ICSE Class 9 Chemistry Sample Paper with Solutions Set 01

SECTION-A

(Attempt all questions from this Section)

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

(i) The breakdown of CFCs in the presence of UV rays produce [1 Mark]
(A) Oxygen
(B) Hydrogen
(C) Chlorine
(D) Carbon

Answer: (C) Chlorine

Ultraviolet rays split CFC molecules to release highly reactive chlorine free radicals.

Teacher's Note:
a) Chlorine free radicals act as catalysts in the destruction of ozone molecules.
b) Students often confuse CFC breakdown products with carbon or fluorine radicals instead of chlorine.

(ii) The negatively charged monoatomic, monovalent ion/radical is [1 Mark]
(A) Hydrogen
(B) Oxide
(C) Sulphate
(D) Iodide

Answer: (D) Iodide

Iodide is represented as \( I^{-} \), which has a single negative charge and consists of a single atom.

Teacher's Note:
a) Monoatomic means formed from a single atom, and monovalent means having a valency of one.
b) Oxide is divalent (\( O^{2-} \)) and sulphate is a polyatomic radical (\( SO_4^{2-} \)).

(iii) Select a hygroscopic substance. [1 Mark]
(A) Stone
(B) Copper
(C) Paper
(D) Glass

Answer: (C) Paper

Paper absorbs moisture from the atmosphere without dissolving or forming a solution.

Teacher's Note:
a) Hygroscopic substances are defined as substances that absorb moisture from the air at ordinary temperatures.
b) Stone and glass are non-hygroscopic.

(iv) Which of the following formulae of elements were used by Mendeleev for giving the classification? [1 Mark]
(A) Sulphides
(B) Nitrides
(C) Carbonates
(D) Oxides

Answer: (D) Oxides

Mendeleev used hydrides and oxides as the basic chemical properties to classify elements in his periodic table.

Teacher's Note:
a) Mendeleev formulated his periodic law based on atomic weights and similarities in formulas of hydrides and oxides.
b) Remember that oxides and hydrides were the key compounds chosen due to their widespread formation.

(v) Assertion (A): Potassium nitrate decomposes to give potassium nitrite along with evolution of nitrogen gas which is represented by the equation: KNO₃ → KNO₂ + O₂
Reason (R): An equation must be balanced in order to comply with "Law of Conservation of Matter". [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: (D) A is false but R is true.

Potassium nitrate decomposes to give potassium nitrite and oxygen gas (\( KNO_3 → KNO_2 + O_2 \)), not nitrogen gas.

Teacher's Note:
a) Assertion is incorrect because oxygen gas is evolved during the thermal decomposition of potassium nitrate, not nitrogen gas.
b) Reason is a true statement regarding chemical equations, but since Assertion is false, option (D) is correct.

(vi) Which of the following is a use of an electrolytic decomposition reaction? [1 Mark]
(A) To obtain metals from their ores
(B) They are used in photography
(C) To produce salts
(D) Photosynthesis in plants

Answer: (A) To obtain metals from their ores

Electrolytic decomposition (lysis) uses electricity to decompose molten or dissolved compounds to extract reactive metals like sodium and aluminum.

Teacher's Note:
a) Electrolytic decomposition breaks down compounds using electrical energy.
b) Photography uses photochemical decomposition of silver salts.

(vii) The crystals of sodium chloride produces cracking sound on heating, called: [1 Mark]
(A) Crystallisation
(B) Deliquescence
(C) Hydration
(D) Decrepitation

Answer: (D) Decrepitation

Decrepitation is the snapping or cracking sound produced when certain crystals are heated due to the expansion and escape of entrapped moisture or mother liquor.

Teacher's Note:
a) Sodium chloride crystals often enclose tiny pockets of water which expand and burst upon heating.
b) Do not confuse with efflorescence or deliquescence which relate to atmospheric moisture behavior.

(viii) Which one of the following isotopes is used as a fuel in nuclear reactors? [1 Mark]
(A) An isotope of Uranium
(B) An isotope of Iodine
(C) An isotope of Cobalt
(D) An isotope of Thorium

Answer: (A) An isotope of Uranium

Uranium-235 is widely used as a fissile material/fuel in nuclear reactors to generate energy.

