ICSE Class 10 Chemistry Board Exam Question Paper 2014 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION - I (40 Marks)

Attempt all questions from this Section

Question 1.

(a) Choose the correct answer from the options given below :
(i) Ionisation Potential increases over a period from left to right because the :
(A) Atomic radius increases and nuclear charge increases
(B) Atomic radius decreases and nuclear charge decreases
(C) Atomic radius increases and nuclear charge decreases
(D) Atomic radius decreases and nuclear charge increases.

Answer: (D) Atomic radius decreases and nuclear charge increases.

Across a period, nuclear charge increases and atomic size decreases, making it harder to remove an electron.

Teacher's Note:
a) Ionisation potential is directly proportional to effective nuclear charge and inversely proportional to atomic radius.
b) Students often confuse trends across periods with trends down groups.

(ii) A compound X consists of only molecules. Hence X will have :
(A) A crystalline hard structure
(B) A low melting point and low boiling point
(C) An ionic bond
(D) A strong force of attraction between its molecules.

Answer: (B) A low melting point and low boiling point

Compounds consisting of molecules are covalently bonded and possess weak intermolecular forces.

Teacher's Note:
a) Covalent compounds exist as gases, liquids, or soft solids with low melting and boiling points.
b) Do not confuse covalent molecules with giant covalent structures like diamond or silica.

(iii) When fused lead bromide is electrolysed we observe :
(A) a silver grey deposit at anode and a reddish brown deposit at cathode
(B) a silver grey deposit at cathode and a reddish brown deposit at anode
(C) a silver grey deposit at cathode and reddish brown fumes at anode
(D) silver grey fumes at anode and reddish brown fumes at cathode.

Answer: (C) a silver grey deposit at cathode and reddish brown fumes at anode

Lead ions discharge at the cathode as silvery-grey lead, and bromide ions discharge at the anode as reddish-brown bromine gas.

Teacher's Note:
a) Lead is deposited at the cathode (negative electrode) as a grey metal.
b) Bromine vapours evolve at the anode (positive electrode) as reddish-brown fumes.

(iv) The main ore used for the extraction of iron is :
(A) Haematite
(B) Calamine
(C) Bauxite
(D) Cryolite

Answer: (A) Haematite

Haematite (Fe₂O₃) is the principal ore of iron.

Teacher's Note:
a) Bauxite is the ore of aluminium and calamine is an ore of zinc.
b) Memorize the chemical formulae and names of chief ores for extraction chapters.

(v) Heating an air in a limited supply of air or in the absence of air at a temperature just below its melting point is known as :
(A) smelting
(B) ore dressing
(C) calcination
(D) bessemerisation

Answer: (C) calcination

Calcination involves heating the ore strongly in the absence of air to expel moisture and volatile impurities.

Teacher's Note:
a) Roasting is done in the presence of excess air, whereas calcination is done in the absence or limited supply of air.
b) Be precise with metallurgical definitions.

(vi) If an element A belongs to Period 3 and Group II then it will have :
(A) 3 shells and 2 valence electrons
(B) 2 shells and 3 valence electrons
(C) 3 shells and 3 valence electrons
(D) 2 shells and 2 valence electrons

Answer: (A) 3 shells and 2 valence electrons

Period number indicates the number of shells (3) and Group number indicates the number of valence electrons (2).

Teacher's Note:
a) Electronic configuration is 2, 8, 2, showing 3 shells and 2 valence electrons.
b) Element A is Magnesium (Mg).

(vii) The molecule containing a triple co-valent bond is :
(A) ammonia
(B) methane
(C) water
(D) nitrogen

Answer: (D) nitrogen

A nitrogen molecule (N₂) has a triple covalent bond between the two nitrogen atoms.

Teacher's Note:
a) Ammonia has three single covalent bonds, water has two single covalent bonds, and methane has four single covalent bonds.
b) Draw Lewis structures to verify bond counts.

(viii) The electrolyte used for electroplating an article with silver is :
(A) silver nitrate solution
(B) silver cyanide solution
(C) sodium argentocyanide solution
(D) nickel sulphate solution

Answer: (C) sodium argentocyanide solution

Sodium argentocyanide is preferred over silver nitrate for uniform and smooth electroplating of silver.

Teacher's Note:
a) Using silver nitrate directly results in a very rapid, non-uniform deposition.
b) Double salts like sodium argentocyanide dissociate slowly, ensuring even coating.

