ICSE Class 10 Chemistry Sample Paper 2023 with Solutions

SECTION A

Question 1

Choose one correct answer to the questions from the given options: [15]

(i) A weak electrolyte is: [1 Mark]
(a) Alcohol
(b) Potassium hydroxide
(c) Ammonium hydroxide
(d) Glucose

Answer: (c) Ammonium hydroxide

Ammonium hydroxide is a weak base that dissociates only partially in aqueous solution.

Teacher's Note:
a) Weak electrolytes dissociate partially, whereas strong electrolytes like KOH dissociate completely.
b) Alcohol and glucose are non-electrolytes as they do not form ions in solution.

(ii) Electron affinity is maximum in: [1 Mark]
(a) Alkaline earth metals
(b) Halogens
(c) Inert gases
(d) Alkali metals

Answer: (b) Halogens

Halogens have a strong tendency to accept an electron to achieve a stable octet configuration.

Teacher's Note:
a) Electron affinity increases across a period from left to right, reaching a maximum in halogens (Group 17).
b) Noble gases have zero electron affinity due to their stable complete octets.

(iii) The main components of bronze are: [1 Mark]
(a) Copper and tin
(b) Copper and iron
(c) Copper and lead
(d) Copper and zinc

Answer: (a) Copper and tin

Bronze is an alloy primarily composed of copper and tin.

Teacher's Note:
a) Memorize common alloy compositions carefully as direct factual questions are frequent.
b) Do not confuse bronze with brass, which is an alloy of copper and zinc.

(iv) A polar covalent compound is: [1 Mark]
(a) Methane
(b) Ammonia
(c) Nitrogen
(d) Chlorine

Answer: (b) Ammonia

Ammonia (NH₃) has polar covalent bonds due to the high electronegativity difference between nitrogen and hydrogen.

Teacher's Note:
a) Polar covalent compounds are formed between atoms with significant electronegativity differences.
b) Methane is non-polar due to its symmetrical tetrahedral structure.

(v) An acid which has two replaceable hydrogen ions: [1 Mark]
(a) Acetic acid
(b) Hydrochloric acid
(c) Phosphoric acid
(d) Carbonic acid

Answer: (d) Carbonic acid

Carbonic acid (H₂CO₃) is a dibasic acid with two replaceable hydrogen ions per molecule.

Teacher's Note:
a) Basicity of an acid is determined by the number of hydronium ions produced per molecule of the acid in aqueous solution.
b) HCl is monobasic, acetic acid is monobasic, and phosphoric acid is tribasic.

(vi) The hydroxide which is soluble in excess of NaOH is: [1 Mark]
(a) Ferric hydroxide
(b) Lead hydroxide
(c) Copper hydroxide
(d) Calcium hydroxide

Answer: (b) Lead hydroxide

Lead hydroxide is amphoteric and dissolves in excess sodium hydroxide to form a soluble plumbate.

Teacher's Note:
a) Amphoteric hydroxides like those of Zn, Pb, and Al dissolve in excess NaOH.
b) Ferric and copper hydroxides are basic and insoluble in excess NaOH.

(vii) If the RMM of carbon dioxide is 44, then its vapour density is: [1 Mark]
(a) 22
(b) 32
(c) 44
(d) 88

Answer: (a) 22

Vapour Density = (Relative Molecular Mass) / 2 = 44 / 2 = 22.

Teacher's Note:
a) Always remember the standard relation: RMM = 2 × Vapour Density.
b) Vapour density has no units since it is a ratio relative to hydrogen.

(viii) Drying agent used to dry Hydrogen chloride gas: [1 Mark]
(a) Concentrated Sulphuric acid
(b) Calcium oxide
(c) Sulphurous acid
(d) Calcium hydroxide

Answer: (a) Concentrated Sulphuric acid

Concentrated sulphuric acid is a strong dehydrating agent that does not react with HCl gas.

Teacher's Note:
a) Calcium oxide cannot be used because it is basic and reacts with acidic HCl gas.
b) Phosphorus pentoxide or anhydrous calcium chloride can also be used, but concentrated H₂SO₄ is standard for laboratory preparation.

