ICSE Class 9 Chemistry Sample Paper with Solutions Set 04

SECTION-A

(Attempt all questions from this Section)

Question 1
Choose one correct answer to the questions from the given options: [15]

(i) All gases have similar kinetic energy at a given pressure and __________ temperature. [1 Mark]
(A) given
(B) standard
(C) absolute
(D) room

Answer: (C) absolute

According to kinetic theory, absolute temperature is directly proportional to the average kinetic energy of gas molecules.

Teacher's Note:
a) Kinetic energy of an ideal gas depends solely on its absolute temperature.
b) Students often confuse standard temperature with absolute temperature in gas laws.

(ii) An oxidizing agent is a substance which: [1 Mark]
(A) Loose oxygen
(B) Gains hydrogen
(C) Gains oxygen
(D) Both A and B

Answer: (D) Both A and B

An oxidizing agent adds oxygen or removes hydrogen, or gains electrons during a redox reaction.

Teacher's Note:
a) Remember the mnemonic OIL RIG or that oxidising agents themselves get reduced.
b) A common mistake is restricting oxidation only to addition of oxygen.

(iii) Which one of the following pollutants is produced by combustion of coal, petrol and diesel? [1 Mark]
(A) Sulphur dioxide
(B) Lead
(C) Suspended particulate matter
(D) Carbon monoxide

Answer: (A) Sulphur dioxide

Fossil fuels like coal, petrol, and diesel contain sulphur impurities which burn to form sulphur dioxide.

Teacher's Note:
a) Sulphur dioxide is a major contributor to acid rain.
b) Ensure you distinguish primary pollutants formed directly from combustion sources.

(iv) A substance which gets oxidized in a redox reaction is a: [1 Mark]
(A) Oxidising agent
(B) Reducing agent
(C) Both oxidizing and reducing agent
(D) None of these

Answer: (B) Reducing agent

A reducing agent undergoes oxidation itself by losing electrons or adding oxygen.

Teacher's Note:
a) The substance oxidized is always the reducing agent.
b) Do not confuse the term with the product formed after oxidation.

(v) Assertion (A): The empirical formula of hydrogen peroxide (H₂O₂) is HO.
Reason (R): The simplest ratio is 1:1 between the hydrogen and oxygen atoms in hydrogen peroxide molecule. [1 Mark]
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is not the correct explanation of A.
(C) A is true but R is false.
(D) A is false but R is true.

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

The molecular formula H₂O₂ simplifies to the empirical formula HO in the ratio of 1:1.

Teacher's Note:
a) Empirical formula gives the simplest whole-number ratio of atoms in a compound.
b) Always divide the subscripts by their highest common factor to find the empirical formula.

(vi) What is the chemical formula of potassium plumbite? [1 Mark]
(A) K₄PbO₄
(B) K₂PbO₂
(C) K₃PbO₃
(D) K₂PbO₃

Answer: (B) K₂PbO₂

Potassium plumbite is formed by the combination of potassium and the plumbite radical (PbO₂^2-).

Teacher's Note:
a) Plumbite radical has a valency of 2 (PbO₂^2-).
b) Balance valencies carefully when writing formulae of complex salts.

(vii) Which of the following need to be constant for Charles' law? [1 Mark]
(A) Pressure
(B) Volume
(C) Temperature
(D) None of the above

Answer: (A) Pressure

Charles' law states that volume is directly proportional to temperature at constant pressure.

Teacher's Note:
a) Boyle's law keeps temperature constant, while Charles' law keeps pressure constant.
b) Memorize the constant parameters for each gas law to avoid confusion.

(viii) Which of these substances is a good reducing agent? [1 Mark]
(A) NaOCl
(B) HI
(C) FeCl₃
(D) KBr

Answer: (B) HI

Hydriodic acid (HI) readily donates hydrogen or electrons, making it a strong reducing agent.

Teacher's Note:
a) Substances with elements in their lower oxidation states act as reducing agents.
b) Halide ions like iodide (I^-) are easily oxidized.

(ix) The addition of certain unwanted chemical substances in the air causing harmful effects is called as: [1 Mark]
(A) Air pollution
(B) Toxicity
(C) Epidemic
(D) Ozone depletion

Answer: (A) Air pollution

Air pollution is defined as the contamination of air by harmful gases, dust, and smoke.

Teacher's Note:
a) Understand standard environmental definitions thoroughly.
b) Do not confuse air pollution with specific phenomena like global warming or ozone depletion.

