ICSE Class 10 Chemistry Board Exam Question Paper 2024 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

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

Q1. Choose the correct answers to the questions from the given options. (Do not copy the questions, write the correct answers only.) [15 Marks]

1.1. The unsaturated hydrocarbons undergo ______. [1 Mark]
1. A substitution reaction
2. An oxidation reaction
3. An addition reaction
4. None of the above
5. Redox reaction

Answer: (3) An addition reaction

Unsaturated hydrocarbons contain double or triple bonds which can break to add other atoms.

Teacher's Note:
a) Remember that compounds with multiple bonds undergo addition reactions easily.
b) Do not confuse addition reactions with substitution reactions which are characteristic of saturated hydrocarbons.

1.2. In the 2^nd period Neon has maximum ionization Potential because ______. [1 Mark]
1. It has unstable electronic configuration.
2. It easily accepts electrons.
3. It easily loses electrons.
4. The outer most shell is completely filled.

Answer: (4) The outer most shell is completely filled.

A completely filled octet provides extreme electronic stability.

Teacher's Note:
a) Ionization potential increases across a period due to decreasing atomic size and increasing nuclear charge.
b) Noble gases have the highest ionization potential in their respective periods.

1.3. Copper, zinc, and Tin are the metals alloyed to form ______. [1 Mark]
1. Duralumin
2. Brass
3. Bronze
4. Solder

Answer: (3) Bronze

Bronze is primarily an alloy of copper and tin, often containing zinc as well.

Teacher's Note:
a) Learn the exact composition of common alloys such as brass, bronze, duralumin, and solder.
b) Brass consists of copper and zinc, while bronze contains copper, tin, and sometimes zinc.

1.4. The metal hydroxide which reacts with both acids and alkalis to form salt and water is ______. [1 Mark]
1. Calcium hydroxide
2. Magnesium hydroxide
3. Aluminium hydroxide
4. Ferric hydroxide

Answer: (3) Aluminium hydroxide

Aluminium hydroxide is amphoteric and reacts with both acids and alkalis.

Teacher's Note:
a) Amphoteric hydroxides include Al(OH)₃, Zn(OH)₂, and Pb(OH)₂.
b) Ensure you write balanced equations for both acidic and basic reactions of amphoteric substances.

1.5. Reaction of an alcohol with a carboxylic acid in the presence of concentrated H₂SO₄ is termed as ______. [1 Mark]
1. Halogenation
2. Esterification
3. Hydrogenation
4. Dehydrohalogenation

Answer: (2) Esterification

Esters are formed with a sweet fruity smell when carboxylic acids react with alcohols.

Teacher's Note:
a) Concentrated sulphuric acid acts as a dehydrating agent in this reaction.
b) Practice writing the structural equations for esterification between ethanol and acetic acid.

1.6. Conversion of ethanol to ethene by the action of concentrated sulphuric acid is an example of _________. [1 Mark]
1. Dehydration
2. Dehydrogenation
3. Dehydrohalogenation
4. Hydrolysis

Answer: (1) Dehydration

Concentrated sulphuric acid removes a molecule of water from ethanol.

Teacher's Note:
a) Note the specific temperature required (170 degrees Celsius) for the catalytic dehydration of ethanol.
b) Distinguish clearly between dehydration (removal of water) and dehydrogenation (removal of hydrogen).

1.7. The oxidizing agent in the equation S + 2H₂SO₄ → 3SO₂ + 2H₂O is ______. [1 Mark]
1. Sulphur
2. Sulphuric acid
3. Sulphur dioxide
4. Water

Answer: (2) Sulphuric acid

Sulphuric acid oxidizes sulphur to sulphur dioxide while getting reduced itself.

Teacher's Note:
a) An oxidizing agent is the substance that gets reduced and brings about oxidation of another species.
b) Look at the change in oxidation states to correctly identify oxidizing and reducing agents in redox reactions.

1.8. Electron Affinity is maximum in ______. [1 Mark]
1. Mg
2. Ar
3. Li
4. Br

Answer: (4) Br

Halogens possess very high electron affinity due to their tendency to complete their octet.

Teacher's Note:
a) Electron affinity generally increases across a period and decreases down a group.
b) Chlorine actually has the highest electron affinity in the periodic table, but among the given options, Bromine is the correct choice.