(ix) The catalyst used in the Haber's Process is: [1 Mark]
(a) Molybdenum
(b) Platinum
(c) Nickel
(d) Finely divided Iron

Answer: (d) Finely divided Iron

Finely divided iron is used as a catalyst with molybdenum acting as a promoter in Haber's process.

Teacher's Note:
a) Do not confuse the catalyst (iron) with the promoter (molybdenum).
b) Contact process uses vanadium pentoxide or platinum as a catalyst.

(x) An aqueous compound which turns colourless phenolphthalein to pink: [1 Mark]
(a) Ammonium hydroxide
(b) Nitric acid
(c) Anhydrous calcium chloride
(d) Sulphuric acid

Answer: (a) Ammonium hydroxide

Ammonium hydroxide is an alkali, and phenolphthalein turns pink in basic or alkaline medium.

Teacher's Note:
a) Phenolphthalein remains colourless in acidic and neutral solutions.
b) Nitric and sulphuric acids are strong acids, while calcium chloride is a neutral salt.

(xi) The gas formed when carbon reacts with concentrated sulphuric acid: [1 Mark]
(a) Hydrogen
(b) Sulphur trioxide
(c) Sulphur dioxide
(d) Oxygen

Answer: (c) Sulphur dioxide

Carbon is oxidized by hot concentrated sulphuric acid to form carbon dioxide, sulphur dioxide, and water.

Teacher's Note:
a) Concentrated H₂SO₄ acts as a strong oxidizing agent with non-metals like carbon and sulphur.
b) The gaseous mixture produced contains both CO₂ and SO₂.

(xii) The organic compound prepared when Ethanol undergoes dehydration: [1 Mark]
(a) Methane
(b) Ethane
(c) Acetylene
(d) Ethene

Answer: (d) Ethene

Heating ethanol with concentrated sulphuric acid at 170 degrees Celsius causes intramolecular dehydration to form ethene.

Teacher's Note:
a) Temperature is crucial: 170 degrees Celsius gives ethene, whereas 140 degrees Celsius gives diethyl ether.
b) Concentrated H₂SO₄ acts as a dehydrating agent in this reaction.

(xiii) The IUPAC name of methyl acetylene is: [1 Mark]
(a) Propyne
(b) Ethene
(c) Propane
(d) Ethyne

Answer: (a) Propyne

Methyl acetylene has three carbon atoms with a triple bond, making it propyne.

Teacher's Note:
a) Acetylene is ethyne; substituting one hydrogen with a methyl group yields propyne.
b) Always count the total number of carbon atoms to determine the root word.

(xiv) The product formed at the cathode in electroplating of an article with Nickel is: [1 Mark]
(a) Hydrogen gas
(b) Nickel ions
(c) Nickel atoms
(d) Oxygen gas

Answer: (c) Nickel atoms

During electroplating, metal ions migrate to the cathode, gain electrons, and get deposited as neutral metal atoms.

Teacher's Note:
a) Reduction always takes place at the cathode (gain of electrons).
b) The article to be electroplated is always made the cathode.

(xv) An alkali metal found in period 3 and group 1 is: [1 Mark]
(a) Magnesium
(b) Lithium
(c) Sodium
(d) Potassium

Answer: (c) Sodium

Sodium has atomic number 11, electronic configuration 2, 8, 1, placing it in Period 3 and Group 1.

Teacher's Note:
a) Period number corresponds to the number of shells, and group number corresponds to the number of valence electrons.
b) Magnesium is in Period 3, Group 2; lithium is in Period 2, Group 1.

Question 2

(i) The diagram shows an experiment set up for the laboratory preparation of a pungent smelling gas. The gas is alkaline in nature. [5 Marks]
[Figure: Laboratory setup showing a round-bottomed flask containing Mixture X on a burner, connected via a delivery tube to a drying tower containing Y, and an inverted gas jar collecting the pungent alkaline gas Ammonia.]
(a) Name the gas collected in the gas jar.
(b) Write a balanced chemical equation for the above preparation.
(c) How is the gas being collected?
(d) What is the purpose of using Y?
(e) How will you find that the jar is full of gas?