(ix) Aluminium powder is used in thermite welding because :
(A) it is a strong reducing agent
(B) it is a strong oxidising agent
(C) it is corrosion resistant
(D) it is a good conductor of heat

Answer: (A) it is a strong reducing agent

Aluminium has a high affinity for oxygen and acts as a powerful reducing agent to reduce metal oxides to metals.

Teacher's Note:
a) The reaction is highly exothermic, producing molten iron for welding.
b) Always mention reducing property rather than just heat production.

(x) The I.U.P.A.C. name of acetylene is :
(A) propane
(B) propyne
(C) ethene
(D) ethyne

Answer: (D) ethyne

Acetylene is the common name for the alkyne with two carbon atoms, which is ethyne (C₂H₂).

Teacher's Note:
a) Ethylene is ethene, and acetylene is ethyne.
b) Learn both common and IUPAC names for important hydrocarbons.

(b) Fill in the blanks from the choices within brackets :

(i) The basicity of Acetic Acid is ............. (3, 1, 4)

Answer: 1

Acetic acid (CH₃COOH) has only one replaceable hydrogen ion per molecule.

Teacher's Note:
a) Basicity refers to the number of hydronium ions produced by one molecule of acid in aqueous solution.
b) Organic acids release only the hydrogen attached to oxygen.

(ii) The compound formed when ethanol reacts with sodium is ............. (sodium ethanoate, sodium ethoxide, sodium propanoate).

Answer: sodium ethoxide

Reaction: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.

Teacher's Note:
a) Sodium ethoxide is formed along with the evolution of hydrogen gas.
b) This reaction is used to test for the presence of alcoholic hydroxyl groups.

(iii) Quicklime is not used to dry HCl gas because ............. (CaO is alkaline, CaO is acidic, CaO is neutral).

Answer: CaO is alkaline

Being a basic oxide, quicklime reacts chemically with acidic hydrogen chloride gas.

Teacher's Note:
a) Concentrated sulfuric acid is used instead to dry HCl gas.
b) Basic drying agents cannot be used to dry acidic gases.

(iv) Ammonia gas is collected by ............. (an upward displacement of air, a downward displacement of water, a downward displacement of air)

Answer: a downward displacement of air

Ammonia is lighter than air and extremely soluble in water.

Teacher's Note:
a) Due to high solubility, water displacement cannot be used.
b) Downward displacement of air means the gas is collected in an inverted jar.

(v) Cold, dilute nitric acid reacts with copper to form ............. (Hydrogen, nitrogen dioxide, nitric oxide).

Answer: nitric oxide

Reaction: 3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 4H₂O + 2NO.

Teacher's Note:
a) Concentrated nitric acid yields nitrogen dioxide (NO₂), while cold dilute acid yields nitric oxide (NO).
b) Hydrogen is not evolved with nitric acid because it is a strong oxidizing agent.

(c) Give one word or phrase for the following :

(i) The ratio of the mass of a certain volume of gas to the mass of an equal volume of hydrogen under the same conditions of temperature and pressure.

Answer: Vapour density

By definition, vapour density is the relative density of a gas with respect to hydrogen.

Teacher's Note:
a) Vapour density = Molecular mass / 2.
b) It has no units as it is a ratio.

(ii) Formation of ions from molecules.

Answer: Ionisation

Ionisation is the process by which covalent polar molecules split into ions in aqueous solution.

Teacher's Note:
a) Distinguish between ionisation (covalent compounds) and dissociation (ionic compounds).
b) Acids undergo ionisation when dissolved in water.

(iii) Electrolytic deposition of a superior metal on a baser metal.

Answer: Electroplating

Electroplating is the process of coating an inferior metal with a superior metal using electricity.

Teacher's Note:
a) The article to be plated is always made the cathode.
b) The anode is made of the pure coating metal.

(iv) Hydrocarbons containing a -C- functional group.

Answer: Ketone or Carbonyl compound

Ketones contain the carbonyl group attached to two alkyl groups.

Teacher's Note:
a) General formula for ketones is R-CO-R'.
b) The simplest ketone is propanone (acetone).

(v) The amount of energy released when an atom in the gaseous state accepts an electron to form an anion.

Answer: Electron affinity

Electron affinity measures the energy released when a gaseous atom gains an electron.

Teacher's Note:
a) It is expressed in electron volts (eV) or kJ/mol.
b) Non-metals generally have high electron affinity values.