1.9. The compound that is not a constituent of the electrolytic mixture used in Hall-Heroult's process is ______. [1 Mark]
1. Al₂O₃
2. NaAlO₂
3. Na₃AlF₆
4. CaF₂

Answer: (2) NaAlO₂

Sodium aluminate is an intermediate in the Bayer's process, not an electrolyte component in Hall-Heroult's process.

Teacher's Note:
a) The Hall-Heroult electrolytic bath consists of alumina, molten cryolite (Na₃AlF₆), and fluorspar (CaF₂).
b) Cryolite lowers the fusion temperature and increases the electrical conductivity of the mixture.

1.10. On passing ammonia gas over heated copper oxide for some time, a reddish-brown residue is left behind. What property of ammonia is demonstrated here? [1 Mark]
1. Basic property
2. Oxidising property
3. Reducing property
4. Acidic property

Answer: (3) Reducing property

Ammonia reduces copper(II) oxide to copper metal.

Teacher's Note:
a) Ammonia acts as a strong reducing agent when reacted with metal oxides.
b) Note the balanced equation: 3CuO + 2NH₃ → 3Cu + 3H₂O + N₂.

1.11. Rotten egg smell is due to the liberation of ______. [1 Mark]
1. HCl gas
2. H₂S gas
3. Cl₂ gas
4. SO₂ gas

Answer: (2) H₂S gas

Hydrogen sulphide gas is well known for its characteristic foul smell of rotten eggs.

Teacher's Note:
a) Memorize the characteristic odors and colors of common laboratory gases.
b) H₂S is also identified by its property of turning lead acetate paper silvery black.

1.12. Ammonia gas is collected by downward displacement of air since ammonia is ______. [1 Mark]
1. Very slightly soluble in water
2. Heavier than air
3. Lighter than air
4. Insoluble in water

Answer: (3) Lighter than air

Ammonia has a vapour density of 8.5, making it much lighter than air.

Teacher's Note:
a) Because ammonia is extremely soluble in water, it cannot be collected over water.
b) Its upward displacement of air method is used due to its lower density compared to air.

1.13. Which of the following would occupy 22.4 litres at S.T.P.?
1. 32 g of oxygen gas
2. 2 moles of hydrogen gas
3. 6.022 × 10²³ molecules of ammonia
[Atomic weights: O = 16, H = 1, N = 14] [1 Mark]
1. 1 and 2
2. 1 and 3
3. 2 and 3
4. 1, 2 and 3

Answer: (2) 1 and 3

32 g of O₂ is 1 mole, and 6.022 × 10²³ molecules of NH₃ is also 1 mole, each occupying 22.4 L at STP.

Teacher's Note:
a) One mole of any ideal gas occupies 22.4 litres at standard temperature and pressure.
b) Two moles of hydrogen gas will occupy 44.8 litres at STP.

1.14. In the molecule of water, oxygen atom has ______. [1 Mark]
1. One shared pair of electrons
2. Three shared pairs of electrons
3. Two lone pairs of electrons
4. One lone pair of electrons

Answer: (3) Two lone pairs of electrons

Oxygen has 6 valence electrons, forms 2 single covalent bonds with hydrogen, leaving 4 non-bonding electrons as two lone pairs.

Teacher's Note:
a) Draw the Lewis dot structure to clearly visualize lone pairs and bond pairs.
b) Water has a bent molecular geometry due to lone pair-lone pair repulsions.

1.15. A mineral from which the metal can be extracted economically and conveniently is known as ______. [1 Mark]
1. Matrix
2. Ore
3. Flux
4. Alloy

Answer: (2) Ore

An ore is a special mineral with a high percentage of metal that makes profitable extraction possible.

Teacher's Note:
a) All ores are minerals, but all minerals are not ores.
b) Gangue or matrix refers to the unwanted earthly impurities present in an ore.

Q2. [25 Marks]

2.1. The following sketch represents the electroplating of an Iron cup with Nickel metal.
Study the diagram and answer the following questions:
a. During electroplating, the iron cup is placed at the cathode. Why?
b. Name the ion that must be present in the electrolyte.
c. State one condition that is necessary to ensure that the deposit is smooth, firm and even.
d. Write the reaction taking place at the cathode.
e. What change would you observe at the anode? [5 Marks]

[Figure: Electroplating cell with an anode (Nickel plate) and cathode (Iron cup) dipped in an electrolyte solution connected to a DC source through an ammeter.]