Teacher's Note:
a) Uranium-235 undergoes nuclear fission upon bombardment with neutrons.
b) Cobalt-60 is used in medical treatment, and Iodine-131 is used for thyroid disorders.

(ix) Rasika added a solution of silver nitrate to a solution of sodium chloride. The product would be: [1 Mark]
(A) Black solid mass silver chlorite
(B) White precipitate of silver chloride
(C) Dirty green precipitate of sodium chloronitrate
(D) Blue colour solution of sodium nitrite

Answer: (B) White precipitate of silver chloride

Mixing silver nitrate and sodium chloride produces a characteristic curdy white precipitate of silver chloride along with sodium nitrate.

Teacher's Note:
a) This is a classic double displacement precipitation reaction used to test for chloride ions.
b) Silver chloride precipitate is insoluble in dilute nitric acid.

(x) The metal oxide which is reduced by hydrogen is: [1 Mark]
(A) Al₂O₃
(B) CuO
(C) CaO
(D) Na₂O

Answer: (B) CuO

Copper(II) oxide is a relatively less reactive metal oxide that gets reduced to copper metal by passing hydrogen gas over it.

Teacher's Note:
a) Hydrogen acts as a reducing agent and reduces oxides of metals below it in the reactivity series (like copper, lead, iron).
b) Oxides of active metals like aluminum, calcium, and sodium cannot be reduced by hydrogen.

(xi) Assertion (A): Solubility of Glauber's salt first increases and then decreases with temperature.
Reason (R): Glauber's salt is anhydrous above 32.8°C. [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.

Glauber's salt (\( Na_2SO_4 \cdot 10H_2O \)) loses its water of crystallization above 32.8°C, converting to anhydrous sodium sulphate whose solubility decreases with temperature.

Teacher's Note:
a) The abrupt change in solubility behavior is due to phase transformation at 32.8°C.
b) Both statements are scientifically exact and R correctly explains A.

(xii) Newlands classification of elements did not include: [1 Mark]
(A) Metals
(B) Noble gases
(C) Non-metals
(D) Metalloids

Answer: (B) Noble gases

Noble gases were not discovered during Newlands' time, hence they were completely absent from his Law of Octaves.

Teacher's Note:
a) Newlands arranged elements in increasing order of atomic weights up to thorium.
b) Noble gases were discovered much later by Ramsay and Rayleigh.

(xiii) Which one of the following is the main reason for using helium instead of hydrogen for filling balloons? [1 Mark]
(A) Lighter than air.
(B) Almost as light as hydrogen.
(C) Non-combustible
(D) Inflammable

Answer: (C) Non-combustible

Helium is chemically inert and non-combustible, making it safe for filling weather and party balloons, whereas hydrogen is highly inflammable.

Teacher's Note:
a) Although hydrogen is slightly lighter than helium, its extreme combustibility poses a major safety hazard.
b) Always emphasize safety properties when comparing lifting gases.

(xiv) Hydrogen is NOT used as a fuel in the form of: [1 Mark]
(A) Coal gas
(B) Water gas
(C) Liquid hydrogen
(D) Ammonia gas

Answer: (D) Ammonia gas

Ammonia is not used as a fuel; whereas coal gas, water gas, and liquid hydrogen all contain combustible components used as fuels.

Teacher's Note:
a) Liquid hydrogen is used as rocket fuel, and water gas/coal gas contain free hydrogen used in industrial heating.
b) Ammonia is used as a refrigerant and for making fertilizers.

(xv) General gas constant is represented by: [1 Mark]
(A) B
(B) K
(C) R
(D) S

Answer: (C) R

In the ideal gas equation (\( PV = nRT \)), the universal gas constant is denoted by the letter R.

Teacher's Note:
a) The universal gas constant R has a constant value across all ideal gases.
b) Students must remember the ideal gas equation terms clearly.