(d) Match the options A to E with the statements (i) to (v) :

Answer:
A - (iv)
B - (iii)
C - (v)
D - (i)
E - (ii)

Teacher's Note:
a) Alkynes have general formula C_nH_2n-2 and alkanes have C_nH_2n+2.
b) Iron forms both ferrous (Fe²⁺) and ferric (Fe³⁺) ions.

(e) Write balanced equations for the following :

(i) Action of heat on a mixture of copper and concentrated nitric acid.

Answer:
Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

Teacher's Note:
a) Concentrated nitric acid produces reddish-brown nitrogen dioxide gas.
b) Ensure proper balancing of water and acid molecules.

(ii) Action of warm water on magnesium nitride.

Answer:
Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

Teacher's Note:
a) This reaction produces magnesium hydroxide and pungent-smelling ammonia gas.
b) Magnesium nitride hydrolyzes readily upon contact with warm water.

(iii) Action of concentrated sulphuric acid on carbon.

Answer:
C + 2H₂SO₄ (conc.) → CO₂ + 2H₂O + 2SO₂

Teacher's Note:
a) Concentrated sulfuric acid acts as a strong oxidizing agent towards non-metals like carbon.
b) Carbon is oxidized to carbon dioxide while sulfuric acid is reduced to sulfur dioxide.

(iv) Action of dilute hydrochloric acid on sodium sulphide.

Answer:
Na₂S + 2HCl (dil.) → 2NaCl + H₂S

Teacher's Note:
a) Hydrogen sulfide gas with the characteristic smell of rotten eggs is evolved.
b) This is a double displacement precipitation reaction producing soluble sodium chloride.

(v) Preparation of ethane from sodium propionate.

Answer:
C₂H₅COONa + NaOH (CaO, Δ) → Na₂CO₃ + C₂H₆

Teacher's Note:
a) This process is called decarboxylation using soda lime (NaOH + CaO).
b) The alkane formed has one carbon atom less than the sodium salt of the carboxylic acid.

(f) Distinguish between the following pairs of compounds using the test given within brackets :

(i) Iron (II) sulphate and iron (III) sulphate (using ammonium hydroxide)

Answer:
- Iron (II) sulphate gives a dirty green precipitate with ammonium hydroxide which is insoluble in excess.
- Iron (III) sulphate gives a reddish-brown precipitate with ammonium hydroxide which is insoluble in excess.

Teacher's Note:
a) Colour of precipitate is the key identification criterion.
b) Mention that both precipitates are insoluble in excess ammonium hydroxide.

(ii) A lead salt and a zinc salt (using excess ammonium hydroxide)

Answer:
- Lead salt gives a chalky white precipitate with ammonium hydroxide which is insoluble in excess.
- Zinc salt gives a gelatinous white precipitate with ammonium hydroxide which is soluble in excess.

Teacher's Note:
a) Both yield white precipitates initially.
b) Solubility in excess NH₄OH distinguishes zinc from lead.

(iii) Sodium nitrate and sodium sulphite (using dilute sulphuric acid)

Answer:
- Sodium nitrate with dilute H₂SO₄ gives no reaction at ordinary temperature.
- Sodium sulphite with dilute H₂SO₄ gives a colourless gas with the smell of burning sulfur (SO₂), which turns acidified potassium dichromate paper green.

Teacher's Note:
a) Nitrates require concentrated H₂SO₄ and copper turnings to produce distinct vapours.
b) Sulfites react readily with dilute acids to evolve sulfur dioxide.

(iv) Dilute sulphuric acid and dilute hydrochloric acid (using barium chloride solution)

Answer:
- Dilute sulphuric acid gives a white precipitate of barium sulphate with barium chloride solution, which is insoluble in all mineral acids.
- Dilute hydrochloric acid gives no precipitate with barium chloride solution.

Teacher's Note:
a) BaSO₄ is insoluble in dilute HCl.
b) This is the standard confirmatory test for sulfate ions.

(v) Ethane and ethene (using alkaline potassium permanganate solution)

Answer:
- Ethane has no effect on alkaline potassium permanganate solution (purple colour remains unchanged).
- Ethene decolorizes alkaline potassium permanganate solution (purple colour turns colourless).

Teacher's Note:
a) Ethene is an unsaturated hydrocarbon containing a double bond.
b) Baeyer's reagent test is used to distinguish between alkanes and alkenes.