Answer:
a. The article to be electroplated is always placed at the cathode so that metal cations get reduced and deposited on it.
b. Nickel ions (\( \text{Ni}^{2+} \)) must be present in the electrolyte.
c. A low electric current should be passed for a longer duration of time.
d. \( \text{Ni}^{2+} + 2e^{-} \rightarrow \text{Ni} \)
e. The nickel anode gradually dissolves into the electrolyte solution.

Teacher's Note:
a) Always connect the article to be plated to the negative terminal (cathode) of the DC source.
b) The concentration of metal ions in the electrolyte remains constant if a rod of the same metal is used as the anode.

2.2. Match the Column A with Column B. [5 Marks]

Teacher's Note:
a) Match each property or category carefully with the corresponding chemical substance.
b) Double-check all pairings to ensure complete accuracy before finalizing.

2.3. Complete the following sentences by choosing the correct answer from the brackets: [5 Marks]

2.3. (a) The salt prepared by the method of direct combination is _______.
1. Iron (II) chloride (\( \text{FeCl}_2 \))
2. Iron (III) chloride (\( \text{FeCl}_3 \)) [1 Mark]

Answer: Iron (III) chloride (\( \text{FeCl}_3 \))

Direct action of dry chlorine gas on heated iron metal produces anhydrous iron (III) chloride.

Teacher's Note:
a) Chlorine is a strong oxidizing agent and oxidizes iron to its higher chloride state, \( \text{FeCl}_3 \).
b) Iron (II) chloride cannot be prepared by this method because it would be oxidized further.

2.3. (b) The metallic oxide which can be reduced by using common reducing agents is ______.
1. \( \text{Fe}_2\text{O}_3 \)
2. \( \text{Al}_2\text{O}_3 \) [1 Mark]

Answer: \( \text{Fe}_2\text{O}_3 \)

Fe₂O₃ is moderately reactive and can be reduced using carbon monoxide or hydrogen.

Teacher's Note:
a) Highly reactive metals like aluminium form oxides with very high thermodynamic stability.
b) Therefore, \( \text{Al}_2\text{O}_3 \) requires electrolytic reduction rather than chemical reduction.

2.3. (c) The metal nitrate which on thermal decomposition forms a black residue is ______.
1. zinc nitrate
2. copper nitrate [1 Mark]

Answer: copper nitrate

Thermal decomposition of copper nitrate yields black copper(II) oxide along with nitrogen dioxide and oxygen.

Teacher's Note:
a) Note the color changes during the thermal decomposition of metal nitrates.
b) Zinc nitrate leaves a yellow residue when hot and white when cold.

2.3. (d) During the electrolysis of copper sulphate solution, if ______ is used as electrodes, the colour of the electrolyte does not fade.
1. copper
2. platinum [1 Mark]

Answer: copper

Using active copper electrodes ensures that copper ions dissolving at the anode replenish those discharged at the cathode.

Teacher's Note:
a) With platinum electrodes, the blue color fades because copper ions are depleted from the solution.
b) Active electrodes participate in the electrolytic reaction.

2.3. (e) The process of heating the concentrated ore in a limited supply or absence of air is ______.
1. Roasting
2. Calcination [1 Mark]

Answer: Calcination

Calcination expels moisture and volatile impurities by heating carbonate or hydroxide ores without melting them in the absence of air.

Teacher's Note:
a) Roasting is done in the presence of excess air, typically for sulphide ores.
b) Calcination is performed in the absence or limited supply of air for carbonate ores.

2.4. State the term for the following: [5 Marks]

2.4. (a) The group obtained by removing one hydrogen atom from the parent alkane. [1 Mark]

Answer: Alkyl group

General formula for an alkyl group is \( \text{C}_n\text{H}_{2n+1} \).

Teacher's Note:
a) Alkyl groups are represented by the symbol R.
b) Examples include methyl, ethyl, and propyl groups.

2.4. (b) Two metal plates or wires through which the current enters and leaves the electrolytic cell. [1 Mark]

Answer: Electrodes

Electrodes consist of the anode and the cathode.