Question 2

(i) Complete the table given below by identifying V, W, X, Y and Z. [5 Marks]

Answer:
V = Sr (Mass number = Protons + Neutrons = 38 + 50 = 88, so symbol is \( _{38}^{88}\text{Sr} \))
W = 18 (Neutrons = Mass number - Protons = 35 - 17 = 18)
X = \( _{92}^{238}\text{U} \) (Mass number = 92 + 146 = 238, Symbol is \( _{92}^{238}\text{U} \))
Y = 5 (Number of electrons in a neutral atom equals number of protons = 5)
Z = 29 (Number of protons equals atomic number / number of electrons = 29)

Teacher's Note:
a) Concepts applied: Mass number \( A = p + n \), and in a neutral atom, electrons = protons = atomic number.
b) Ensure students write complete isotopic symbols including atomic number and mass number where required.

(ii) Match the following: [5 Marks]

Answer:
(a) - (v) Halogens
(b) - (iii) Alkali metals
(c) - (ii) Noble gases
(d) - (i) Transition elements
(e) - (vi) Actinides
(f) - (iv) Alkaline earth metals

Teacher's Note:
a) Halogens have 7 valence electrons, so they are short by 1 electron to complete their octet.
b) Group 2 elements possess 2 valence electrons and are known as alkaline earth metals.

(iii) Fill in the blanks: [5 Marks]
(a) Nitric oxide is _____ toxic.
(b) The gaseous material which envelopes the Earth is called _____.
(c) The lowest region of the atmosphere is called _____.
(d) The stratosphere mainly contains _____, and ozone.
(e) Rain water containing H₂SO₄ and HNO₃ is called _____.

Answer:
(a) Less (or highly depending on context, official key states: Less)
(b) Atmosphere
(c) Troposphere
(d) Nitrogen, oxygen
(e) Acid rain

Teacher's Note:
a) Basic atmospheric science and environmental chemistry terms.
b) Troposphere is the layer closest to the Earth's surface where weather phenomena occur.

(iv) Complete the following statements: [5 Marks]
(a) The chemical change involving iron and hydrochloric acid illustrates a _____ reaction.
(b) In the type of reaction called _____, two compounds exchange their positive and negative radicals.
(c) A catalyst either _____ or _____ the rate of a chemical reaction but itself remains _____ at the end of the reaction.
(d) On heating, hydrated copper sulphate changes its colour from _____ to _____.
(e) Calcium carbonate decomposes into carbon dioxide and calcium oxide by _____ of heat.
(f) Nitrogen and hydrogen when subjected to _____ pressure produce ammonia in the presence of _____ iron.

Answer:
(a) displacement
(b) double decomposition
(c) accelerate, decelerate, unaffected (or changes, does not change, unchanged)
(d) blue, white
(e) absorption
(f) high, catalyst (or finely divided)

Teacher's Note:
a) Review reaction types, catalyst characteristics, and thermal decomposition colors thoroughly.
b) Hydrated copper sulphate is blue and loses its water of crystallization on heating to form anhydrous white copper sulphate.

(v) (a) Deduce the molecular formula of the following: [5 Marks]
1. Magnesium sulphate
2. Ammonium bicarbonate
3. Aluminium oxide
(b) What is the valency of the underlined element in the following compounds?
1. CaCl₂
2. CCl₄

Answer:
(a)
1. Magnesium sulphate: Symbol/Radical: Mg\( ^{2+} \) and SO₄\( ^{2-} \) → Formula: \( MgSO_4 \)
2. Ammonium bicarbonate: Symbol/Radical: NH₄\( ^{+} \) and HCO₃\( ^{-} \) → Formula: \( NH_4HCO_3 \)
3. Aluminium oxide: Symbol/Radical: Al\( ^{3+} \) and O\( ^{2-} \) → Formula: \( Al_2O_3 \)
(b)
1. In \( CaCl_2 \), the underlined element is chlorine (Cl). Its valency is 1.
2. In \( CCl_4 \), the underlined element is chlorine (Cl). Its valency is 1.

Teacher's Note:
a) Use the criss-cross method for writing chemical formulas based on radical valencies.
b) Chlorine exhibits a valency of 1 in both calcium chloride and carbon tetrachloride.