Answer:
(a) Ammonia gas (NH₃).
(b) 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃.
(c) By downward displacement of air.
(d) Y is quicklime (Calcium oxide, CaO) used as a drying agent for ammonia.
(e) Bring a moist red litmus paper near the mouth of the gas jar; it turns blue.

Teacher's Note:
a) Ammonia cannot be dried using concentrated H₂SO₄, P₂O₅, or CaCl₂ because it reacts with them.
b) Downward displacement of air is used because ammonia is much lighter than air (vapor density is 8.5).

(ii) Match the following Column A with Column B. [5 Marks]

Answer:
(a) Acid Salt - 4. Sodium Hydrogen Carbonate
(b) Copper Oxide - 1. Black in colour
(c) Zinc hydroxide - 5. Soluble in excess sodium hydroxide
(d) Copper Metal - 2. Reddish brown
(e) Polar compound - 3. Hydrogen chloride

Teacher's Note:
a) Read both columns carefully before matching to avoid confusion.
b) Acid salts contain replaceable hydrogen ions, such as NaHCO₃.

(iii) Complete the following by choosing the correct answers from the bracket: [5 Marks]
(a) Ammonia in the liquefied form is _________. [neutral / basic]
(b) Organic compounds are generally insoluble in _________. [Water / Organic solvents]
(c) An inert electrode used in electrolysis of acidified water is _________. [iron / platinum]
(d) Hydrocarbons having double bond is _________. [alkenes / alkynes]
(e) An alkaline gas gives dense white fumes of _________. [NH₄OH / NH₄Cl] with hydrogen chloride gas.

Answer:
(a) neutral
(b) Water
(c) platinum
(d) alkenes
(e) NH₄Cl

Teacher's Note:
a) Liquid ammonia does not ionize and is neutral, unlike aqueous ammonia which is basic.
b) Platinum or graphite is commonly used as an inert electrode in aqueous electrolysis.

(iv) Identify the following: [5 Marks]
(a) The property by which carbon bonds with itself to form a long chain.
(b) A substance that conducts electricity in molten or aqueous state.
(c) The energy required to remove an electron from the valence shell of a neutral isolated gaseous atom.
(d) The name of the process by which the Bauxite ore is concentrated.
(e) The bond formed by a shared pair of electrons with both electrons coming from the same atom.

Answer:
(a) Catenation.
(b) Electrolyte.
(c) Ionization potential (or Ionization energy).
(d) Baeyer's process (or Hall's process).
(e) Coordinate bond (or Dative bond).

Teacher's Note:
a) Catenation is the unique ability of carbon atoms to link together forming long chains and rings.
b) A coordinate bond requires one atom to have a lone pair and the other to have a vacant orbital.

(v) (a) Draw the structural formula for the following: [5 Marks]
1. 2-pentanol
2. Ethanal
3. 1-butene
(b) Name the following organic compounds in IUPAC system:
1. H - C(H)(H) - C(H)(H) - C(H)(H) - C(H)(H) - O - H (drawn as CH₃-CH₂-CH₂-CH₂-OH)
2. H - C ≡ C - C(H)(H) - H (drawn as CH≡C-CH₃)

Answer:
(a) 1. 2-pentanol: CH₃-CH(OH)-CH₂-CH₂-CH₃
2. Ethanal: CH₃-CHO
3. 1-butene: CH₂=CH-CH₂-CH₃
(b) 1. Butan-1-ol (or 1-butanol)
2. Propyne

Teacher's Note:
a) When drawing structural formulas, ensure every carbon atom forms four bonds and every hydrogen forms one.
b) Number the carbon chain from the end closer to the functional group.

SECTION B

(Attempt any four questions.)

Question 3

(i) Identify the Anion present in each of the following compounds. [2 Marks]
(a) When Barium Chloride Solution is added to a solution of compound B, a white precipitate insoluble in dilute Hydrochloric acid is formed.
(b) When dilute Sulphuric acid is added to compound D, a gas is produced which turns lime water milky but has no effect on acidified potassium dichromate solution.

Answer:
(a) Sulphate anion (SO₄^2-).
(b) Carbonate anion (CO₃^2-) or Bicarbonate anion (HCO₃^-).