Teacher's Note:
a) Oxidation occurs at the anode while reduction occurs at the cathode.
b) Electrodes can be inert or active depending on the electrolytic process.

2.4. (c) The amount of substance which contains the same number of units as the number of atoms in carbon-12. [1 Mark]

Answer: Mole

One mole contains Avogadro's number (\( 6.022 \times 10^{23} \)) of elementary entities.

Teacher's Note:
a) The mole is the SI unit for amount of substance.
b) One mole of any gas occupies 22.4 dm³ at STP.

2.4. (d) The tendency of an atom to pull a shared pair of electrons towards itself in a compound. [1 Mark]

Answer: Electronegativity

Fluorine is the most electronegative element in the periodic table.

Teacher's Note:
a) Electronegativity increases across a period and decreases down a group.
b) Do not confuse electronegativity with electron affinity, which is the energy released when an electron is added.

2.4. (e) The formula which represents the simplest ratio between the atoms of elements present in a compound. [1 Mark]

Answer: Empirical formula

It gives the simplest whole number ratio of atoms of each element in a compound.

Teacher's Note:
a) Molecular formula is a simple integer multiple of the empirical formula.
b) Empirical formula mass is calculated using atomic masses of the simplest ratio.

2.5. (a)
1. Give the IUPAC name of the organic compound represented by the structural formula given below:
\( \text{H-C(H)(Cl)-C(H)(Cl)-C(H}_2\text{)-C(H}_2\text{)-C(H}_3 \) (represented via structural lines)
2. Give the IUPAC name of the organic compound represented by the structural formula given below:
\( \text{CH}_3-\text{CH}_2-\text{COOH} \) [5 Marks]

[Figure: 1. Pentane chain with chlorine atoms at positions 2 and 3. 2. Propanoic acid structure.]

Answer:
1. 2, 3-Dichloropentane
2. Propanoic acid (or propan-1-oic acid)

Teacher's Note:
a) Number the carbon chain from the end that gives substituents the lowest possible locants.
b) Identify the principal functional group to determine the correct suffix (-oic acid for carboxylic acids).

2.5. (b)
1. Draw the structural diagram for the following organic compound: 3-methyl pentane
2. Write the structural formula of propyne.
3. Give the structural formula of the following organic compound: Methanal [5 Marks]

Answer:
1. Pentane chain with a methyl group attached to carbon 3: \( \text{CH}_3-\text{CH}_2-\text{CH(CH}_3)-\text{CH}_2-\text{CH}_3 \)
2. Propyne: \( \text{CH}_3-\text{C}\equiv\text{CH} \)
3. Methanal: \( \text{H-CHO} \) (formaldehyde with a double bond between carbon and oxygen)

Teacher's Note:
a) Verify valency of carbon (four bonds) in all drawn structures.
b) Show all covalent bonds clearly when drawing full structural diagrams.

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

Q3. [10 Marks]

3.1. (a) Rewrite the following statement by adding the correct word, as shown in the example:
Example:
Given Statement: Ammonia changes moist red litmus to blue.
Correct Statement: Aqueous ammonia changes moist red litmus to blue.
Sulphuric acid acts as a dehydrating agent. [1 Mark]

Answer: Concentrated sulphuric acid acts as a dehydrating agent.

Only concentrated sulphuric acid has a strong affinity for water to act as a dehydrating agent.

Teacher's Note:
a) Dilute sulphuric acid acts as a typical dibasic acid and does not dehydrate compounds.
b) Always specify concentration when describing chemical properties of acids.

3.1. (b) Rewrite the following statement by adding the correct word, as shown in the example:
Example:
Given Statement: Ammonia changes moist red litmus to blue.
Correct Statement: Aqueous ammonia changes moist red litmus to blue.
Ammonia reacts with chlorine to give ammonium chloride and nitrogen. [1 Mark]

Answer: Excess ammonia reacts with chlorine to give ammonium chloride and nitrogen.

When chlorine is in excess, it forms an explosive nitrogen trichloride instead.

Teacher's Note:
a) Learn both reactions of ammonia with chlorine depending on which reactant is in excess.
b) Excess ammonia yields nitrogen and ammonium chloride.