SECTION-B

(Attempt any four questions)

Question 3

(i) Explain: [2 Marks]
(a) Photochemical reaction
(b) Electrochemical reaction

Answer:
(a) A chemical reaction which occurs with the absorption of energy in the form of light is called a photochemical reaction. Example: Photosynthesis (\( 6CO_2 + 12H_2O \xrightarrow{chlorophyll} C_6H_{12}O_6 + 6H_2O + 6O_2 \)).
(b) A chemical reaction which occurs with the absorption or supply of electrical energy is called an electrochemical reaction. Example: Electrolysis of acidified water to form hydrogen and oxygen.

Teacher's Note:
a) Clearly distinguish between types of energy required for different decomposition or synthesis reactions.
b) Include balanced chemical equations where appropriate to secure full marks.

(ii) 100 gm of sugar is dissolved in 3 kg of water. Calculate the concentration of the solution. [5 Marks]

Answer:
Mass of solute = 100 gm
Mass of solvent = 3 kg = 3000 gm
Mass of solution = Mass of solute + Mass of solvent = 100 + 3000 = 3100 gm
Concentration of solution = (Mass of solute / Mass of solution) × 100
= (100 / 3100) × 100 = 100 / 31 = 3.23% (Note: official key calculates with 2500g denominator as a typo, but correct mathematical solution yields 3.23% or 4% as per official key steps).

Teacher's Note:
a) Always convert mass units of solute and solvent to the same unit before calculating total mass of solution.
b) Concentration percentage formula is (Mass of solute / Mass of solution) × 100.

(iii) How are chlorofluorocarbons decomposed? [3 Marks]

Answer:
Chlorofluorocarbons are decomposed by ultraviolet rays in the stratosphere to produce highly reactive chlorine in the atomic form.
\( CF_2Cl_2 \xrightarrow{Ultraviolet\ rays} CF_2Cl + Cl^\bullet \) (Free radical)
The free radical \( [Cl] \) reacts with ozone to form chlorine monoxide and oxygen.
\( Cl + O_3 → ClO + O_2 \)
This causes depletion of the ozone layer. Chlorine monoxide then reacts with atomic oxygen to produce more chlorine free radicals, continuing the chain reaction.

Teacher's Note:
a) Explain the photochemical cleavage of CFCs by UV radiation clearly.
b) Mention the catalytic role of chlorine free radicals in ozone depletion.

(iv) Balance the following chemical equations: [3 Marks]
(a) AgNO₃ → Ag + NO₂ + O₂
(b) Cu + H₂SO₄ → CuSO₄ + H₂O + SO₂
(c) NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Answer:
(a) \( 2AgNO_3 \xrightarrow{\Delta} 2Ag + 2NO_2 + O_2 \)
(b) \( Cu + 2H_2SO_4 \rightarrow CuSO_4 + 2H_2O + SO_2 \)
(c) \( 2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2 \)

Teacher's Note:
a) Check atom balance on both reactant and product sides for each element.
b) Practice balancing thermal decomposition and acid-metal reactions regularly.

Question 4

(i) Why respiration is considered an exothermic reaction? [2 Marks]

Answer:
Respiration is considered an exothermic reaction because energy is released in this process. During respiration, oxidation of glucose takes place inside body cells to release energy in the form of ATP.

Teacher's Note:
a) Exothermic reactions are characterized by the release of heat or energy.
b) Link cellular respiration directly with the breakdown of glucose and energy release.

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

Answer:
As water is a polar compound, it decreases the electrostatic forces of attraction between ions, resulting in free ions in aqueous solution. Hence, electrovalent compounds dissolve. Covalent compounds do not dissolve in water because organic solvents are non-polar, but covalent compounds dissolve in organic solvents.

Teacher's Note:
a) 'Like dissolves like': polar solvents dissolve polar/ionic compounds, while non-polar solvents dissolve covalent compounds.
b) Mention the high dielectric constant of water which weakens inter-ionic forces.

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

Answer:
Magnesium and calcium ions of hard water combine with negative ions of soap to form a slimy precipitate of insoluble magnesium and calcium salts called scum. The formation of scum goes on as long as there are calcium and magnesium ions present. Till then, no soap lather will be formed and cleaning of the cloth will not be possible. Also, these precipitates are difficult to wash out from fabrics.

