ICSE Class 10 Chemistry Board Exam Question Paper 2011 with Solutions

ICSE Board Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION I (40 Marks)

Attempt all questions from this section.

(1) Choose from the following list of substances as to what matches the description from (i) to (v) given below: [5]
[Acetylene gas, aqua fortis, coke, brass, barium chloride, bronze, platinum].
i. An aqueous salt solution used for testing sulphate radical.
ii. A catalyst used in the manufacture of nitric acid by Ostwald's process.
iii. A black powdery substance used for the reduction of zinc oxide during its extraction.
iv. A gaseous hydrocarbon commonly used for welding purposes.
v. The substance is an alloy of zinc, copper and tin

Answer:
i. Barium chloride
ii. Platinum
iii. Coke
iv. Acetylene gas
v. Bronze

Teacher's Note:
a) Read the given list carefully and associate each property or usage with its specific chemical or physical characteristic.
b) Ensure exact spelling from the bracketed list is used in the answers.

(2) Ammonium hydroxide is first added in a small quantity and then in excess to a solution of copper sulphate. [1]

Answer:
A pale blue precipitate is formed on the addition of a small quantity of ammonium hydroxide, which dissolves in excess ammonium hydroxide to give a deep inky blue (or clear deep blue) solution.

Teacher's Note:
a) The pale blue precipitate is copper hydroxide, and the deep blue solution is due to the formation of a complex salt, tetraamminecopper(II) sulphate.
b) Students must mention both the initial precipitate and its solubility in excess along with the final colour.

(3) Sugar crystals are added to a hard glass test tube containing concentrated sulphuric acid. [1]

Answer:
Sugar turns black (charring) and a spongy mass of carbon rises up, accompanied by frothing due to the evolution of steam and sulphur dioxide/carbon dioxide gases.

Teacher's Note:
a) Concentrated sulphuric acid acts as a strong dehydrating agent and removes water of crystallization from cane sugar (C₁₂H₂₂O₁₁).
b) Mentioning the black spongy mass of carbon is crucial for full credit.

(4) Copper is heated with concentrated nitric acid in a hard glass test tube. [1]

Answer:
Copper dissolves to form a blue/green solution and dense reddish - brown vapours of nitrogen dioxide gas are evolved.

Teacher's Note:
a) Concentrated nitric acid acts as an oxidizing agent, oxidizing copper to copper(II) nitrate while getting reduced itself to nitrogen dioxide.
b) The reddish - brown gas (NO₂) is a characteristic observation for reactions involving concentrated nitric acid.

(5) Water is added to the product formed when aluminium is burnt in a jar of nitrogen gas. [1]

Answer:
A pungent smelling gas (ammonia gas) is evolved which turns moist red litmus paper blue.

Teacher's Note:
a) Aluminium burns in nitrogen to form aluminium nitride (AlN). Addition of water hydrolyses it to form aluminium hydroxide and ammonia gas.
b) The pungent smell and its basic nature (turning red litmus blue) identify the gas as ammonia.

(6) When carbon monoxide is passed over heated copper oxide. [1]

Answer:
The black copper oxide changes to a reddish - brown metallic copper.

Teacher's Note:
a) Carbon monoxide acts as a reducing agent, removing oxygen from copper oxide.
b) This reaction demonstrates the reducing property of carbon monoxide at high temperatures.

(7) The electrolysis of acidulated water is considered an example of catalysis. [1]

Answer:
Pure water is a poor conductor of electricity. Adding a small amount of dilute sulphuric acid increases the concentration of ions, thereby catalysing the electrical conduction and decomposition of water without undergoing any permanent chemical change itself.

Teacher's Note:
a) The dilute acid acts effectively to facilitate the process, fulfilling the role of an electrolyte-cum-catalyst in this context.
b) Keep the explanation concise and focused on the poor conductivity of pure water.

(8) Almost 90% of all known compounds are organic in nature. [1]

Answer:
This is due to the unique properties of carbon atoms, namely catenation (self-linking property to form long chains and rings) and tetravalency.