(x) The element which is virtually inactive towards water is: [1 Mark]
(A) Au
(B) Al
(C) Ag
(D) Both A and C

Answer: (D) Both A and C

Gold (Au) and Silver (Ag) are noble metals that do not react with water or steam even at high temperatures.

Teacher's Note:
a) Metals below hydrogen in the reactivity series do not displace hydrogen from water.
b) Aluminium reacts with steam, whereas Au and Ag show no reaction.

(xi) Assertion (A): An element is a pure substance which can neither be formed nor decomposed into simple substances by ordinary physical or chemical methods.
Reason (R): A molecule is the smallest particle of an element that exhibits all the properties of that element. [1 Mark]
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is not the correct explanation of A.
(C) A is true but R is false.
(D) A is false but R is true.

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

An atom is the smallest particle of an element that may or may not exist independently, while a molecule can be of a compound too.

Teacher's Note:
a) An element consists of only one kind of atoms.
b) Read statements carefully to distinguish between definitions of atoms and molecules.

(xii) Which one of the following solutions is used for the removal of carbon dioxide gas in the purification of hydrogen gas? [1 Mark]
(A) Caustic potash solution
(B) Lead nitrate solution
(C) Silver nitrate solution
(D) Sulphuric acid

Answer: (A) Caustic potash solution

Caustic potash (KOH) absorbs carbon dioxide gas efficiently to form potassium carbonate.

Teacher's Note:
a) KOH is used as an absorbent for acidic gases like CO₂ and SO₂.
b) Impurities must be scrubbed during the preparation of pure hydrogen.

(xiii) X (atomic mass = 37), Y (atomic mass = 81), and Z are three members of Dobereiner's triads. Atomic mass Z =? [1 Mark]
(A) 120
(B) 125
(C) 130
(D) 135

Answer: (B) 125

According to Dobereiner's law of triads, the atomic mass of the middle element is the arithmetic mean of the other two: Y = (X + Z) / 2 → 81 = (37 + Z) / 2 → Z = 162 - 37 = 125.

Teacher's Note:
a) Apply the arithmetic mean formula for triads: Middle = (First + Third) / 2.
b) Double-check simple algebraic calculations to avoid silly errors.

(xiv) What is blue vitriol? [1 Mark]
(A) CuSO₄.5H₂O
(B) FeSO₄.5H₂O
(C) CuSO₄.7H₂O
(D) FeSO₄.7H₂O

Answer: (A) CuSO₄.5H₂O

Hydrated copper(II) sulphate pentahydrate is commonly known as blue vitriol.

Teacher's Note:
a) Green vitriol is FeSO₄.7H₂O.
b) Memorize common names and chemical formulae of important hydrated salts.

(xv) Which of the following is an example of a combination reaction between two compounds? [1 Mark]
(A) Formation of water
(B) Formation of limestone
(C) Water contracts when cooled up to 4°C
(D) The specific heat capacity of water is 1 calorie/(gram°C)

Answer: (B) Formation of limestone

Quicklime (CaO, a compound) reacts with carbon dioxide (CO₂, a compound) to form limestone (CaCO₃), which is a combination reaction between two compounds.

Teacher's Note:
a) Combination reactions involve two or more reactants forming a single product.
b) Distinguish between elements and compounds when analyzing chemical reactions.

Question 2

(i) The table given below shows the mass number and atomic number of five elements A, B, C, D and E. [5 Marks]

(a) To which group and period does element A belong?
(b) Choose from A, B, C, D and E, metal, non-metal and inert gas.
(c) Give the electronic configuration of elements A, B, C, D and E.

Answer:
(a) Element A (Atomic number 17) has electronic configuration 2, 8, 7. It belongs to Period 3 and Group 17.
(b) Metal: B; Non-metals: A, C, D; Inert gas: E.
(c) Electronic configurations:
- Element A (17): 2, 8, 7
- Element B (11): 2, 8, 1
- Element C (6): 2, 4
- Element D (8): 2, 6
- Element E (18): 2, 8, 8

Teacher's Note:
a) Period number equals the number of shells; group number for elements with 3 or more valence electrons is 10 plus the number of valence electrons.
b) Metals have 1 to 3 valence electrons, non-metals have 4 to 7, and inert gases have a complete octet (or duplet).