(g) (i) Oxygen oxidises ethyne to carbon dioxide and water as shown by the equation:
\( 2C_{2}H_{2} + 5O_{2} \rightarrow 4CO_{2} + 2H_{2}O \)
What volume of ethyne gas at s.t.p. is required to produce \( 8.4\text{ dm}^{3} \) of carbon dioxide ? [H = 1, C = 12, O = 16] [3 Marks]

Answer:
From the equation:
\( 2\text{ volumes} \) of \( C_{2}H_{2} \) produce \( 4\text{ volumes} \) of \( CO_{2} \).
Therefore, \( 4\text{ dm}^{3} \) of \( CO_{2} \) is produced by \( 2\text{ dm}^{3} \) of \( C_{2}H_{2} \).
Volume of \( C_{2}H_{2} \) required to produce \( 8.4\text{ dm}^{3} \) of \( CO_{2} \) = \( \frac{2}{4} \times 8.4 = 4.2\text{ dm}^{3} \).

Teacher's Note:
a) Apply Gay-Lussac's Law of Combining Volumes directly using stoichiometric coefficients.
b) Ensure units (\( \text{dm}^{3} \)) are written clearly in the final step.

(ii) A compound made up of two elements X and Y has an empirical formula \( X_{2}Y \). If the atomic weight of X is 10 and that of Y is 5 and the compound has a vapour density 25, find its molecular formula. [2 Marks]

Answer:
Empirical formula = \( X_{2}Y \)
Empirical formula mass = \( (2 \times 10) + (1 \times 5) = 20 + 5 = 25 \)
Molecular mass = \( 2 \times \text{Vapour density} = 2 \times 25 = 50 \)
\( n = \frac{\text{Molecular mass}}{\text{Empirical formula mass}} = \frac{50}{25} = 2 \)
Molecular formula = \( (\text{Empirical formula})_{n} = (X_{2}Y)_{2} = X_{4}Y_{2} \).

Teacher's Note:
a) Always calculate empirical formula mass before finding \( n \).
b) Multiply each subscript in the empirical formula by \( n \) to get the molecular formula.

SECTION - II (40 Marks)

Answer any four questions from this section

Question 2.

(a) State your observation in each of the following cases :
(i) When dilute hydrochloric acid is added to sodium carbonate crystals. [1 Mark]

Answer:
Brisk effervescence with the release of a colourless, odourless gas that turns lime water milky and extinguishes a burning splint (Carbon dioxide gas is evolved).

Teacher's Note:
a) Carbonates react with dilute acids to liberate carbon dioxide gas.
b) Lime water test is the standard confirmatory test for carbon dioxide.

(ii) When excess sodium hydroxide is added to calcium nitrate solution. [1 Mark]

Answer:
A white precipitate of calcium hydroxide is obtained that remains insoluble in excess NaOH.

Teacher's Note:
a) Calcium hydroxide is sparingly soluble, yielding a white precipitate.
b) Unlike amphoteric hydroxides, calcium hydroxide does not dissolve in excess alkali.

(iii) At the cathode when acidified aqueous copper sulphate solution is electrolyzed with copper electrodes. [1 Mark]

Answer:
A pinkish-red deposit of copper metal is formed at the cathode.

Teacher's Note:
a) Copper ions are preferentially discharged at the cathode due to their lower position in the electrochemical series.
b) The blue colour of the electrolyte remains unchanged as copper dissolves at the anode at the same rate.

(iv) When calcium hydroxide is heated with ammonium chloride crystals. [1 Mark]

Answer:
A colourless, pungent-smelling gas that turns moist red litmus blue is evolved (Ammonia gas).

Teacher's Note:
a) Ammonium salts react with alkalis upon heating to release ammonia gas.
b) This is the standard laboratory preparation method for ammonia.

(v) When moist starch iodide paper is introduced into chlorine gas. [1 Mark]

Answer:
The moist starch iodide paper turns blue-black.

Teacher's Note:
a) Chlorine displaces iodine from potassium iodide, and free iodine turns starch paper blue-black.
b) This demonstrates the strong oxidizing property of chlorine.

(b) Study the figure given below and answer the questions that follow :

[Figure: Fountain experiment setup showing a round-bottom flask filled with Gas Y, a jet tube dipping into a trough of water with blue litmus, a spray of water from a dropper producing a red fountain inside the flask.]

(i) Identify the gas Y. [1 Mark]

Answer:
Hydrogen chloride gas (HCl).

Teacher's Note:
a) The red fountain indicates an acidic gas.
b) Hydrogen chloride is extremely soluble in water.