Teacher's Note:
a) Catenation and tetravalency are the two primary reasons for the vast number of carbon compounds.
b) Both terms must be clearly stated to secure full marks.

(9) It is dangerous to burn methane in an insufficient supply of air. [1]

Answer:
Burning methane in an insufficient supply of air leads to incomplete combustion, producing highly toxic and poisonous carbon monoxide gas instead of carbon dioxide.

Teacher's Note:
a) Incomplete combustion produces CO, which is lethal as it binds with haemoglobin in the blood.
b) Emphasize the formation of carbon monoxide over carbon dioxide.

(10) Hydrogen chloride can be termed a polar covalent compound. [1]

Answer:
Hydrogen and chlorine share electrons to form a covalent bond, but chlorine has a much higher electronegativity than hydrogen, causing the shared pair of electrons to shift towards chlorine, creating partial positive and negative poles.

Teacher's Note:
a) Unequal sharing of the electron pair due to a difference in electronegativity results in polarity.
b) Mention electronegativity difference to explain the polarity.

(11) The oxidising power of elements increases on moving from left to right along a period in the periodic table. [1]

Answer:
Across a period from left to right, the nuclear charge increases and atomic size decreases, making it easier for elements to accept electrons and act as oxidizing agents.

Teacher's Note:
a) Oxidizing power depends on the ease of gaining electrons (electron affinity/electronegativity).
b) Link the trend to increasing nuclear charge and decreasing atomic radius.

(12) In covalent compounds, the bond is formed due to the_______ (sharing/transfer) of electrons. [1]

Answer: sharing

Teacher's Note:
a) Covalent bonds are formed by mutual sharing of electrons between non-metals.
b) Transfer of electrons takes place in electrovalent/ionic compounds.

(13) Electrovalent compounds have _______ (low/high) boiling point. [1]

Answer: high

Teacher's Note:
a) Electrovalent compounds have strong electrostatic forces of attraction between oppositely charged ions.
b) A large amount of energy is required to break these strong ionic bonds, leading to high boiling points.

(14) A molecule of _______ contains a triple bond (hydrogen, ammonia, nitrogen). [1]

Answer: nitrogen

Teacher's Note:
a) A nitrogen molecule (N₂) is held together by a triple covalent bond.
b) Hydrogen has a single covalent bond and ammonia has three single covalent bonds.

(15) Across a period, the ionisation potential _______ (increases, decreases, remains the same). [1]

Answer: increases

Teacher's Note:
a) As we move across a period, atomic size decreases and nuclear charge increases, making it harder to remove an electron.
b) Ionisation potential is directly proportional to the effective nuclear charge.

(16) Down the group, the electron affinity _______ (increases, decreases, remains the same). [1]

Answer: decreases

Teacher's Note:
a) Down a group, atomic size increases, reducing the nuclear attraction for an incoming electron.
b) Thus, electron affinity generally decreases down a group.

(17) Answer the following question: [5]
i. Calculate the volume of 320 g of SO₂ at STP (Atomic mass: S = 32 and O = 16).
ii. State Gay-Lussac's Law of combining volumes.
iii. Calculate the volume of oxygen required for the complete combustion of 8.8 g of propane (C₃H₈) (Atomic mass: C = 12, O = 16, H = 1, Molar volume = 22.4 dm³ at STP).

Answer:
i. Molar mass of SO₂ = 32 + (16 × 2) = 64 g/mol.
Number of moles = 320 g / 64 g/mol = 5 moles.
Volume at STP = 5 moles × 22.4 dm³ = 112 dm³.
ii. Gay - Lussac's Law of combining volumes states that when gases react, they do so in volumes which bear a simple whole number ratio to one another and to the volume of the products, provided all volumes are measured under the same conditions of temperature and pressure.
iii. Balanced equation for combustion of propane:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Molar mass of C₃H₈ = (3 × 12) + (8 × 1) = 44 g/mol.
Moles of C₃H₈ = 8.8 / 44 = 0.2 moles.
From the equation, 1 mole of C₃H₈ requires 5 moles of O₂.
Moles of O₂ required = 0.2 × 5 = 1.0 mole.
Volume of O₂ at STP = 1.0 × 22.4 dm³ = 22.4 dm³.

