallic conduction involves electrons without chemical change, whereas electrolytic conduction involves ions with chemical decomposition.

(ii) Equal volumes of ammonia gas and chlorine gas are kept in two different containers under the same conditions of temperature and pressure.
Find the number of molecules contained in chlorine gas when the mass of ammonia is 34 g.
\([Atomic weight: \text{Cl}=35.5, \text{H}=1, \text{N}=14]\) [2 Marks]

Answer:
Molar mass of \(\text{NH}_3 = 14 + 3(1) = 17\text{ g/mol}\).
Moles of \(\text{NH}_3 = \frac{34\text{ g}}{17\text{ g/mol}} = 2\text{ moles}\).
Since equal volumes of gases under identical conditions contain equal number of moles (Avogadro's Law), moles of \(\text{Cl}_2 = 2\text{ moles}\).
Number of molecules in chlorine gas \(= 2 \times 6.022 \times 10^{23} = 1.2044 \times 10^{24}\) molecules.

Teacher's Note:
a) Apply Avogadro's hypothesis: equal volumes of all gases under same temperature and pressure contain equal number of molecules.
b) Multiply moles by Avogadro's number to get total molecules.

(iii) Elements P, Q, and R are in the same period of the modern periodic table. [3 Marks]
- P readily loses its one valence electron to form a stable ion.
- Q shares its electrons in bonding but does not form ions easily.
- R has high electronegativity.
Answer the following questions based on the above information:
(a) Which element would be most difficult to reduce among P, Q and R?
(b) Which element is expected to have the smallest atomic radius among P, Q and R?
(c) Arrange P, Q and R in order of decreasing ionization potential.

Answer:
(a) P (since P is a strongly electropositive alkali metal and easily oxidized, hence most difficult to reduce).
(b) R (Electronegativity is highest for halogens on the right, which have the smallest atomic radii in a period).
(c) Decreasing order of ionization potential: \(\text{R} \gt \text{Q} \gt \text{P}\).

Teacher's Note:
a) Ionization potential increases across a period from left to right.
b) Alkali metals (P) have lowest IP, while halogens/non-metals (R) have highest IP.

(iv) Given below are some chemicals. [3 Marks]

Using suitable chemicals from the box given above, write balanced chemical equations for the preparation of the following salts.
(Note: The chemicals may be used more than once, if needed.)
(a) Lead (II) carbonate
(b) Copper (II) chloride
(c) Iron (II) chloride

Answer:
(a) Lead(II) carbonate: \(\text{Pb(NO}_3)_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{PbCO}_3\downarrow + 2\text{NaNO}_3\)
(b) Copper(II) chloride: \(\text{CuCO}_3 + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow\)
(c) Iron(II) chloride: \(\text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2\uparrow\)

Teacher's Note:
a) Insoluble salts like lead carbonate are prepared by precipitation (double decomposition).
b) Active metals like iron react with dilute \(\text{HCl}\) to form iron(II) chloride and hydrogen gas.

Question 5

(i) An unsaturated hydrocarbon having two carbon atoms was made to react with two moles of hydrogen gas in the presence of a catalyst to give a saturated hydrocarbon. [3 Marks]
Using the above information, answer the following questions:
(a) The type of reaction that occurred is ____________. (addition / substitution)
(b) Name a suitable catalyst which is used in the reaction.
(c) Write a balanced chemical equation for the above reaction.

Answer:
(a) Addition
(b) Nickel (Ni) or Platinum (Pt) or Palladium (Pd)
(c) \(\text{C}_2\text{H}_2 + 2\text{H}_2 \xrightarrow{\text{Ni}} \text{C}_2\text{H}_6\)

Teacher's Note:
a) Since two moles of hydrogen reacted with a two-carbon unsaturated hydrocarbon, it is ethyne undergoing complete hydrogenation to ethane.
b) Addition reactions convert unsaturated compounds to saturated compounds.

(ii) During the manufacture of sulphuric acid, a chemist collected the gas formed after catalytic oxidation of sulphur dioxide. [3 Marks]
(a) Name the gas collected.
(b) Which acid is formed when the gas collected by the chemist is dissolved directly in water?
(c) Why should we not dissolve this gas directly in water?

Answer:
(a) Sulphur trioxide (\(\text{SO}_3\))
(b) Sulphuric acid (\(\text{H}_2\text{SO}_4\))
(c) Dissolution is highly exothermic, producing a dense, corrosive mist of acid droplets that is difficult to condense.

Teacher's Note:
a) Catalytic oxidation of \(\text{SO}_2\) produces \(\text{SO}_3\).
b) In Contact process, \(\text{SO}_3\) is absorbed in concentrated \(\text{H}_2\text{SO}_4\) to formoleum first.

(iii) The diagram given below shows the electroplating of a spoon with silver. With reference to the diagram, answer the following questions: [4 Marks]

[Figure: Electroplating setup showing a spoon as cathode, silver strip as anode, and sodium argento cyanide solution as electrolyte.]