3.2. (a) Identify only the anion present in the following compound:
The compound, on heating, produces a colourless, odourless gas which turns lime water milky and has no effect on acidified potassium dichromate solution. [1 Mark]

Answer: Carbonate (\( \text{CO}_3^{2-} \))

Carbonates evolve carbon dioxide on heating, which turns lime water milky.

Teacher's Note:
a) Carbon dioxide does not affect acidified potassium dichromate solution, distinguishing it from sulphur dioxide.
b) Write the formula of the anion clearly as requested.

3.2. (b) Identify only the anion present in the following compound:
The solution of the compound which on treating with concentrated sulphuric acid and freshly prepared ferrous sulphate solution produces a brown ring. [1 Mark]

Answer: Nitrate (\( \text{NO}_3^{-} \))

The brown ring test is specific for the confirmation of nitrate ions.

Teacher's Note:
a) The brown ring is formed due to the complex compound \( [\text{Fe}(\text{H}_2\text{O})_5\text{NO}]\text{SO}_4 \).
b) Ferrous sulphate must be freshly prepared for the test to succeed.

3.3. Mohan has three solutions P, Q and R having a pH of 13, 5 and 2 respectively.
(a) Which of the above solutions P, Q or R will react with magnesium to liberate hydrogen gas?
(b) Which of the above solutions P, Q or R will liberate ammonia gas when it reacts with ammonium chloride?
(c) Which of the above solutions P, Q or R will contain molecules as well as ions? [3 Marks]

Answer:
(a) Solution R (pH = 2)
(b) Solution P (pH = 13)
(c) Solution Q (pH = 5)

Teacher's Note:
a) Acids react with active metals to liberate hydrogen gas.
b) Weak acids (like Q with pH 5) are partially dissociated and contain both molecules and ions in solution.

3.4. The following table is related to an industrial process of an acid.
Identify (a), (b) and (c). [3 Marks]

Answer:
(a) Contact process
(b) Vanadium pentoxide (\( \text{V}_2\text{O}_5 \)) (or platinised asbestos)
(c) Sulphuric acid (\( \text{H}_2\text{SO}_4 \))

Teacher's Note:
a) The Contact process is used for the large-scale industrial manufacture of sulphuric acid.
b) Platinum is also a catalyst, but vanadium pentoxide is preferred because it is less susceptible to poisoning.

Q4. [10 Marks]

4.1. (a) Define the term: Molar volume [1 Mark]

Answer: The volume occupied by one mole of any gas at standard temperature and pressure (STP), which is equal to 22.4 dm³.

Teacher's Note:
a) State the exact numerical value along with units.
b) Avogadro's law forms the basis of molar volume.

4.1. (b) Define normal salt. [1 Mark]

Answer: A normal salt is formed by the complete replacement of all the ionizable hydrogen atoms of an acid by a metallic or an ammonium ion.

Teacher's Note:
a) Examples include sodium chloride (\( \text{NaCl} \)) and potassium sulphate (\( \text{K}_2\text{SO}_4 \)).
b) They do not contain any replacement-capable hydrogen atoms in their molecules.

4.2. (a) Draw the electron dot structure of Methane molecule.
[Atomic number: N = 7, C = 6, H = 1] [2 Marks]

Answer:
Carbon shares its 4 valence electrons with 4 hydrogen atoms to form four single covalent bonds, achieving a stable octet for carbon and duplet for each hydrogen atom.

Teacher's Note:
a) Use dots and crosses clearly to represent electrons from different atoms.
b) Check that carbon completes its octet in the final drawing.

4.2. (b) Draw the electron dot structure of Nitrogen molecule.
[Atomic number: N = 7, C = 6, H = 1] [2 Marks]

Answer:
Two nitrogen atoms share three pairs of electrons between them, forming a triple covalent bond with one lone pair on each nitrogen atom.

Teacher's Note:
a) Nitrogen gas molecule consists of a stable triple covalent bond.
b) Both atoms attain a stable octet configuration.