Teacher's Note:
a) Ensure correct molar masses are used for calculations.
b) State Gay-Lussac's law precisely including the temperature and pressure condition.

(18) This metal is a liquid at room temperature. [0.5]
(A) Potassium
(B) Zinc
(C) Gold
(D) Mercury

Answer: (D) Mercury

Mercury is the only metallic element that is liquid at standard conditions for temperature and pressure.

Teacher's Note:
a) Recall physical properties of metals.
b) Mercury is famously used in thermometers due to this property.

(19) Hydroxide of this metal is soluble in sodium hydroxide solution. [0.5]
(A) Magnesium
(B) Lead
(C) Silver
(D) Copper

Answer: (B) Lead

Lead hydroxide forms a white precipitate with NaOH which dissolves in excess sodium hydroxide to form sodium plumbate.

Teacher's Note:
a) Lead, zinc, and aluminium form amphoteric hydroxides which dissolve in excess NaOH.
b) Magnesium, silver, and copper hydroxides are insoluble in excess NaOH.

(20) In the periodic table, alkali metals are placed in the group_______. [0.5]
(A) 1
(B) 11
(C) 17
(D) 18

Answer: (A) 1

Group 1 elements (excluding hydrogen) are known as alkali metals.

Teacher's Note:
a) Group 1 contains alkali metals, Group 2 contains alkaline earth metals.
b) Group 17 contains halogens and Group 18 contains noble gases.

(21) Hydrogen chloride gas being highly soluble in water is dried by [0.5]
(A) Anhydrous calcium chloride
(B) Phosphorus pentoxide
(C) Quick lime
(D) Concentrated sulphuric acid

Answer: (D) Concentrated sulphuric acid

Concentrated sulphuric acid is a non-volatile acid and a dehydrating agent used to dry HCl gas.

Teacher's Note:
a) Quick lime (CaO) cannot be used because it is basic and reacts with acidic HCl gas.
b) Concentrated H₂SO₄ is the standard drying agent for HCl gas.

(22) The brown ring test is used for the detection of [0.5]
(A) CO₃^2-
(B) NO₃^-
(C) SO₃^2-
(D) Cl^-

Answer: (B) NO₃^-

Nitrate ions give a characteristic brown ring at the junction of two liquids in the brown ring test.

Teacher's Note:
a) The brown ring is formed due to the formation of nitroso-iron(II) sulphate complex.
b) This is a standard confirmatory test for nitrates.

(23) When dilute sulphuric acid reacts with iron sulphide, the gas evolved is _______. [0.5]
(A) Hydrogen sulphide
(B) Sulphur dioxide
(C) Sulphur trioxide
(D) Vapour of sulphuric acid

Answer: (A) Hydrogen sulphide

FeS + H₂SO₄ (dilute) → FeSO₄ + H₂S

Teacher's Note:
a) Metal sulphides react with dilute acids to evolve hydrogen sulphide gas with a rotten egg smell.
b) Do not confuse with sulphites which evolve sulphur dioxide.

(24) The functional group present in acetic acid is [0.5]
(A) ketonic >C=O
(B) Hydroxyl -OH
(C) Aldehydic -CHO
(D) Carboxylic -COOH

Answer: (D) Carboxylic -COOH

Acetic acid (ethanoic acid) has the formula CH₃COOH, containing the carboxyl group.

Teacher's Note:
a) Carboxylic acids contain the -COOH functional group.
b) Acetic acid is a weak organic mono-carboxylic acid.