Teacher's Note:
a) BaSO₄ is a white precipitate insoluble in dil. HCl.
b) Carbon dioxide turns lime water milky but does not change the orange color of acidified K₂Cr₂O₇ (unlike SO₂ gas).

(ii) Write the products and balance the equation. [2 Marks]
(a) S + Conc HNO₃ →
(b) ZnS + HCl →

Answer:
(a) S + 6HNO₃ (conc.) → H₂SO₄ + 6NO₂ + 2H₂O
(b) ZnS + 2HCl (dil.) → ZnCl₂ + H₂S

Teacher's Note:
a) Concentrated nitric acid oxidizes non-metals like sulphur to sulphuric acid and itself gets reduced to nitrogen dioxide.
b) Metal sulphides react with dilute acids to evolve hydrogen sulphide gas with the smell of rotten eggs.

(iii) Arrange the following as per the instruction given in the brackets: [3 Marks]
(a) Na, K, Cl, Si, S (increasing order of electro negativity)
(b) Be, Li, F, C, B, N, O (increasing order of metallic character)
(c) Br, F, I, Cl (increasing order of atomic size)

Answer:
(a) K < Na < Si < S < Cl
(b) F < O < N < C < B < Be < Li
(c) F < Cl < Br < I

Teacher's Note:
a) Electronegativity increases across a period from left to right and decreases down a group.
b) Metallic character decreases across a period and increases down a group.

(iv) Fill in the blanks selecting the appropriate word from the given choice: [3 Marks]
(a) In a covalent compound the bond is formed due to _________ of electrons (sharing / transfer)
(b) A molecule which has a single lone pair of electrons _________ (NH₃ / H₂O)
(c) Electrovalent compounds do not conduct electricity in their _________ state. (molten / solid)

Answer:
(a) sharing
(b) NH₃
(c) solid

Teacher's Note:
a) Covalent bonds involve mutual sharing of electrons, whereas electrovalent bonds involve complete transfer of electrons.
b) Solid ionic compounds have fixed ions that cannot move, hence they do not conduct electricity in solid state.

Question 4

(i) For each of the substances given below, what is the role played in the extraction of Aluminum. [2 Marks]
(a) Cryolite
(b) Graphite

Answer:
(a) Cryolite (Na₃AlF₆) lowers the fusion temperature of the mixture and increases the electrical conductivity of alumina.
(b) Graphite acts as the anode, which gets oxidized to carbon dioxide gas during the electrolytic reduction.

Teacher's Note:
a) Pure alumina is a bad conductor and has a very high melting point (2050 degrees Celsius); cryolite solves both issues.
b) Graphite anodes need periodic replacement as they burn away due to oxygen gas evolved at the anode.

(ii) Calculate: [2 Marks]
(a) A gas cylinder is filled with hydrogen and it holds 5 gms of gas X. The same cylinder holds 85 gms of gas X under same temperature and pressure. Calculate the vapour density of gas X.
(b) Give the empirical formula of CH₃COOH.

Answer:
(a) According to Avogadro's Law, equal volumes of all gases under the same conditions of temperature and pressure contain the equal number of molecules.
Mass of 1 volume of hydrogen = 5 g (since 5 g of hydrogen is taken as reference volume).
Mass of same volume of gas X = 85 g.
Vapour Density = (Mass of 1 volume of gas X) / (Mass of 1 volume of hydrogen) = 85 / 5 = 17.
(b) Empirical formula of CH₃COOH is CH₂O.

Teacher's Note:
a) Vapour density is defined as the ratio of the mass of a certain volume of gas to the mass of the same volume of hydrogen under similar conditions.
b) Molecular formula of acetic acid is C₂H₄O₂, so dividing by the common factor 2 gives the empirical formula CH₂O.

(iii) The following questions are pertaining to the laboratory preparation of Hydrogen chloride gas. [3 Marks]
(a) Write a balanced chemical equation for its preparation mentioning the condition required.
(b) Why is concentrated Nitric Acid not used in the preparation of Hydrogen Chloride gas?
(c) How is Hydrogen Chloride gas collected?