4.3. Complete and balance the following equation:
(a) \( \text{Al}_2\text{O}_3 + \text{NaOH} \rightarrow \)
(b) \( \text{C}_2\text{H}_5\text{COONa} + \text{NaOH} \xrightarrow{\Delta/\text{CaO}} \)
(c) \( \text{C}_2\text{H}_4\text{Br}_2 + \text{alcoholic KOH} \xrightarrow{\Delta} \) [3 Marks]

Answer:
(a) \( \text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O} \)
(b) \( \text{C}_2\text{H}_5\text{COONa} + \text{NaOH} \xrightarrow{\Delta/\text{CaO}} \text{C}_2\text{H}_6 + \text{Na}_2\text{CO}_3 \)
(c) \( \text{C}_2\text{H}_4\text{Br}_2 + 2\text{alc. KOH} \xrightarrow{\Delta} \text{C}_2\text{H}_2 + 2\text{KBr} + 2\text{H}_2\text{O} \)

Teacher's Note:
a) Equation (b) represents the decarboxylation reaction used for preparing alkanes.
b) Equation (c) is a dehydrohalogenation reaction yielding an alkyne.

4.4. Choose the organic compound from the list given below to answer the following questions:
List: Ethene, Ethanoic acid, Ethanol, Methanal
(a) The compound which does not have a double bond in its structure.
(b) The compound in its pure form turns into an ice like solid on cooling.
(c) The compound which is used for artificial ripening of fruits. [3 Marks]

Answer:
(a) Ethanol
(b) Ethanoic acid
(c) Ethene

Teacher's Note:
a) Pure ethanoic acid freezes at 16.6 degrees Celsius into an ice-like solid, earning it the name glacial acetic acid.
b) Ethene is a plant hormone used commercially for fruit ripening.

Q5. [10 Marks]

5.1. Name the main constituent metal in the following alloy:
(a) Duralumin
(b) Stainless steel [2 Marks]

Answer:
(a) Aluminium
(b) Iron

Teacher's Note:
a) Duralumin contains about 95% aluminium along with copper, magnesium, and manganese.
b) Stainless steel is primarily iron alloyed with chromium, nickel, and carbon.

5.2. Differentiate between the following pairs based on the criteria given:
(a) Sulphuric acid and Nitric acid (using barium chloride solution)
(b) Unsaturated and Saturated hydrocarbons (type of bond present) [2 Marks]

Answer:
(a) Sulphuric acid gives a white precipitate of barium sulphate with barium chloride, whereas nitric acid does not give any precipitate.
(b) Saturated hydrocarbons contain only single covalent bonds between carbon atoms, whereas unsaturated hydrocarbons contain at least one double or triple bond.

Teacher's Note:
a) Barium chloride test is a confirmatory test for sulphate ions.
b) Saturated hydrocarbons are less reactive due to strong sigma bonds.

5.3. Calcium carbonate react with dilute hydrochloric acid as given below:
\( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \)
(a) What is the mass of 5 moles of calcium carbonate? (Relative molecular mass of calcium carbonate is 100)
(b) How many moles of HCl will react with 5 moles of calcium carbonate?
(c) What is the volume of carbon dioxide liberated at S.T.P. at the same time? [3 Marks]

Answer:
(a) Mass = 5 moles × 100 g/mole = 500 g
(b) From the equation, 1 mole of CaCO₃ reacts with 2 moles of HCl. Therefore, 5 moles of CaCO₃ will react with 10 moles of HCl.

(c) 1 mole of CaCO₃ liberates 1 mole of CO₂ (22.4 L at STP). 5 moles of CaCO₃ will liberate 5 × 22.4 = 112 litres of CO₂.

Teacher's Note:
a) Always use stoichiometry from the balanced chemical equation for mole-to-mole conversions.
b) One mole of any gas occupies 22.4 dm³ at STP.

5.4. Identify the gas evolved in the following reaction:
(a) Methane undergoes complete combustion.
(b) Copper carbonate is heated.
(c) MnO₂ reacts with concentrated HCl. [3 Marks]

Answer:
(a) Carbon dioxide (\( \text{CO}_2 \))
(b) Carbon dioxide (\( \text{CO}_2 \))
(c) Chlorine (\( \text{Cl}_2 \))

Teacher's Note:
a) Complete combustion of hydrocarbons yields carbon dioxide and water vapour.
b) Action of concentrated hydrochloric acid on manganese dioxide is a standard laboratory method for preparing chlorine gas.