(25) The unsaturated hydrocarbons undergo [0.5]
(A) A substitution reaction
(B) An oxidation reaction
(C) An addition reaction
(D) None of the above

Answer: (C) An addition reaction

Unsaturated hydrocarbons (alkenes and alkynes) possess double or triple bonds and readily undergo addition reactions.

Teacher's Note:
a) Saturated hydrocarbons (alkanes) undergo substitution reactions.
b) Unsaturated hydrocarbons undergo addition reactions due to the presence of multiple bonds.

(26) The number of C-H bonds in ethane molecule are [0.5]
(A) Four
(B) Six
(C) Eight
(D) Ten

Answer: (B) Six

Ethane (C₂H₆) has 6 carbon-hydrogen single covalent bonds and 1 carbon-carbon single covalent bond.

Teacher's Note:
a) Structure of ethane: CH₃-CH₃.
b) Total C-H bonds = 6, total covalent bonds = 7.

(27) Which of the following properties do not match with elements of the halogen family [0.5]
(A) They have seven electrons in their valence shell.
(B) They are highly reactive chemically.
(C) They are metallic in nature.
(D) They are diatomic in their molecular form.

Answer: (C) They are metallic in nature.

Halogens are non-metals, not metals.

Teacher's Note:
a) Halogens (Group 17) are typical non-metals.
b) They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) and have 7 valence electrons.

(28) Write the balanced chemical equation for each of the following reactions: [5]
i. Sodium thiosulphate is reacted with dilute hydrochloric acid.
ii. Calcium bicarbonate reacts with dilute hydrochloric acid.
iii. Dilute sulphuric acid is poured over sodium sulphite.
iv. Lead nitrate solution is added to sodium chloride solution.
v. Zinc is heated with sodium hydroxide solution.

Answer:
i. Na₂S₂O₃ + 2HCl (dil.) → 2NaCl + H₂O + SO₂ + S
ii. Ca(HCO₃)₂ + 2HCl (dil.) → CaCl₂ + 2H₂O + 2CO₂
iii. Na₂SO₃ + H₂SO₄ (dil.) → Na₂SO₄ + H₂O + SO₂
iv. Pb(NO₃)₂ + 2NaCl → PbCl₂ (down arrow) + 2NaNO₃
v. Zn + 2NaOH → Na₂ZnO₂ + H₂ (up arrow)

Teacher's Note:
a) Ensure all chemical equations are balanced with proper states or precipitate/gas arrows where applicable.
b) Practice writing equations for the action of acids on salts and amphoteric metals reacting with bases.

SECTION II (40 Marks)

Attempt any four questions from this section.

(29) Differentiate between electrical conductivity of copper sulphate solution and copper metal. [3]

Answer:

Teacher's Note:
a) Distinguish clearly between metallic conduction and electrolytic conduction.
b) Mentioning the charge carriers (ions vs. electrons) is essential.

(30) Sodium hydroxide solution is added to the solutions containing the ions mentioned in List X. List Y gives the details of the precipitate. Match the ions with their coloured precipitates. [3.5]

Teacher's Note:
a) Note the numbering anomaly in the question paper where (vi) is printed twice; match each ion to its correct characteristic precipitate with NaOH.
b) Amphoteric hydroxides (like Pb²⁺ and Zn²⁺) dissolve in excess NaOH to form clear solutions.

(31) During the electrolysis of copper (II) sulphate solution using platinum as cathode and carbon as anode: [3.5]
i. What do you observe at the cathode and at the anode?
ii. What change is noticed in the electrolyte?
iii. Write the reactions at the cathode and at the anode.

Answer:
i. At the cathode, a reddish - brown deposit of copper is observed. At the anode, bubbles of oxygen gas are evolved.
ii. The blue colour of the copper(II) sulphate solution fades gradually and the solution eventually becomes colourless as Cu²⁺ ions are depleted, and sulphuric acid is left behind.

iii. Reaction at the cathode:
Cu²⁺ + 2e^- → Cu
Reaction at the anode:
4OH^- - 4e^- → 2H₂O + O₂ (or 2H₂O → 4H⁺ + O₂ + 4e^-)

Teacher's Note:
a) Emphasize that platinum and carbon are inert electrodes, so the anode does not dissolve.
b) Clearly state the colour change of the electrolyte.