Answer:
(a) NaCl + H₂SO₄ (conc.) < 200 degrees Celsius → NaHSO₄ + HCl
(b) Concentrated nitric acid is volatile and would distill over along with hydrogen chloride gas.
(c) By upward displacement of air.

Teacher's Note:
a) Temperature is maintained below 200 degrees Celsius to save fuel, prevent damage to glass apparatus, and avoid formation of hard sodium sulphate.
b) HCl gas is heavier than air (vapor density is 18.25), so it is collected by upward displacement of air.

(iv) Explain the following: [3 Marks]
(a) Concentrated Nitric acid appears yellow when it is left standing in a glass bottle.
(b) An inverted Funnel is used to dissolve Hydrogen Chloride gas in water.
(c) All apparatus made of glass is used in the laboratory preparation of Nitric acid.

Answer:
(a) Concentrated nitric acid decomposes slowly in the presence of sunlight to form yellowish-brown nitrogen dioxide gas, which dissolves in the acid and turns it yellow.
(b) An inverted funnel prevents back-suction of water and provides a large surface area for the rapid absorption of HCl gas.
(c) Glass is used because nitric acid is highly corrosive and attacks cork, rubber, and metallic components.

Teacher's Note:
a) Reaction for yellow coloration: 4HNO₃ → 2H₂O + 4NO₂ + O₂.
b) Back-suction occurs because HCl is extremely soluble in water, causing a sudden drop in pressure inside the delivery tube.

Question 5

(i) (a) State one property of Ammonia demonstrated in the Fountain Experiment. [2 Marks]
(b) Give the ionic equation when Ammonium Hydroxide is dissolved in water.

Answer:
(a) Extremely high solubility of ammonia gas in water (or basic nature of ammonia solution).
(b) NH₄OH ⇌ NH₄⁺ + OH^-

Teacher's Note:
a) The fountain experiment proves both high solubility and alkalinity of ammonia gas.
b) Use reversible arrows for weak electrolytes like ammonium hydroxide.

(ii) Name a probable Cation present based on the following Observations: [2 Marks]
(a) Reddish brown precipitate insoluble in Ammonium Hydroxide.
(b) Blue coloured sulphate solution.

Answer:
(a) Ferric ion (Fe³⁺).
(b) Cupric ion (Cu²⁺).

Teacher's Note:
a) Ferric hydroxide is a reddish-brown precipitate insoluble in excess NH₄OH.
b) Copper sulphate solutions are characteristic blue due to hydrated Cu²⁺ ions.

(iii) Give balanced chemical equation for the following: [3 Marks]
(a) Laboratory Preparation of Methane from Sodium Acetate.
(b) Preparation of Ethyne from 1, 2 dibromoethane.
(c) Ethene reacting with Chlorine.

Answer:
(a) CH₃COONa + NaOH (CaO) → (heat) Na₂CO₃ + CH₄
(b) BrCH₂-CH₂Br + 2KOH (alc.) → (heat) CH≡CH + 2KBr + 2H₂O
(c) CH₂=CH₂ + Cl₂ → CH₂Cl-CH₂Cl (1, 2-dichloroethane)

Teacher's Note:
a) Soda lime (NaOH + CaO) is used in decarboxylation of sodium acetate to prepare methane.
b) Alcoholic KOH causes dehydrohalogenation to yield ethyne from vicinal dihalides.

(iv) State one relevant observation for each of the following reactions: [3 Marks]
(a) When excess Ammonia is passed through an aqueous solution of Lead Nitrate.
(b) Copper Sulphate solution is electrolysed using Copper electrodes.
(c) Ammonium hydroxide is added to Ferrous Sulphate solution.

Answer:
(a) A chalky white precipitate is formed which is insoluble in excess ammonia.
(b) The blue color of the copper sulphate solution remains unchanged, while the anode diminishes in size and copper gets deposited at the cathode.
(c) A dirty green precipitate is formed which is insoluble in excess ammonium hydroxide.

Teacher's Note:
a) Lead hydroxide precipitate is insoluble in excess NH₄OH (unlike zinc hydroxide which dissolves).
b) In electrorefining with active copper electrodes, Cu²⁺ ions discharged at the cathode are continuously replaced by Cu atoms dissolving from the anode.