Q6. [10 Marks]

6.1. From the reactions below, identify the reaction which exhibits:
X: \( \text{HCl} \rightleftharpoons \text{H}^{+} + \text{Cl}^{-} \) (in solution state)
Y: \( \text{PbBr}_2 \rightleftharpoons \text{Pb}^{2+} + 2\text{Br}^{-} \) (in molten state)
(a) electrolytic dissociation
(b) ionization [2 Marks]

Answer:
(a) Reaction Y
(b) Reaction X

Teacher's Note:
a) Ionization is the process where a polar covalent compound dissolves in water to form ions.
b) Electrolytic dissociation is the separation of ions already present in an ionic compound when it is melted or dissolved.

6.2. Give a reason for the following:
(a) Inert gases do not form ions.
(b) Covalent compounds have a low melting and boiling point. [2 Marks]

Answer:
(a) Inert gases possess a completely filled valence shell (stable octet or duplet), making them extremely stable and chemically unreactive.
(b) Covalent compounds are held together by weak intermolecular forces (van der Waals forces), which require minimal thermal energy to break.

Teacher's Note:
a) Noble gases have zero electron affinity and very high ionization energy.
b) Do not confuse intramolecular covalent bonds with intermolecular forces when explaining melting points.

6.3. Arrange the following as per the instruction given in the bracket:
(a) Carbon, Fluorine, Beryllium (decreasing order of atomic size).
(b) Sulphuric acid, Phosphoric acid, Acetic acid (increasing order of number of replaceable H atoms per molecule).
(c) Potassium, Lithium, Sodium (increasing order of ionization potential). [3 Marks]

Answer:
(a) Beryllium > Carbon > Fluorine
(b) Acetic acid < Sulphuric acid < Phosphoric acid
(c) Potassium < Sodium < Lithium

Teacher's Note:
a) Atomic size decreases across a period from left to right due to increasing effective nuclear charge.
b) Ionization potential decreases down a group as atomic size increases, making Lithium have a higher IP than Sodium and Potassium.

6.4. Identify the following:
(a) An element in Period 1 which can be placed in both Group 1 and Group 17 of the Periodic Table.
(b) The element having electronic configuration 2, 8, 6.
(c) The most electronegative element of Period 3. [3 Marks]

Answer:
(a) Hydrogen
(b) Sulphur
(c) Chlorine

Teacher's Note:
a) Hydrogen resembles alkali metals in having one valence electron and halogens in needing one electron to achieve a stable duplet.
b) Electronegativity increases across a period, making chlorine the most electronegative element in Period 3 (excluding argon).

Q7. [10 Marks]

7.1. Rita was given an unknown salt for identification. She prepared a solution of the salt and divided it into two parts.
- To the first part of the salt solution, she added a few drops of ammonium hydroxide and obtained a reddish-brown precipitate.
- To the second part of the salt solution, she added a few drops of silver nitrate solution and obtained a white precipitate.
Name:
(a) The cation present and
(b) The anion present in the salt given for identification. [2 Marks]

Answer:
(a) Ferric ion (\( \text{Fe}^{3+} \))
(b) Chloride ion (\( \text{Cl}^{-} \))

Teacher's Note:
a) Ferric hydroxide precipitate formed with ammonium hydroxide is reddish-brown in color.
b) Silver nitrate gives a white curdy precipitate of silver chloride with chloride ions.

7.2. Fill in the blanks by choosing the correct answer from the bracket:
(a) Carbon tetrachloride is a ______ covalent molecule.
1. Polar
2. Non-polar
(b) During electrolysis of acidulated water, the gas liberated at the anode is ______.
1. Oxygen
2. Hydrogen [2 Marks]

Answer:
(a) Non-polar
(b) Oxygen

Teacher's Note:
a) Symmetrical tetrahedral molecules like carbon tetrachloride have zero net dipole moment, making them non-polar.
b) During the electrolysis of acidified water, oxygen gas is oxidized and liberated at the anode.