(ii) What property of gas Y does this experiment demonstrate ? [1 Mark]

Answer:
Extreme solubility of hydrogen chloride gas in water and its acidic nature.

Teacher's Note:
a) The high solubility creates a partial vacuum inside the flask, drawing water up as a fountain.
b) Acidic nature turns blue litmus red.

(iii) Name another gas which has the same property and can be demonstrated through this experiment. [1 Mark]

Answer:
Ammonia gas (NH₃).

Teacher's Note:
a) Ammonia creates a blue fountain since it is alkaline in nature.
b) Both gases exhibit high solubility in water.

(c) (i) Name the other ion formed when ammonia dissolves in water. [1 Mark]

Answer:
Hydroxyl ion (\( \text{OH}^{-} \)).

Teacher's Note:
a) Reaction: \( \text{NH}_{3} + \text{H}_{2}\text{O} \rightarrow \text{NH}_{4}^{+} + \text{OH}^{-} \).
b) Presence of hydroxyl ions makes aqueous ammonia alkaline.

(ii) Give one test that can be used to detect the presence of the ion produced. [1 Mark]

Answer:
Red litmus paper turns blue or methyl orange turns pink/yellow in alkaline medium.

Teacher's Note:
a) Hydroxyl ions turn red litmus blue.
b) Phenolphthalein turns pink in the presence of alkali.

Question 3.

(a) State the conditions required for the following reactions to take place :
(i) Catalytic hydrogenation of ethyne. [1 Mark]

Answer:
Presence of catalyst like Nickel, Platinum, or Palladium at moderate temperature.

Teacher's Note:
a) Hydrogenation converts unsaturated hydrocarbons to saturated ones.
b) Ni/Pt/Pd act as hydrogenation catalysts.

(ii) Preparation of ethyne from ethylene dibromide. [1 Mark]

Answer:
Heating with alcoholic potassium hydroxide (alcoholic KOH).

Teacher's Note:
a) Alcoholic KOH brings about dehydrohalogenation.
b) Two molecules of HBr are eliminated from ethylene dibromide to form ethyne.

(iii) Catalytic oxidation of ammonia to nitric oxide. [1 Mark]

Answer:
Platinum catalyst at \( 800^{\circ}\text{C} \).

Teacher's Note:
a) This is the first step of the Ostwald process.
b) The reaction is exothermic once initiated.

(iv) Any two conditions for the conversion of sulphur dioxide to sulphur trioxide. [2 Marks]

Answer:
1. Catalyst: Vanadium pentoxide (\( \text{V}_{2}\text{O}_{5} \)) or Platinized asbestos.
2. Temperature: \( 450^{\circ}\text{C} \) to \( 500^{\circ}\text{C} \).

Teacher's Note:
a) This is the Contact Process step.
b) Mentioning pressure (1 - 2 atm) is also acceptable as an optional condition.

(b) State the main components of the following alloys :
(i) Brass. [1 Mark]

Answer:
Copper (Cu) and Zinc (Zn).

Teacher's Note:
a) Brass is harder than pure copper.
b) Typical composition is about 60% to 80% copper.

(ii) Duralumin. [1 Mark]

Answer:
Aluminium (Al), Copper (Cu), Magnesium (Mg), and Manganese (Mn).

Teacher's Note:
a) Duralumin is lightweight and corrosion resistant.
b) Used extensively in aircraft construction.

(iii) Bronze. [1 Mark]

Answer:
Copper (Cu) and Tin (Sn).

Teacher's Note:
a) Bronze is strong and resistant to corrosion.
b) Used for making statues, coins, and medals.

(c) Give balanced equations for the following :
(i) Laboratory preparation of nitric acid. [1 Mark]

Answer:
\( \text{NaNO}_{3} + \text{H}_{2}\text{SO}_{4} \xrightarrow{<200^{\circ}\text{C}} \text{NaHSO}_{4} + \text{HNO}_{3} \)

Teacher's Note:
a) Temperature must be maintained below \( 200^{\circ}\text{C} \) to prevent damage to glassware and formation of hard sodium sulfate crust.
b) All-glass apparatus is used because nitric acid vapor corrodes cork and rubber.

(ii) Preparation of ethanol from monochloroethane and aq. sodium hydroxide. [1 Mark]

Answer:
\( \text{C}_{2}\text{H}_{5}\text{Cl} + \text{NaOH}_{\text{(aq)}} \rightarrow \text{C}_{2}\text{H}_{5}\text{OH} + \text{NaCl} \)

Teacher's Note:
a) This is a nucleophilic substitution (hydrolysis) reaction.
b) Aqueous alkali must be used; alcoholic alkali would cause dehydrohalogenation to ethene.