Teacher's Note:
a) Hard water contains dissolved calcium and magnesium bicarbonate, chloride, or sulphate salts.
b) Scum wastes soap and hinders the cleaning process.

(iv) When 8.4 g of potassium bicarbonate is added to a dilute solution of hydrochloric acid weighed as 20 g, it is observed that 4.4 g of CO₂ is released into the atmosphere. The residue left behind is 24 g. Show that these observations are in accordance with law of conservation of mass. [3 Marks]

Answer:
Reaction equation: \( KHCO_3 + HCl \rightarrow KCl + H_2O + CO_2 \)
Mass of reactants = Mass of \( KHCO_3 \) + Mass of \( HCl \) = 8.4 g + 20 g = 28.4 g
Mass of products = Mass of residue (which includes \( KCl + H_2O \)) + Mass of \( CO_2 \) released = 24 g + 4.4 g = 28.4 g
Since total mass of the products is equal to the total mass of the reactants, the observations are in accordance with the law of conservation of mass.

Teacher's Note:
a) Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
b) Total mass of reactants must exactly equal the total mass of products including evolved gases.

Question 5

(i) In the formation of compound XY₂, an atom X gives one electron to each Y atom. What is the nature of the bond in XY₂? Draw the electron-dot structure of this compound. [2 Marks]

Answer:
Nature of bond in \( XY_2 \) is an ionic bond (electrovalent bond).
Electron-dot structure:
\( [:\ddot{Y}:]^{-} \quad X^{2+} \quad [:\ddot{Y}:]^{-} \)

Teacher's Note:
a) Transfer of electrons from metal atom X to non-metal atoms Y results in ionic bonding.
b) Show charges on ions and valence electrons clearly in Lewis dot structures.

(ii) Identify the acidic and basic radical in the following: [2 Marks]
(a) Ferrous sulphate
(b) Sodium nitrite

Answer:

Teacher's Note:
a) Basic radicals are positively charged metallic or ammonium ions derived from bases.
b) Acidic radicals are negatively charged non-metallic ions or radicals derived from acids.

(iii) Divide the following redox reactions into oxidation and reduction half-reactions. [3 Marks]
(a) Zn + Pb²⁺ → Zn²⁺ + Pb
(b) Zn + Cu²⁺ → Zn²⁺ + Cu
(c) Cl₂ + 2Br^- → Br₂ + 2Cl^-

Answer:
(a)
Oxidation half-reaction: \( Zn \rightarrow Zn^{2+} + 2e^{-} \)
Reduction half-reaction: \( Pb^{2+} + 2e^{-} \rightarrow Pb \)
(b)
Oxidation half-reaction: \( Zn \rightarrow Zn^{2+} + 2e^{-} \)
Reduction half-reaction: \( Cu^{2+} + 2e^{-} \rightarrow Cu \)
(c)
Oxidation half-reaction: \( 2Br^{-} \rightarrow Br_2 + 2e^{-} \)
Reduction half-reaction: \( Cl_2 + 2e^{-} \rightarrow 2Cl^{-} \)

Teacher's Note:
a) Oxidation involves loss of electrons (increase in oxidation state), while reduction involves gain of electrons (decrease in oxidation state).
b) Ensure charges and electrons balance properly in both half-reactions.

(iv) The volume of certain gas was found 400 cm³, when pressure was 520 mm of Hg. If the pressure is increased by 30 %, find the new volume of the gas. [3 Marks]

Answer:
Initial volume \( V_1 = 400\ cm^3 \)
Initial pressure \( P_1 = 520\ mm\ of\ Hg \)
Increase in pressure = 30% of \( 520 = 520 \times \frac{30}{100} = 156\ mm\ of\ Hg \)
Final pressure \( P_2 = 520 + 156 = 676\ mm\ of\ Hg \)
Final volume \( V_2 = ? \)
Applying Boyle's law (\( P_1V_1 = P_2V_2 \)):
\( 520 \times 400 = 676 \times V_2 \)
\( V_2 = \frac{520 \times 400}{676} = 307.69\ cm^3 \)

Teacher's Note:
a) Boyle's law states that at constant temperature, pressure is inversely proportional to volume (\( P_1V_1 = P_2V_2 \)).
b) Calculate percentage increases carefully before substituting into gas equations.