(ii) Match the following: [5 Marks]

Answer:
1. Aluminate - (e) AlO₂^-2
2. Chromate - (c) CrO₄^-2
3. Caustic Potash - (a) KOH
4. Lime stone - (b) CaCO₃
5. Silica - (d) SiO₂

Teacher's Note:
a) Match each chemical name with its standard formula accurately.
b) Cross-verify charges of complex ions like aluminate and chromate.

(iii) Fill in the blanks: [5 Marks]
(a) Pollutants such as NO₂, SO₂ and SO₃ dissolved in the moisture of air are the cause of __________.
(b) Excessive release of carbon dioxide in the atmosphere is the cause of __________ effect which produces global warming.
(c) The ozone layer prevents the harmful __________ radiation of the sun from reaching the earth.
(d) Decrease of the concentration of ozone in the stratosphere is the cause of formation of __________ holes.
(e) Ozone depletion is mainly caused by the active __________ atoms generated from CFC in the presence of UV radiation.

Answer:
(a) acid rain
(b) greenhouse
(c) ultraviolet
(d) ozone
(e) chlorine

Teacher's Note:
a) Memorize key environmental terms and their direct causes.
b) Chlorine free radicals from CFCs catalyze the destruction of ozone molecules.

(iv) Find the valency of the given elements: [5 Marks]
(a) An element A atomic number 7 mass numbers 14
(b) B electronic configuration 2, 8, 8
(c) C electrons 13, neutrons 14
(d) D Protons 18 neutrons 22
(e) E Electronic configuration 2, 8, 8, 1

Answer:
(a) Element A: Atomic number = 7, electrons = 2, 5. Valency = 8 - 5 = 3.
(b) Element B: Electronic configuration = 2, 8, 8. Valence electrons = 8. Valency = 0.
(c) Element C: Electrons = 13 (Atomic number = 13), configuration = 2, 8, 3. Valency = 3.
(d) Element D: Protons = 18 (Atomic number = 18), configuration = 2, 8, 8. Valency = 0.
(e) Element E: Electronic configuration = 2, 8, 8, 1. Valence electron = 1. Valency = 1.

Teacher's Note:
a) Valency is the combining capacity, equal to the number of valence electrons or 8 minus the valence electrons for non-metals.
b) Elements with a complete octet have a valency of zero.

(v)
(a) Fill in the blanks. [5 Marks]
1. K = °C + __________
2. 1 dm³ = __________ cm³
3. 1 torr = __________ mm of Hg
(b) Define:
1. Boyles' law
2. Charles' law

Answer:
(a)
1. 273
2. 1000
3. 1
(b)
1. Boyle's law: At constant temperature, the volume of a given mass of a dry gas is inversely proportional to its pressure.
2. Charles' law: At constant pressure, the volume of a given mass of a dry gas is directly proportional to its absolute temperature.

Teacher's Note:
a) Always state temperature and pressure conditions clearly when defining gas laws.
b) Ensure conversion factors like 1 dm³ = 1000 cm³ are memorized accurately.

SECTION-B

(Attempt any four questions)

Question 3

(i) Which of the following changes are endothermic or exothermic? [2 Marks]
(a) Dissolution of quick lime in water
(b) Dissolution of ammonium chloride in water

Answer:
(a) Dissolution of quick lime in water: Exothermic reaction.
(b) Dissolution of ammonium chloride in water: Endothermic reaction.

Teacher's Note:
a) Exothermic processes release heat (container becomes hot), while endothermic processes absorb heat (container becomes cold).
b) Slaking of lime is a vigorously exothermic combination reaction.

(ii) What are physical and chemical tests for water? [2 Marks]

Answer:
Physical test: Pure water is a clear, colorless, odorless liquid with a boiling point of 100°C and freezing point of 0°C.
Chemical test: Pure water turns white anhydrous copper sulphate blue, and turns blue crystals of cobalt chloride pink.

Teacher's Note:
a) Anhydrous CuSO₄ is a standard reagent used to detect the presence of moisture.
b) Clearly distinguish between physical characteristics and chemical confirmation tests.

(iii)
(a) Which of the following is true/false? If the answer is false, then explain the correct answer. [3 Marks]
1. The valency of an element with atomic number 3 is 3.
2. The ionisation energy tends to increase as one move from left to right across a period.
(b) Name any one pollutant, its origin, and its harmful effect.