(32) Answer the following questions: [5]
i. Name a metal which is found abundantly in the Earth's crust.
ii. What is the difference between calcination and roasting?
iii. Name the process used for the enrichment of sulphide ore.
iv. Write the chemical formulae of one main ore of iron and aluminium.
v. Write the constituents of electrolyte for the extraction of aluminium.

Answer:
i. Aluminium.
ii. Calcination involves heating the ore strongly in the absence of air below its melting point, whereas roasting involves heating the ore strongly in the presence of excess air below its melting point.
iii. Froth floatation process.
iv. Iron: Haematite (Fe₂O₃) [or Magnetite, Iron pyrites]; Aluminium: Bauxite (Al₂O₃.2H₂O).
v. Pure alumina (Al₂O₃), molten cryolite (Na₃AlF₆), and fluorspar (CaF₂).

Teacher's Note:
a) Differentiate calcination and roasting based on the presence or absence of air.
b) Mention all three constituents of the electrolyte mixture in Hall-Héroult process for aluminium extraction.

(33) The diagram shows an experimental setup for the laboratory preparation of a pungent smelling gas. The gas is alkaline in nature. [5]

[Figure: Laboratory preparation of ammonia gas showing a round bottom flask clamped slantingly containing a mixture X (ammonium chloride and calcium hydroxide) heated with a burner, connected to a drying tower Y (containing quick lime CaO), and the dry ammonia gas collected in an inverted gas jar by downward displacement of air.]

i. Name the gas collected in the jar.
ii. Write the balanced equation for the above preparation.
iii. How is the gas being collected?
iv. Name the drying agent used.
v. How will you find that the jar is full of gas?

Answer:
i. Ammonia gas (NH₃).
ii. 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃
iii. By the downward displacement of air (upward delivery).
iv. Quick lime (Calcium oxide, CaO).
v. Bring a glass rod dipped in concentrated hydrochloric acid near the mouth of the gas jar; dense white fumes are formed.

Teacher's Note:
a) Quick lime is the only suitable drying agent for ammonia as it is basic and does not react with it.
b) The test for ammonia using conc. HCl giving dense white fumes of ammonium chloride is standard.

(34) An organic compound with vapour density = 94 contains C = 12.67%, H = 2.13% and Br = 85.11%. Find the molecular formula. [3]
[Atomic mass: C = 12, H = 1, Br = 80]

Answer:

Empirical formula = CH₂Br.
Empirical formula mass = (1 × 12) + (2 × 1) + (1 × 80) = 12 + 2 + 80 = 94 g.
Molecular mass = 2 × Vapour Density = 2 × 94 = 188 g.
n = Molecular mass / Empirical formula mass = 188 / 94 = 2.
Molecular formula = (Empirical formula)_n = (CH₂Br)₂ = C₂H₄Br₂.

Teacher's Note:
a) Always set up a clear calculation table for empirical formula determination.
b) Molecular mass is calculated as twice the vapour density.

(35) Calculate the mass of [3.5]
i. 10²² atoms of sulphur
ii. 0.1 mole of carbon dioxide
[Atomic mass: S = 32, C = 12 and O = 16 and Avogadro's number = 6 × 10²³]

Answer:
i. Mass of 6 × 10²³ atoms of sulphur = 32 g.
Mass of 10²² atoms of sulphur = (32 / (6 × 10²³)) × 10²²
= 32 / 60 = 0.533 g.
ii. Molar mass of CO₂ = 12 + (16 × 2) = 44 g/mol.
Mass of 0.1 mole of CO₂ = Number of moles × Molar mass
= 0.1 × 44 = 4.4 g.

Teacher's Note:
a) Use the unitary method or mole formula correctly for atomic and molar mass calculations.
b) Check powers of 10 carefully during division.