Question 4.

(a) Give the structural formula of the following :
(i) ethanol. [1 Mark]

Answer:
\( \text{CH}_{3}-\text{CH}_{2}-\text{OH} \)

Teacher's Note:
a) Shows two carbon atoms with an -OH functional group attached.
b) Draw all single covalent bonds clearly for full structural formulas.

(ii) 1-propanal. [1 Mark]

Answer:
\( \text{CH}_{3}-\text{CH}_{2}-\text{CHO} \)

Teacher's Note:
a) Propanal contains three carbon atoms with an aldehyde group at the terminal position.
b) The aldehyde group has a double-bonded oxygen (\( \text{C}=\text{O} \)) and a hydrogen atom.

(iii) ethanoic acid. [1 Mark]

Answer:
\( \text{CH}_{3}-\text{COOH} \)

Teacher's Note:
a) Contains a methyl group attached to a carboxylic acid group.
b) Commonly known as acetic acid.

(iv) 1, 2, dichloroethane. [1 Mark]

Answer:
\( \text{CH}_{2}\text{Cl}-\text{CH}_{2}\text{Cl} \)

Teacher's Note:
a) Two chlorine atoms substituted on adjacent carbon atoms.
b) Formed by the addition of chlorine to ethene.

(b) Draw the structure of the stable positive ion formed when an acid dissolves in water. [2 Marks]

Answer:
Hydronium ion (\( \text{H}_{3}\text{O}^{+} \)) structure:
\( \text{H}^{+}\text{O}\text{H} \rightarrow \text{H}_{3}\text{O}^{+} \) containing a coordinate covalent bond.
Structure: Central oxygen atom bonded to three hydrogen atoms with a lone pair and overall positive charge.

Teacher's Note:
a) Formed by coordinate (dative) bonding between a hydrogen ion and a water molecule.
b) Clearly show the coordinate bond arrow pointing from oxygen to the coordinated proton.

(c) State the inference drawn from the following observations :
(i) On carrying out the flame test with a salt P a brick red flame was obtained. What is the cation in P ? [1 Mark]

Answer:
Calcium ion (\( \text{Ca}^{2+} \)).

Teacher's Note:
a) Brick red flame is characteristic of calcium salts.
b) Flame tests help identify basic radicals.

(ii) A gas Q turns moist lead acetate paper silvery black. Identify the gas Q. [1 Mark]

Answer:
Hydrogen sulphide gas (\( \text{H}_{2}\text{S} \)).

Teacher's Note:
a) Hydrogen sulfide reacts with lead acetate to form black lead sulfide.
b) This is the standard test for sulfide salts.

(iii) pH of liquid R is 10. What kind of substance is R ? [1 Mark]

Answer:
Alkali / Base.

Teacher's Note:
a) pH greater than 7 indicates a basic medium.
b) pH 10 represents a weak-to-moderate alkali.

(iv) Salt S is prepared by reacting dilute sulphuric acid with copper oxide. Identify S. [1 Mark]

Answer:
Copper(II) sulphate (\( \text{CuSO}_{4} \)).

Teacher's Note:
a) Reaction: \( \text{CuO} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{CuSO}_{4} + \text{H}_{2}\text{O} \).
b) Insoluble metal oxides react with dilute acids to form salt and water.

Question 5.

(a) Name the following :
(i) The property possessed by metals by which they can be beaten into sheets. [1 Mark]

Answer:
Malleability.

Teacher's Note:
a) Malleability allows metals to be hammered into thin sheets.
b) Gold and silver exhibit the highest malleability.

(ii) A compound added to lower the fusion temperature of the electrolytic bath in the extraction of aluminium. [1 Mark]

Answer:
Cryolite (\( \text{Na}_{3}\text{AlF}_{6} \)) (Fluororspar can also be accepted).

Teacher's Note:
a) Pure alumina has a very high melting point; cryolite lowers it from \( 2050^{\circ}\text{C} \) to about \( 950^{\circ}\text{C} \).
b) It also acts as a solvent for pure alumina and increases conductivity.

(iii) The ore of zinc containing its sulphide. [1 Mark]

Answer:
Zinc blende (\( \text{ZnS} \)).