Answer:
(a)
1. False. The element with atomic number 3 is Lithium, having electronic configuration 2, 1. Its valency is 1.
2. True.
(b) Pollutant: Carbon monoxide (CO). Origin: Incomplete combustion of fuels in vehicles and industries. Harmful effect: It reduces the oxygen-carrying capacity of blood by forming carboxyhaemoglobin.

Teacher's Note:
a) Always correct the false statement by providing the scientifically accurate fact.
b) Ionization energy increases across a period due to a decrease in atomic size and increase in nuclear charge.

(iv) Name the following: [3 Marks]
(a) Two metals that react with very dilute nitric acid to liberate hydrogen.
(b) A salt that is insoluble in all mineral acids.
(c) A gas having a rotten egg smell.

Answer:
(a) Magnesium and Manganese.
(b) Barium sulphate (BaSO₄) or Lead sulphate (PbSO₄).
(c) Hydrogen sulphide (H₂S).

Teacher's Note:
a) Very dilute nitric acid (about 1%) reacts with Mg and Mn to give hydrogen gas because it acts as a normal acid in extreme dilution.
b) H₂S gas is easily recognized by its characteristic rotten egg odor.

Question 4

(i) Balance the following reactions: [2 Marks]
(a) NH₃ + Cl₂ → NH₄Cl + N₂
(b) CaOCl₂ + NH₃ → CaCl₂ + N₂ + H₂O

Answer:
(a) 8NH₃ + 3Cl₂ → 6NH₄Cl + N₂
(b) 3CaOCl₂ + 2NH₃ → 3CaCl₂ + N₂ + 3H₂O

Teacher's Note:
a) Balance nitrogen and chlorine atoms systematically before balancing hydrogen.
b) Verify that the total number of atoms of each element is equal on both sides.

(ii) Electrovalent compounds have high melting and boiling points, while covalent compounds have low melting and boiling points. [2 Marks]

Answer:
Electrovalent compounds consist of ions held together by strong electrostatic forces of attraction, requiring large amounts of thermal energy to break. Covalent compounds consist of molecules held by weak intermolecular forces, requiring very little energy to separate.

Teacher's Note:
a) Mention electrostatic forces for ionic compounds and weak intermolecular (van der Waals) forces for covalent compounds.
b) This is a standard comparative reasoning question frequently asked in board exams.

(iii) What are stalagmites and stalactites? How are they formed? [3 Marks]

Answer:
Stalactites and stalagmites are pillar-like deposits found in limestone caves. They are formed when water containing dissolved calcium bicarbonate drips continuously from the rocks. Upon release of pressure, calcium bicarbonate decomposes into calcium carbonate, water, and carbon dioxide: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. The deposits growing downwards from the roof are called stalactites, and those growing upwards from the floor are called stalagmites.

Teacher's Note:
a) Clearly differentiate the direction of growth for stalactites (top down) and stalagmites (bottom up).
b) Write the chemical equation for the thermal decomposition / precipitation of calcium carbonate.

(iv) Arrange the following as per the instructions given in the brackets: [3 Marks]
(a) Cs, Na, Li, K, Rb (increasing order of metallic character).
(b) Mg, Cl, Na, S, Si (decreasing order of atomic size).
(c) Cl, F, Br, I (increasing order of electron affinity).

Answer:
(a) Li < Na < K < Rb < Cs
(b) Na > Mg > Si > S > Cl
(c) I < Br < Cl < F (Note: Chlorine has the highest electron affinity; fluorine is slightly lower due to small size and inter-electronic repulsion).

Teacher's Note:
a) Metallic character increases down a group as atomic size increases.
b) Atomic size decreases across a period from left to right due to increasing effective nuclear charge.

Question 5

(i) Electrovalent compounds dissolve in water, whereas covalent compounds do not. Explain. [2 Marks]

Answer:
Water is a polar solvent with a high dielectric constant, which weakens the electrostatic forces between ions in electrovalent compounds, causing them to dissociate into mobile ions. Covalent compounds are generally non-polar and insoluble in polar solvents like water, though they dissolve in non-polar organic solvents.

Teacher's Note:
a) Mention the role of water as a polar solvent in dissolving ionic compounds.
b) "Like dissolves like" is the fundamental principle governing solubility.

(ii) Calculate the percentage of phosphorus in Ca₃(PO₄)₂. [2 Marks]

Answer:
Relative molecular mass of Ca₃(PO₄)₂ = (40.07 × 3) + (30.9 × 2) + (16 × 8) = 120.21 + 61.8 + 128 = 310.01 g/mol.
Mass of phosphorus in one mole = 2 × 30.9 = 61.8 g.
Percentage of phosphorus = (61.8 / 310.01) × 100 = 19.93% (approx 20%).