(36) In the laboratory preparation of hydrochloric acid, HCl gas is dissolved in water. [3.5]
i. Draw a diagram to show the arrangement used for the absorption of HCl in water.
ii. Why is such an arrangement necessary? Give two reasons.
iii. Write the chemical equations for the laboratory preparation of HCl gas when the reactants are
(A) below 200°C
(B) above 200°C

Answer:
i. [Figure: Diagram showing a funnel inverted and just dipping into the surface of water in a trough, connected to a delivery tube carrying hydrogen chloride gas to prevent back suction and provide a large surface area for absorption.]
ii. Two reasons:
1. It prevents back-suction of water into the generator flask.
2. It provides a large surface area for the rapid and complete absorption of hydrogen chloride gas.
iii. (A) Below 200°C:
NaCl + H₂SO₄ → NaHSO₄ + HCl
(B) Above 200°C:
2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl

Teacher's Note:
a) The inverted funnel arrangement is a standard setup to avoid the high solubility of HCl causing water to rush back into the hot glass apparatus.
b) Clearly distinguish between the two temperature ranges for sodium bisulphate and sodium sulphate formation.

(37) Choose the correct word/phrase from within the brackets to complete the following sentences: [5]
i. The catalyst used for conversion of ethene to ethane is commonly _____ (nickel/iron/cobalt).
ii. When acetaldehyde is oxidised with acidified potassium dichromate, it forms______(ester/ethanol/acetic acid).
iii. Ethanoic acid reacts with ethanol in the presence of concentrated H₂SO₄, so as to form a compound and water. The chemical reaction which takes place is called_____ (dehydration/hydrogenation/esterification).
iv. Write the equation for the reaction taking place between 1, 2-dibromoethane and alcoholic potassium hydroxide.
v. The product formed when ethene gas reacts with water in the presence of sulphuric acid is______(ethanol/ethanal/ethanoic acid).

Answer:
i. nickel
ii. acetic acid
iii. esterification
iv. CH₂Br-CH₂Br + 2KOH (alc.) → C₂H₂ + 2KBr + 2H₂O
v. ethanol

Teacher's Note:
a) Ensure precise selection of terms from the given brackets.
b) Reaction (iv) is a dehydrohalogenation reaction leading to the formation of an alkyne (acetylene/ethyne).

(38) Write balanced chemical equations for the following: [5]
i. Monochloroethane is hydrolysed with aqueous KOH.
ii. A mixture of soda lime and sodium acetate is heated.
iii. Ethanol under high pressure and low temperature is treated with acidified potassium dichromate.
iv. Water is added to calcium carbide.
v. Ethanol reacts with sodium at room temperature.

Answer:
i. C₂H₅Cl + KOH (aq.) → C₂H₅OH + KCl
ii. CH₃COONa + NaOH (CaO, heat) → CH₄ + Na₂CO₃
iii. 3C₂H₅OH + 2O (acidified K₂Cr₂O₇) → 3CH₃CHO + 3H₂O
iv. CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂
v. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ (up arrow)

Teacher's Note:
a) Equation (ii) is the laboratory preparation of methane using soda lime.
b) Equation (v) shows the acidic nature of alcohol through reaction with active sodium metal.

(39) Answer the following question: [3.5]
i. With the help of equations, give an outline for the manufacture of sulphuric acid by the contact process.
ii. What property of sulphuric acid is shown by the reaction of concentrated sulphuric acid when heated with
(A) Potassium nitrate
(B) Carbon

Answer:
i. Step 1: Burning of sulphur or pyrites in air to form sulphur dioxide.
S + O₂ → SO₂
Step 2: Catalytic oxidation of sulphur dioxide to sulphur trioxide.
2SO₂ + O₂ ⇌ (V₂O₅, 450°C) 2SO₃
Step 3: Dissolution of sulphur trioxide in concentrated sulphuric acid to form oleum.
SO₃ + H₂SO₄ → H₂S₂O₇
Step 4: Dilution of oleum with water to get sulphuric acid.
H₂S₂O₇ + H₂O → 2H₂SO₄
ii. (A) Volatile acid property (or displacement of a more volatile acid by a less volatile acid).
(B) Oxidising property.