Teacher's Note:
a) Zinc blende is concentrated by froth floatation.
b) Calamine is zinc carbonate, another important zinc ore.

(b) Give one equation each to show the following properties of sulphuric acid :
(i) Dehydrating property. [1 Mark]

Answer:
\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\text{Conc. H}_{2}\text{SO}_{4}} 12\text{C} + 11\text{H}_{2}\text{O} \)

Teacher's Note:
a) Concentrated sulfuric acid removes water of crystallization or elements of water from organic compounds.
b) Sugar is charred to a spongy black mass of carbon.

(ii) Acidic nature. [1 Mark]

Answer:
\( \text{NaOH} + \text{H}_{2}\text{SO}_{4} \xrightarrow{<200^{\circ}\text{C}} \text{NaHSO}_{4} + \text{H}_{2}\text{O} \)

Teacher's Note:
a) Demonstrates neutralization of an alkali by the acid.
b) Can form both acid salts and normal salts depending on stoichiometry.

(iii) As a non-volatile acid. [1 Mark]

Answer:
\( \text{KNO}_{3} + \text{H}_{2}\text{SO}_{4} \xrightarrow{<200^{\circ}\text{C}} \text{KHSO}_{4} + \text{HNO}_{3} \)

Teacher's Note:
a) Non-volatile acids displace more volatile acids from their salts upon heating.
b) Sulfuric acid has a high boiling point compared to nitric or hydrochloric acid.

(c) Give balanced chemical equations to prepare the following salts :
(i) Lead sulphate from lead carbonate. [1 Mark]

Answer:
Step 1: \( \text{PbCO}_{3} + 2HNO_{3} \rightarrow \text{Pb(NO}_{3})_{2} + \text{H}_{2}\text{O} + \text{CO}_{2} \)
Step 2: \( \text{Pb(NO}_{3})_{2} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{PbSO}_{4}\downarrow + 2\text{HNO}_{3} \)

Teacher's Note:
a) Lead sulfate is insoluble, so it is prepared by precipitation.
b) Direct reaction between lead carbonate and dilute sulfuric acid forms an insoluble protective layer halting the reaction, hence a two-step method is used.

(ii) Sodium sulphate using dilute sulphuric acid. [1 Mark]

Answer:
\( 2\text{NaOH} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{Na}_{2}\text{SO}_{4} + 2\text{H}_{2}\text{O} \)

Teacher's Note:
a) Prepared by neutralization of sodium hydroxide with dilute sulfuric acid.
b) Forms a normal soluble salt.

(iii) Copper chloride using copper carbonate. [1 Mark]

Answer:
\( \text{CuCO}_{3} + 2HCl \rightarrow \text{CuCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}\uparrow \)

Teacher's Note:
a) Insoluble metal carbonate reacts with dilute hydrochloric acid to yield a soluble copper salt, water, and carbon dioxide.
b) Excess carbonate is filtered off after the reaction stops.

Question 6.

(a) (i) State Avogadro's law. [1 Mark]

Answer:
Under the same conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

Teacher's Note:
a) Fundamental gas law connecting volume and molecular content.
b) Forms the basis for deducing atomicity of gases.

(ii) A cylinder contains 68g of ammonia gas at s.t.p.
(1) What is the volume occupied by this gas ? [1 Mark]
(2) How many moles of ammonia are present in the cylinder ? [1 Mark]
(3) How many molecules of ammonia are present in the cylinder ? [1 Mark]
[N=14, H=1]

Answer:
Molar mass of \( \text{NH}_{3} = 14 + (3 \times 1) = 17\text{ g/mol} \).
(1) Volume occupied = \( \frac{68}{17} \times 22.4 = 4 \times 22.4 = 89.6\text{ litres} \).
(2) Number of moles = \( \frac{\text{Mass}}{\text{Molar mass}} = \frac{68}{17} = 4\text{ moles} \).
(3) Number of molecules = \( 4 \times 6.023 \times 10^{23} = 2.409 \times 10^{24}\text{ molecules} \).

Teacher's Note:
a) Always calculate the molar mass first for mole-concept numericals.
b) Use Avogadro's number \( 6.023 \times 10^{23} \) for calculating total molecules.

(b) (i) Why do covalent compounds exist as gases, liquids or soft solids ? [1 Mark]

Answer:
Because the molecules are held together by weak intermolecular forces (Van der Waals forces).