Teacher's Note:
a) Formula for percentage composition: (Total mass of the element / Molecular mass of the compound) × 100.
b) Always show intermediate steps clearly for full credit.

(iii) From the knowledge of the activity series, name a metal that shows the following properties: [3 Marks]
(a) It reacts readily with cold water.
(b) It displaces hydrogen from hot water.
(c) It displaces hydrogen from dilute HCl.

Answer:
(a) Sodium (or Potassium / Calcium)
(b) Magnesium
(c) Zinc (or Iron / Aluminium)

Teacher's Note:
a) Highly reactive metals like Na and K react with cold water vigorously.
b) Moderately active metals like Mg react with boiling water or steam, while metals above hydrogen react with dilute acids.

(iv) A gas occupies a volume of 116 ml at 180°C and 8 atm. What will be the volume of the sample of the gas at STP? [3 Marks]

Answer:
Given:
\( V_1 = 116 \text{ ml} \)
\( T_1 = 180^{\circ}\text{C} + 273 = 453 \text{ K} \)
\( P_1 = 8 \text{ atm} \)
At STP: \( T_2 = 273 \text{ K} \), \( P_2 = 1 \text{ atm} \), \( V_2 = ? \)
Using the gas equation: \(\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}\)
\( V_2 = \frac{P_1 V_1 T_2}{T_1 P_2} = \frac{8 \times 116 \times 273}{453 \times 1} \)
\( V_2 = \frac{253344}{453} = 559.25 \text{ ml} \).

Teacher's Note:
a) Always convert Celsius temperatures to Kelvin by adding 273 before substituting into gas equations.
b) Double-check units of volume and pressure to maintain consistency.

Question 6

(i) Answer the following questions in one word: [2 Marks]
(a) What is the charge on canal rays?
(b) e/m ratio is constant in which rays?

Answer:
(a) Positive
(b) Cathode rays

Teacher's Note:
a) Canal rays consist of positively charged positive ions.
b) The e/m ratio for anode rays depends on the gas used, whereas for cathode rays (electrons) it is constant.

(ii) How acid rains are formed? [2 Marks]

Answer:
Acid rain is formed when atmospheric pollutants like sulphur dioxide (SO₂) and nitrogen oxides (NO_x), released from industrial emissions and combustion of fossil fuels, dissolve in rainwater to form sulphuric acid and nitric acid.

Teacher's Note:
a) Mention pH values below 5.6 for acid rain.
b) State both natural and anthropogenic sources of the pollutant gases.

(iii) Complete and balance the following equations: [3 Marks]
(a) Al + NaOH + H₂O →
(b) Fe + HCl →
(c) H₂S + Cl₂ →

Answer:
(a) 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑
(b) Fe + 2HCl → FeCl₂ + H₂↑
(c) H₂S + Cl₂ → 2HCl + S↓

Teacher's Note:
a) Aluminium reacts with alkalis like NaOH to produce sodium meta-aluminate and hydrogen gas.
b) Reaction of iron with HCl yields ferrous chloride (FeCl₂), not ferric chloride.

(iv) A gas occupies 200 cm³ at temperature 30°C and pressure 720 mm. Find the volume of the gas at 5°C and 740 mm of Hg. [3 Marks]

Answer:
Given:
\( V_1 = 200 \text{ cm}^3 \)
\( T_1 = 30^{\circ}\text{C} + 273 = 303 \text{ K} \)
\( P_1 = 720 \text{ mm of Hg} \)
\( T_2 = 5^{\circ}\text{C} + 273 = 278 \text{ K} \)
\( P_2 = 740 \text{ mm of Hg} \)
\( V_2 = ? \)
Using gas equation: \(\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}\)
\( V_2 = \frac{P_1 V_1 T_2}{T_1 P_2} = \frac{720 \times 200 \times 278}{303 \times 740} \)
\( V_2 = \frac{40032000}{224220} = 178.53 \text{ cm}^3 \).

Teacher's Note:
a) Ensure temperature is converted to Kelvin in all gas law problems.
b) Verify units of pressure and volume before final calculation.