(a) Write an equation for the reaction that occurs at the cathode.
(b) Silver nitrate solution is not preferred as an electrolyte in the above process. Give a reason.
(c) Why is alternating current not used in the process?
(d) What is the observation at the anode?

Answer:
(a) \(\text{Ag}^+ + \text{e}^- \rightarrow \text{Ag}\)
(b) Silver nitrate causes very rapid deposition of silver, resulting in a non-smooth, non-adherent coating.
(c) Alternating current causes continuous reversal of polarity, leading to no net electrodeposition.
(d) The silver anode gradually dissolves/decreases in mass.

Teacher's Note:
a) Use double cyanides like sodium argento cyanide for even and smooth electroplating.
b) Direct current (DC) is mandatory for electrolysis and electroplating.

Question 6

(i) Give reasons why: [2 Marks]
(a) Hydrochloric acid cannot form an acid salt.
(b) Electronegativity increases across a period.

Answer:
(a) Hydrochloric acid is a monobasic acid having only one replaceable hydrogen ion per molecule.
(b) Across a period, nuclear charge increases and atomic radius decreases, thereby increasing the attraction for shared electrons.

Teacher's Note:
a) Only polybasic acids form acid salts by partial replacement of hydrogen ions.
b) Electronegativity measures atom's tendency to attract shared electrons.

(ii) State whether the following statements are True or False: [2 Marks]
(a) Organic compounds having different molecular formula, but same structural formula are called isomers.
(b) A salt is a compound formed by partial or complete replacement of the hydrogen ion of an acid by a metal or electro positive ion.

Answer:
(a) False
(b) True

Teacher's Note:
a) Isomers have the *same* molecular formula but *different* structural formulas.
b) Statement (b) is the standard definition of a salt.

(iii) Anil is a laboratory assistant in a college. He prepared nitric acid but stored it in a transparent container exposed to sunlight. After a few days, brown fumes were observed in the container and the acid turned slightly yellowish in colour. [3 Marks]
(a) What was the error made by Anil while storing the acid?
(b) Why were brown fumes observed in the container?
(c) How can the yellowish tinge in the acid be removed?

Answer:
(a) He stored nitric acid in a transparent container exposed to sunlight instead of an amber-coloured bottle.
(b) Nitric acid undergoes photochemical decomposition to form nitrogen dioxide (brown gas), which dissolves in the acid.
(c) By bubbling air or carbon dioxide through the warm acid to drive out dissolved nitrogen dioxide.

Teacher's Note:
a) Nitric acid is photosensitive and decomposes as \(4\text{HNO}_3 \rightarrow 2\text{H}_2\text{O} + 4\text{NO}_2 + \text{O}_2\).
b) Amber bottles block light and prevent decomposition.

(iv) \(\text{Zn} + 4\text{HNO}_3 \rightarrow \text{Zn}(\text{NO}_3)_2 + 2\text{H}_2\text{O} + 2\text{NO}_2\)
32.5 g of zinc reacts with concentrated nitric acid as given in the above equation. [3 Marks]
(a) How many moles of zinc was required in the reaction?
(b) Find the mass of nitric acid needed to react with 32.5 g of zinc.
(c) Find the volume of nitrogen dioxide liberated in (b).
\([Atomic weight: \text{H}=1, \text{N}=14, \text{O}=16, \text{Zn}=65]\)

Answer:
(a) Moles of \(\text{Zn} = \frac{32.5\text{ g}}{65\text{ g/mol}} = 0.5\text{ moles}\).
(b) From equation, 1 mole of \(\text{Zn}\) requires 4 moles of \(\text{HNO}_3\). Therefore, 0.5 moles of \(\text{Zn}\) requires \(0.5 \times 4 = 2\text{ moles}\) of \(\text{HNO}_3\).
Mass of \(\text{HNO}_3 = 2 \times [1 + 14 + 48] = 2 \times 63 = 126\text{ g}\).
(c) From equation, 1 mole of \(\text{Zn}\) gives 2 moles of \(\text{NO}_2\). So 0.5 moles of \(\text{Zn}\) gives \(0.5 \times 2 = 1\text{ mole}\) of \(\text{NO}_2\).
Volume of \(\text{NO}_2\) at \(\text{STP} = 1 \times 22.4 = 22.4\text{ litres}\).

Teacher's Note:
a) Always calculate moles of the given substance first using its atomic mass.
b) Use stoichiometric ratios directly for mass and volume calculations.

Question 7

(i) Carbon reacts with an acid to form carbon dioxide, water and nitrogen dioxide. [2 Marks]
(a) Name the acid used in the reaction.
(b) Write a balanced chemical equation for the reaction that occurs.

Answer:
(a) Concentrated nitric acid (\(\text{HNO}_3\))
(b) \(\text{C} + 4\text{HNO}_3 (\text{conc.}) \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 4\text{NO}_2\)

Teacher's Note:
a) Concentrated nitric acid is a strong oxidizing agent that oxidizes non-metals like carbon and sulphur.
b) Check balancing of oxygen and nitrogen atoms carefully.