Teacher's Note:
a) Write all four key steps of the Contact process clearly with reaction conditions.
b) Identify the property of H₂SO₄ accurately for each reaction in part (ii).

(40) Give the answers of questions given below: [3.5]
i. What is the special feature of the apparatus which is used in the laboratory preparation of nitric acid?
ii. Why should the temperature of the reaction mixture of nitric acid not be allowed to rise above 200°C?

Answer:
i. An all-glass apparatus (glass retort and glass condenser) is used because nitric acid vapours are highly corrosive and attack cork, rubber, or metal parts.
ii. If the temperature rises above 200°C, it may damage the glass apparatus, cause wastage of fuel, and lead to the decomposition of nitric acid into nitrogen dioxide, water, and oxygen.

Teacher's Note:
a) Glass is chosen due to the corrosive nature of nitric acid vapour.
b) Temperature control is critical to prevent thermal decomposition of HNO₃.

(41) Write the balanced chemical equations for the following: [3]
i. Chlorine reacts with excess of ammonia.
ii. Ferric hydroxide reacts with nitric acid.
iii. Zinc oxide dissolves in sodium hydroxide.

Answer:
i. 8NH₃ (excess) + 3Cl₂ → 6NH₄Cl + N₂
ii. Fe(OH)₃ + 3HNO₃ → Fe(NO₃)₃ + 3H₂O
iii. ZnO + 2NaOH → Na₂ZnO₂ + H₂O

Teacher's Note:
a) Note the difference between chlorine in excess and ammonia in excess; here ammonia is in excess.
b) ZnO is an amphoteric oxide that dissolves in sodium hydroxide to form sodium zincate and water.

(42) [5]
i. Give the number of the group and period of the element having three shells with three electrons in the valence shell.
ii. By drawing an electron dot diagram, show the lone pair effect leading to the formation of ammonium ion from ammonia gas and hydrogen ion.
iii. What happens to the crystals of washing soda when exposed to air? Name the phenomenon exhibited.

Answer:
i. Period: 3 (since it has three shells); Group: 13 (since valence electrons = 3, group = 10 + 3 = 13).
ii. [Figure: Electron dot diagram showing the nitrogen atom in ammonia (NH₃) with a lone pair of electrons sharing it with a hydrogen ion (H⁺) having no electrons, forming a coordinate covalent bond in the ammonium ion (NH₄⁺).]
iii. Crystals of washing soda lose 10 molecules of water of crystallization when exposed to air and turn into a white powdery substance. The phenomenon is called efflorescence.

Teacher's Note:
a) Group number for elements with 3 to 8 valence electrons is calculated as 10 + number of valence electrons.
b) Efflorescence is the loss of water of crystallization from hydrated crystals to the atmosphere.

(43) Name the method used for the preparation of the following salts from the list given below: [5]
i. Sodium nitrate
ii. Iron (III) chloride
iii. Lead chloride
iv. Zinc sulphate
v. Sodium hydrogen sulphate
List:
(A) Simple displacement
(B) Neutralisation
(C) Decomposition by acid
(D) Double decomposition
(E) Direct synthesis

Answer:
i. Sodium nitrate - (B) Neutralisation
ii. Iron (III) chloride - (E) Direct synthesis
iii. Lead chloride - (D) Double decomposition
iv. Zinc sulphate - (A) Simple displacement (or (C) Decomposition by acid)
v. Sodium hydrogen sulphate - (C) Decomposition by acid

Teacher's Note:
a) Match each salt preparation method based on whether it is soluble/insoluble and the nature of the reactants.
b) Lead chloride is an insoluble salt prepared by precipitation (double decomposition).