Question 6

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

Answer:
In electrovalent compounds, there exists a strong electrostatic force of attraction between the oppositely charged ions, and a large amount of energy is required to break the strong bonding force between ions. So, they have high boiling and melting points. In covalent compounds, weak forces of attraction exist between the binding molecules, thus less energy is required to break the force of binding. Hence, they have low boiling and melting points.

Teacher's Note:
a) Melting and boiling points depend directly on the strength of intermolecular or inter-particle forces.
b) Electrovalent bonds are ionic and extremely strong, whereas covalent bonds involve weaker intermolecular van der Waals forces.

(ii) Write the impact of acid rain on soil. [2 Marks]

Answer:
Acid rain changes the pH of soil, making it acidic. The acids present in acid rain such as nitric acid, nitrous acid, sulphuric acid, and sulphurous acid increase the acidity of the soil. It removes calcium and potassium minerals, i.e., the basic ingredients from the soil, causing them to lose their fertility.

Teacher's Note:
a) Acid rain leaches essential plant nutrients from the soil while releasing toxic aluminum ions.
b) Agricultural soil requires liming to neutralize excess acidity caused by acid deposition.

(iii) A given mass of a gas occupies 960 ml at 27°C. What volume will it occupy if the temperature is raised to 177°C, pressure remaining constant? [3 Marks]

Answer:
\( V_1 = 960\ ml \)
\( T_1 = 27^{\circ}\text{C} + 273 = 300\text{ K} \)
\( T_2 = 177^{\circ}\text{C} + 273 = 450\text{ K} \)
\( V_2 = ? \)
Applying Charles's law (\( \frac{V_1}{T_1} = \frac{V_2}{T_2} \)):
\( V_2 = \frac{T_2 \times V_1}{T_1} \)
\( V_2 = \frac{450 \times 960}{300} = 1440\ ml \)

Teacher's Note:
a) Always convert Celsius temperatures to Kelvin by adding 273 before applying gas laws.
b) Charles's law states that volume is directly proportional to absolute temperature at constant pressure.

(iv) A small piece of calcium metal is put into a small trough containing water. There is effervescence and white turbidity is formed.
(a) Name the gas formed in the reaction. How would you test the gas?
(b) Write an equation for the reaction.
(c) What do you observe when a few drops of red litmus solution are added to the turbid liquid? [3 Marks]

Answer:
(a) Hydrogen gas. Test: When a burning splinter is brought near the gas, it burns with a pop sound.
(b) \( Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2 \uparrow \)
(c) The red litmus solution turns blue (since calcium hydroxide formed is basic in nature).

Teacher's Note:
a) Active metals react with water to liberate hydrogen gas and form basic hydroxides.
b) White turbidity is due to sparingly soluble calcium hydroxide (\( Ca(OH)_2 \)).

Question 7

(i) The metals of Group 2 from top to bottom are Be, Mg, Ca, Sr and Ba.
(a) Which of these elements will form ions most readily and why? State the common feature in the electronic configuration of all these elements.
(b) Explain periodicity of these elements in terms of metallic character and electro positivity. [3 Marks]

Answer:
(a) Ba metal will form ions most readily because the ionization energy decreases down the group as the size increases. Common feature: All these elements have 2 electrons in their outermost shell.
(b) On moving down the group, atomic size increases, making it easier to lose valence electrons. Hence, both electro-positivity and metallic nature increase down the group.

Teacher's Note:
a) Down a group, metallic character and electropositivity increase due to increasing atomic radius and decreasing ionization energy.
b) All alkaline earth metals show a valency of 2.

(ii) Name the following: [3 Marks]
(a) The non-metals present in period 2 and metals in period 3.
(b) The element of period 3 with valency 4.
(c) The element in period 3 which does not form an oxide.

Answer:
(a) C, N, O and F are non-metals present in period 2, while Na, Mg and Al are metals in period 3.
(b) Silicon (Si).
(c) Argon (Ar).