Question 6

(i) Define: [2 Marks]
(a) Gay Lussac's law of combining volume.
(b) Vapour Density

Answer:
(a) When gases react, they do so in volumes which bear a simple ratio to one another and to the volume of the gaseous products, provided all volumes are measured at the same temperature and pressure.
(b) Vapour density is defined as the ratio of the mass of a certain volume of gas or vapour to the mass of the same volume of hydrogen, measured under the same conditions of temperature and pressure.

Teacher's Note:
a) Mentioning constant temperature and pressure is compulsory in Gay Lussac's law.
b) Vapour density has no units.

(ii) Solve: [2 Marks]
1250cc of oxygen was burnt with 300cc of ethane (C₂H₆). Calculate the volume of the unused oxygen and the volume of the carbon dioxide formed.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Answer:
From the equation: 2 volumes of ethane require 7 volumes of oxygen to produce 4 volumes of carbon dioxide.
Therefore, 300 cc of ethane will require: (7 / 2) × 300 = 1050 cc of oxygen.
Volume of unused oxygen = Initial oxygen - Oxygen used = 1250 cc - 1050 cc = 200 cc.
Volume of carbon dioxide formed = (4 / 2) × 300 = 600 cc.

Teacher's Note:
a) Always apply Gay Lussac's law directly using the stoichiometric coefficients for gaseous reactions.
b) Clearly state the limiting reactant (ethane) and calculate excess oxygen properly.

(iii) State the conditions required for the following reactions: [3 Marks]
(a) Conversion of Sulphur dioxide to Sulphur trioxide.
(b) Conversion of Ammonia to Nitric acid
(c) Conversion of Nitrogen to Ammonia

Answer:
(a) Temperature: 450 to 500 degrees Celsius, Pressure: 1 to 2 atmospheres, Catalyst: Vanadium pentoxide (V₂O₅) or Platinized asbestos.
(b) Temperature: 800 degrees Celsius, Catalyst: Platinum, Pressure: 5 to 10 atmospheres.
(c) Temperature: 450 to 500 degrees Celsius, Pressure: 200 to 900 atmospheres, Catalyst: Finely divided iron with molybdenum as promoter.

Teacher's Note:
a) These industrial conditions (Contact process, Ostwald process, Haber process) must be memorized accurately.
b) Mentioning temperature, pressure, and catalyst explicitly fetches full marks.

(iv) Choose the role played by concentrated Sulphuric acid as A, B, C which is responsible for the reactions 1 to 3. [3 Marks]
A. Oxidizing agent
B. Non Volatile Acid
C. Dehydrating agent
1. NaNO₃ + H₂SO₄ →(<200 degrees Celsius) NaHSO₄ + HNO₃
2. CuSO₄.5H₂O →(H₂SO₄) CuSO₄ + 5H₂O
3. S + 2H₂SO₄ → 3SO₂ + 2H₂O

Answer:
1. B. Non Volatile Acid
2. C. Dehydrating agent
3. A. Oxidizing agent

Teacher's Note:
a) Concentrated H₂SO₄ has high boiling point, making it non-volatile compared to nitric or hydrochloric acid.
b) It removes water of crystallization from blue vitriol acting as a dehydrating agent.

Question 7

(i) Find the empirical formula and molecular formula of an organic compound from the data given below: [2 Marks]
C = 75.92% H = 6.32%, N = 17.76% its vapour density is 39.5
(At.wt: C=12, H=1, N=14)

Answer:
C: moles = 75.92 / 12 = 6.326; ratio = 6.326 / 1.268 = 5
H: moles = 6.32 / 1 = 6.32; ratio = 6.32 / 1.268 = 5
N: moles = 17.76 / 14 = 1.268; ratio = 1.268 / 1.268 = 1
Empirical formula = C₅H₅N.
Empirical formula mass = (5 × 12) + (5 × 1) + 14 = 60 + 5 + 14 = 79.
Molecular mass = 2 × Vapour Density = 2 × 39.5 = 79.
n = Molecular mass / Empirical formula mass = 79 / 79 = 1.
Molecular formula = C₅H₅N.