Question 7

(i) Give the names of the following compounds: [3 Marks]
(a) HClO
(b) HClO₃
(c) HClO₄

Answer:
(a) Hypochlorous acid
(b) Chloric acid
(c) Perchloric acid

Teacher's Note:
a) Memorize the oxoacids of chlorine with their respective prefixes and suffixes.
b) HClO₂ is chlorous acid; distinguish carefully between -ous and -ic acid series.

(ii) Explain why? [3 Marks]
(a) Water is a very good cooling agent to use in cooling systems.
(b) A solution always appears clear and transparent.
(c) Lakes and rivers do not suddenly freeze in the winters.

Answer:
(a) Water has a very high specific heat capacity, allowing it to absorb large amounts of heat with a minimal rise in temperature.
(b) Solute particles in a true solution are extremely small (less than 1 nm) and do not scatter light.
(c) Water has a high specific latent heat of fusion, requiring large amounts of heat loss before it freezes completely.

Teacher's Note:
a) Emphasize the high specific heat capacity of water in cooling applications.
b) True solutions are homogeneous and transparent due to ultra-small particle size.

(iii) A gas of volume 22.4 L weighs 70 g at STP. Calculate the weight of the gas if it occupies a volume of 20 L at 27 °C and 700 mm Hg of pressure. [4 Marks]

Answer:
At STP, 22.4 L weighs 70 g. Thus, molecular mass = 70 g/mol.
For the given conditions:
\( P_1 = 760 \text{ mm} \), \( V_1 = 22.4 \text{ L} \), \( T_1 = 273 \text{ K} \)
\( P_2 = 700 \text{ mm} \), \( V_2 = 20 \text{ L} \), \( T_2 = 27 + 273 = 300 \text{ K} \)
Let us find the volume \( V \) of this gas at STP: \(\frac{P_1 V_{\text{STP}}}{T_1} = \frac{P_2 V_2}{T_2}\)
\( V_{\text{STP}} = \frac{700 \times 20 \times 273}{300 \times 760} = \frac{3822000}{228000} = 16.76 \text{ L} \).
Mass of 22.4 L at STP = 70 g.
Mass of 16.76 L at STP = \(\frac{70 \times 16.76}{22.4} = 52.38 \text{ g}\).

Teacher's Note:
a) Convert the given state to STP volume first using the combined gas equation.
b) Use molar mass or direct proportion to determine the final weight of the gas.

Question 8

(i) Name the following. [2 Marks]
(a) An alkali metal in period 3 and halogen in period 2.
(b) The noble gas with 3 shells.

Answer:
(a) Sodium (Na) and Fluorine (F)
(b) Argon (Ar)

Teacher's Note:
a) Sodium is in period 3, group 1; fluorine is in period 2, group 17.
b) Noble gases with 3 shells (2, 8, 8) correspond to atomic number 18 (Argon).

(ii) Convert the following on the kelvin scale: [2 Marks]
(a) 100°C
(b) 20°C
(c) -273°C
(d) 0°C

Answer:
(a) 100 + 273 = 373 K
(b) 20 + 273 = 293 K
(c) -273 + 273 = 0 K
(d) 0 + 273 = 273 K

Teacher's Note:
a) Kelvin temperature = Celsius temperature + 273.
b) Absolute zero is 0 K or -273°C.

(iii) Give the electron structure of the following compounds: [3 Marks]
(a) Magnesium chloride
(b) Calcium chloride
(c) Ethyne

Answer:
(a) Magnesium chloride (MgCl₂): Mg (2,8,2) transfers two electrons to two chlorine atoms (2,8,7), forming Mg²⁺ and two [Cl]^- ions.
(b) Calcium chloride (CaCl₂): Ca (2,8,8,2) transfers two electrons to two chlorine atoms, forming Ca²⁺ and two [Cl]^- ions.
(c) Ethyne (C₂H₂): Contains a carbon-carbon triple bond and two single C-H covalent bonds sharing six electrons between carbon atoms.

Teacher's Note:
a) Show electron transfer with arrows for ionic compounds and shared pairs for covalent compounds.
b) Ethyne features a triple covalent bond between the two carbon atoms.

(iv) Explain why the hardness of water makes it unfit for washing purposes. [3 Marks]

Answer:
Hard water contains calcium and magnesium ions which react with soap to form insoluble precipitates (scum) instead of lather. As a result, soap is wasted and cleaning action is hindered until all calcium and magnesium ions are precipitated.

Teacher's Note:
a) Mention the formation of sticky scum which wastes soap.
b) Soft water does not contain dissolved calcium and magnesium salts and forms lather readily.