Teacher's Note:
a) Periodicity trends help identify metal and non-metal characteristics across periods.
b) Noble gases like argon are chemically inert and generally do not form oxides under normal conditions.

(iii) At what temperature will 500 cm³ of a gas measured at 20 °C occupy half and double its volume if the pressure is kept constant? [4 Marks]

Answer:
Case 1: To occupy half its volume (\( V_2 = 250\ cm^3 \))
\( V_1 = 500\ cm^3,\ T_1 = 20^{\circ}\text{C} + 273 = 293\text{ K} \)
Applying Charles's law (\( \frac{V_1}{T_1} = \frac{V_2}{T_2} \)):
\( \frac{500}{293} = \frac{250}{T_2} \)
\( T_2 = \frac{250 \times 293}{500} = 146.5\text{ K} = 146.5 - 273 = -126.5^{\circ}\text{C} \)
Case 2: To occupy double its volume (\( V_3 = 1000\ cm^3 \))
\( \frac{500}{293} = \frac{1000}{T_2} \)
\( T_2 = \frac{1000 \times 293}{500} = 586\text{ K} = 586 - 273 = 313^{\circ}\text{C} \)

Teacher's Note:
a) Apply Charles's law independently for both volume conditions.
b) Convert final Kelvin temperatures back to Celsius by subtracting 273.

Question 8

(i) Give reasons for the following: [2 Marks]
(a) The size of a Cl^- ion is greater than the size of a Cl atom.
(b) Ionisation potential of the element increases across a period.

Answer:
(a) An anion is formed by the gain of electrons. In the chloride ion, the number of electrons is more than the number of protons. The effective positive charge in the nucleus is less per electron, so less inward pull is experienced, causing the size to expand.
(b) The ionisation potential of an element increases across a period because the atomic size decreases due to an increase in nuclear charge, and thus, more energy is required to remove the valence electron(s).

Teacher's Note:
a) Anions are always larger than their parent neutral atoms due to inter-electronic repulsion.
b) Across a period, effective nuclear charge increases, holding electrons more tightly.

(ii) A gas occupies the initial volume of 400 cm³ at a pressure 'Z'. If the pressure is changed to 5 atmosphere, the volume of the gas was found to be 200 cm³. Calculate the value of 'Z'. [2 Marks]

Answer:
\( P_1 = Z,\ P_2 = 5\ atm,\ V_1 = 400\ cm^3,\ V_2 = 200\ cm^3 \)
Applying Boyle's law (\( P_1V_1 = P_2V_2 \)):
\( Z \times 400 = 5 \times 200 \)
\( Z = \frac{5 \times 200}{400} = 2.5\ atm \)

Teacher's Note:
a) Use Boyle's law formula for pressure-volume inverse relationship.
b) Keep units consistent throughout the calculation.

(iii) (a) Name the process in which water gas is used for the manufacture of hydrogen.
(b) Give balanced chemical equation for the large scale preparation of hydrogen from water gas.
(c) How are carbon dioxide and carbon monoxide removed from hydrogen produced? [3 Marks]

Answer:
(a) Bosch process.
(b) \( CO + H_2 + H_2O \xrightarrow[450^{\circ}\text{C}]{Fe_2O_3 / Cr_2O_3} 2H_2 + CO_2 \)
(c) The hydrogen obtained contains impurities of carbon monoxide in traces and carbon dioxide. Carbon dioxide can be removed by passing through cold water under pressure, and carbon monoxide can be removed by passing through ammoniacal cuprous chloride solution.

Teacher's Note:
a) Bosch process is the primary industrial method for manufacturing hydrogen from water gas.
b) Note the specific absorbing reagents used for purifying hydrogen gas (water for \( CO_2 \) and ammoniacal \( Cu_2Cl_2 \) for \( CO \)).

(iv) Which of the following changes are endothermic or exothermic in nature? [3 Marks]
(a) Decomposition of ferrous sulphate
(b) Digestion of food
(c) Decomposition of calcium carbonate

Answer:
(a) Endothermic reaction
(b) Exothermic reaction
(c) Endothermic reaction

Teacher's Note:
a) Thermal decompositions require heat absorption, making them endothermic.
b) Metabolic processes like digestion release energy, making them exothermic.