(ii) Bauxite is the principal ore used in the commercial extraction of aluminium. The Bayer's process is used to refine bauxite into pure alumina, with caustic soda playing a crucial role in the initial stage of the process. [2 Marks]
Based on this, answer the following questions:
(a) Explain the reason behind the addition of caustic soda during the Bayer's process.
(b) Write a balanced chemical equation representing the reaction between bauxite and caustic soda during the Bayer's process.

Answer:
(a) To dissolve amphoteric aluminium oxide from bauxite, leaving behind insoluble impurities like iron(III) oxide.
(b) \(\text{Al}_2\text{O}_3\cdot 2\text{H}_2\text{O} + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2\text{O}\)

Teacher's Note:
a) Caustic soda (\(\text{NaOH}\)) selectively leaches out alumina as soluble sodium aluminate.
b) Impurities (red mud) are filtered off.

(iii) Give one relevant observation for the following: [3 Marks]
(a) Sodium hydroxide is added dropwise to Calcium nitrate solution.
(b) Dilute Hydrochloric acid is added to Iron (II) sulphide.
(c) An amphoteric metal is added to hot concentrated alkali.

Answer:
(a) A sparingly soluble white precipitate of calcium hydroxide is formed.
(b) A gas having the smell of rotten eggs (hydrogen sulphide, \(\text{H}_2\text{S}\)) is evolved.
(c) Effervescence is observed and hydrogen gas is evolved.

Teacher's Note:
a) Calcium hydroxide is sparingly soluble, hence a white precipitate is formed only on dropwise addition.
b) Metal sulphides react with dilute acids to evolve \(\text{H}_2\text{S}\) gas.

(iv) Study the reaction scheme shown below and identify the reactants A, B and C. [3 Marks]

[Figure: Flowchart showing Ethanol reacting with A to form Ethene, Ethanol reacting with B to form Ethyl acetate, and Ethanol reacting with C to form Carbon dioxide and water.]

Answer:
A: Concentrated sulphuric acid (\(\text{conc. }\text{H}_2\text{SO}_4\)) at \(170^{\circ}\text{C}$
B: Acetic acid (\(\text{CH}_3\text{COOH}\)) in the presence of acid catalyst
C: Oxygen (\(\text{O}_2\)) / Air (Combustion)

Teacher's Note:
a) Dehydration of ethanol with conc. \(\text{H}_2\text{SO}_4\) gives ethene.
b) Esterification with acetic acid gives ethyl acetate.

Question 8

(i) 5.6 litres of gas Z weighs 32 g at STP. What is the molecular weight and vapour density of Z? [2 Marks]

Answer:
Moles of gas Z \(= \frac{5.6\text{ litres}}{22.4\text{ litres/mol}} = 0.25\text{ moles}\).
Molecular weight \(= \frac{\text{Mass}}{\text{Moles}} = \frac{32\text{ g}}{0.25\text{ mol}} = 128\text{ g/mol}\).
Vapour density \(= \frac{\text{Molecular weight}}{2} = \frac{128}{2} = 64\).

Teacher's Note:
a) Use molar volume at STP (\(22.4\text{ litres}\)) to find moles.
b) Vapour density is half of the relative molecular mass.

(ii) Name the following: [2 Marks]
(a) The most electronegative element of Period 2.
(b) The largest atom of Period 3.

Answer:
(a) Fluorine (\(\text{F}\))
(b) Sodium (\(\text{Na}\))

Teacher's Note:
a) Fluorine is the most electronegative element in the entire periodic table.
b) Atomic size decreases across a period from left to right, making Group 1 elements the largest in their respective periods.

(iii) Given below are a few elements. [3 Marks]

Identify the element which:
(a) has the least atomic radius.
(b) has two valence electrons.
(c) is the most electropositive.

Answer:
(a) Fluorine (\(\text{F}\))
(b) Calcium (\(\text{Ca}\))
(c) Potassium (\(\text{K}\)))

Teacher's Note:
a) Fluorine is in period 2, hence has the smallest atomic radius among the given elements.
b) Potassium is down Group 1, making it more electropositive than lithium.

(iv) K, L and M are beakers containing three different solutions as shown below. [3 Marks]

[Figure: Three beakers labeled K (Aqueous acetic acid), L (Sodium chloride solution), and M (Distilled water).]

Name the beaker which:
(a) contains only ions.
(b) contains only molecules.
(c) has pH less than 7.

Answer:
(a) L (Sodium chloride solution - strong electrolyte completely dissociated into ions)
(b) M (Distilled water - covalent molecules with negligible ionization)
(c) K (Aqueous acetic acid - acidic solution with \(\text{pH} < 7\))

Teacher's Note:
a) Strong electrolytes like \(\text{NaCl}\) exist almost entirely as ions in aqueous solution.
b) Acetic acid is a weak acid containing mostly unionized molecules along with a few ions, and its solution is acidic (\(\text{pH} < 7\)).