Teacher's Note:
a) Always set up a clear tabular format for calculating empirical formulas with elements, percentages, atomic weights, moles, atomic ratios, and simplest whole number ratios.
b) Molecular formula = n × Empirical formula.

(ii) Identify the functional group in the following organic compounds: [2 Marks]
(a) HCHO
(b) C₂H₅COOH

Answer:
(a) Aldehyde group (-CHO)
(b) Carboxylic acid group (-COOH)

Teacher's Note:
a) HCHO is formaldehyde (methanal), containing the aldehydic functional group.
b) C₂H₅COOH is propionic acid (propanoic acid), containing the carboxyl functional group.

(iii) During the Electrolysis of Copper II Sulphate solution using platinum as cathode and graphite as anode. [3 Marks]
(a) State what you observe at the cathode.
(b) State the change noticed in the electrolyte.
(c) Write the reaction at the cathode.

Answer:
(a) A reddish-brown coating of copper metal is deposited at the cathode.
(b) The blue color of the copper sulphate solution gradually fades and eventually becomes colorless, and the solution becomes acidic due to accumulation of sulphuric acid.
(c) Cu²⁺ + 2e^- → Cu

Teacher's Note:
a) Since platinum is inert, copper ions are discharged at the cathode in preference to hydrogen ions.
b) Cu²⁺ ions are depleted from the solution, turning the blue solution colorless.

(iv) Choose the answer from the list which fits the description. [3 Marks]
[CaO, CO₂, NaOH, Fe(OH)₃, CO]
(a) A basic oxide.
(b) An oxide which is acidic.
(c) An Alkali.

Answer:
(a) CaO
(b) CO₂
(c) NaOH

Teacher's Note:
a) Metallic oxides like CaO are generally basic, whereas non-metallic oxides like CO₂ are acidic.
b) Alkalis are water-soluble bases, such as NaOH.

Question 8

(i) Draw the electron dot structure for the following. [2 Marks]
(a) H₃O⁺
(b) CH₄

Answer:
(a) H₃O⁺ structure shows oxygen bonded to three hydrogen atoms by single covalent bonds with one lone pair on oxygen and a coordinate bond formed with H⁺.
(b) CH₄ structure shows a central carbon atom sharing four electron pairs with four hydrogen atoms in a tetrahedral arrangement.

Teacher's Note:
a) Hydronium ion (H₃O⁺) contains both covalent and coordinate covalent bonds.
b) Use dots and crosses clearly to distinguish electrons of different atoms in Lewis dot structures.

(ii) Distinguish between the following as directed: [2 Marks]
(a) Sodium Carbonate and Sodium Sulphate by using dilute HCl
(b) Ammonium Sulphate and Sodium Sulphate by using Calcium hydroxide.

Answer:
(a) Sodium carbonate reacts with dilute HCl to produce brisk effervescence of colorless, odorless carbon dioxide gas, whereas sodium sulphate does not react with dilute HCl.
(b) Ammonium sulphate on warming with calcium hydroxide releases pungent smelling ammonia gas which turns moist red litmus blue, whereas sodium sulphate does not evolve ammonia gas.

Teacher's Note:
a) Carbonates react with acids to liberate CO₂ gas.
b) Ammonium salts evolve ammonia gas when heated with a strong alkali like Ca(OH)₂.

(iii) Name the particles present in: [3 Marks]
(a) Strong Electrolyte
(b) Weak Electrolyte
(c) Non Electrolyte

Answer:
(a) Only free mobile ions.
(b) Both molecules and ions.
(c) Only molecules.

Teacher's Note:
a) Strong electrolytes dissociate completely into ions.
b) Non-electrolytes consist solely of covalent molecules and do not contain ions.

(iv) An element X has atomic number 17. Answer the following questions. [3 Marks]
(a) State the period & group to which it belongs.
(b) Is it a Metal or Non Metal?
(c) Write the formula between X and Hydrogen.

Answer:
(a) Period 3, Group 17.
(b) Non-metal.
(c) HX (or HCl).

Teacher's Note:
a) Electronic configuration of element with atomic number 17 is 2, 8, 7, placing it in Period 3 and Group 17 (Halogens).
b) Elements with 5, 6, or 7 valence electrons are typically non-metals.