Teacher's Note:
a) Absence of strong electrostatic forces results in low melting and boiling points.
b) Contrast with ionic compounds which have strong crystal lattices.

(ii) Which electrode : anode or cathode is the oxidising electrode ? Why ? [2 Marks]

Answer:
Anode. Because anions lose electrons at the anode, and loss of electrons is oxidation.

Teacher's Note:
a) Anode is connected to the positive terminal and attracts anions.
b) Oxidation always occurs at the anode during electrolysis.

(c) Name the kind of particles present in :
(i) Sodium Hydroxide solution. [1 Mark]

Answer:
Sodium ions (\( \text{Na}^{+} \)) and Hydroxyl ions (\( \text{OH}^{-} \)) (along with water molecules).

Teacher's Note:
a) Sodium hydroxide is a strong electrolyte that dissociates completely in water.
b) Mention both cations and anions.

(ii) Carbonic acid. [1 Mark]

Answer:
Hydronium ions (\( \text{H}_{3}\text{O}^{+} \)), bicarbonate ions (\( \text{HCO}_{3}^{-} \)), carbonate ions (\( \text{CO}_{3}^{2-} \)), and unionized molecules of \( \text{H}_{2}\text{CO}_{3} \).

Teacher's Note:
a) Carbonic acid is a weak acid, hence it ionizes partially.
b) Both ions and molecules are present in its solution.

(iii) Sugar solution. [1 Mark]

Answer:
Molecules of sugar (\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \)) and water molecules.

Teacher's Note:
a) Sugar is a non-electrolyte and does not dissociate into ions.
b) Only neutral molecules exist in solution.

Question 7.

(a) An element Z has atomic number 16. Answer the following questions on Z :
(i) State the period and group to which Z belongs. [1 Mark]

Answer:
Period number: 3, Group number: VI A (or Group 16).

Teacher's Note:
a) Electronic configuration of Z (Sulfur, atomic number 16) is 2, 8, 6.
b) Three shells imply Period 3, and six valence electrons imply Group 16 (VI A).

(ii) Is Z a metal or a non-metal ? [1 Mark]

Answer:
Non-metal.

Teacher's Note:
a) Elements with 5, 6, or 7 valence electrons are typically non-metals.
b) Element Z is Sulfur.

(iii) State the formula between Z and Hydrogen. [1 Mark]

Answer:
\( \text{H}_{2}\text{Z} \) (Hydrogen sulphide, \( \text{H}_{2}\text{S} \)).

Teacher's Note:
a) Valency of Z is 2, and valency of hydrogen is 1.
b) Cross-valency method gives \( \text{H}_{2}\text{Z} \).

(iv) What kind of a compound is this ? [1 Mark]

Answer:
Polar covalent compound.

Teacher's Note:
a) Formed by sharing of electrons between non-metals with different electronegativities.
b) Molecules have partial positive and negative poles.

(b) M is a metal above hydrogen in the activity series and its oxide has the formula \( \text{M}_{2}\text{O} \). This oxide when dissolved in water forms the corresponding hydroxide which is a good conductor of electricity. In the above context answer the following :
(i) What kind of combination exists between M and O ? [1 Mark]

Answer:
Electrovalent bond / Ionic bond.

Teacher's Note:
a) Formed between a metal and a non-metal by complete transfer of electrons.
b) Results in an electrovalent compound.

(ii) How many electrons are there in the outermost shell of M ? [1 Mark]

Answer:
1 electron.

Teacher's Note:
a) Since the oxide formula is \( \text{M}_{2}\text{O} \), M exhibits a valency of 1.
b) Alkali metals have 1 valence electron.

(iii) Name the group to which M belongs. [1 Mark]

Answer:
Group 1 (or Alkali metals / Group I A).

Teacher's Note:
a) Elements with 1 valence electron belong to Group 1.
b) Examples include sodium and potassium.

(iv) State the reaction taking place at the cathode. [1 Mark]

Answer:
\( \text{M}^{+} + e^{-} \rightarrow \text{M} \) (Reduction).

Teacher's Note:
a) Cations migrate to the cathode and undergo reduction by gaining electrons.
b) Metal atoms are deposited at the cathode.

(v) Name the product at the anode. [1 Mark]

Answer:
Oxygen gas (\( \text{O}_{2} \)).

Teacher's Note:
a) Hydroxyl ions migrate to the anode, lose electrons, and form oxygen gas and water.
b) This occurs during the electrolysis of aqueous solutions containing metallic hydroxides or oxygen-containing anions.