7.3. Ammonia burns in oxygen, as shown below:
\( 4\text{NH}_3 + 3\text{O}_2 \rightarrow 2\text{N}_2 + 6\text{H}_2\text{O} \)
If 240 cc of ammonia is burnt in 300 cc of oxygen, find out the composition of the resultant gaseous mixture at room temperature. [3 Marks]

Answer:
From the equation, 4 volumes of NH₃ react with 3 volumes of O₂.
240 cc of NH₃ will require \( \frac{3}{4} \times 240 = 180 \) cc of O₂.
Oxygen left unreacted = \( 300 - 180 = 120 \) cc.
Volume of N₂ produced = \( \frac{2}{4} \times 240 = 120 \) cc.
Water vapour condenses at room temperature.
Composition of the resultant mixture: 120 cc of Nitrogen and 120 cc of unreacted Oxygen.

Teacher's Note:
a) Apply Gay-Lussac's Law of combining volumes directly to gaseous reactions.
b) Remember to account for water condensing into liquid at room temperature when calculating the final gas mixture.

7.4. The following table shows the electronic configuration of the atoms A, B, C and D.
Element: A | B | C | D
Electronic configuration: 2, 8, 8, 2 | 2, 6 | 2, 8, 7 | 2, 4
(a) Write the formula of the compound formed between:
1. A and B
2. D and C
(b) Which of the above elements will exhibit catenation? [3 Marks]

Answer:
(a) 1. Element A has valency +2, Element B has valency -2. Formula: AB
2. Element D has valency 4, Element C has valency -1. Formula: DC₄
(b) Element D

Teacher's Note:
a) Determine valency from the number of valence electrons (8 minus valence electrons for non-metals).
b) Carbon and silicon-like elements with 4 valence electrons exhibit strong catenation.

Q8. [10 Marks]

8.1. Choose the correct answer from the list given below:
List: Zinc blende, \( \text{C}_2\text{H}_2 \), Calamine, CH, Haematite
(a) The ore which can be concentrated by magnetic separation.
(b) Empirical formula of Ethyne. [2 Marks]

Answer:
(a) Haematite
(b) CH

Teacher's Note:
a) Magnetic separation is used when either the ore or the impurities are magnetic (e.g., Haematite, Fe₂O₃).
b) The molecular formula of ethyne is C₂H₂, so its simplest ratio empirical formula is CH.

8.2. (a) Give a balanced equation for the following reaction: Copper reacts with concentrated nitric acid.
(b) Write the equation for the reaction: Aluminium nitride is treated with warm water. [2 Marks]

Answer:
(a) \( \text{Cu} + 4\text{HNO}_3\text{(conc.)} \rightarrow \text{Cu}(\text{NO}_3)_2 + 2\text{NO}_2 + 2\text{H}_2\text{O} \)
(b) \( \text{AlN} + 3\text{H}_2\text{O} \rightarrow \text{Al}(\text{OH})_3 + \text{NH}_3 \)

Teacher's Note:
a) Concentrated nitric acid reacts with copper to produce brown nitrogen dioxide gas.
b) Metal nitrides react with warm water to liberate ammonia gas.

8.3. Match the salts underlined in Column A with the most suitable method of preparation given in Column B.
Column A:
(a) from \( \text{ZnCl}_2 \) from Zn
(b) from \( \text{KNO}_3 \) from KOH
(c) from \( \text{CaCO}_3 \) from \( \text{CaCl}_2 \)
Column B:
1. Precipitation
2. Direct combination
3. Displacement reaction
4. Neutralization [3 Marks]

Teacher's Note:
a) Active metals above hydrogen displace hydrogen from dilute acids to form salts.
b) Insoluble salts like calcium carbonate are conveniently prepared via precipitation (double displacement).

8.4. Hydrogen chloride gas is prepared in the laboratory by the action of concentrated sulphuric acid on sodium chloride.
(a) Give a balanced chemical equation for the above reaction.
(b) State the method of collection of the gas formed above.
(c) What is the property of sulphuric acid that makes it a suitable reagent for the reaction? [3 Marks]

Answer:
(a) \( 2\text{NaCl} + \text{H}_2\text{SO}_4\text{(conc.)} \xrightarrow{< 200^{\circ}\text{C}} \text{Na}_2\text{SO}_4 + 2\text{HCl} \)
(b) Upward displacement of air (or downward delivery)
(c) Its non-volatile nature and high boiling point.

Teacher's Note:
a) Concentrated sulphuric acid displaces more volatile acids from their salts.
b) HCl gas cannot be collected over water due to its extremely high solubility in water.