ICSE Class 9 Chemistry Sample Paper with Solutions Set 02

SECTION-A

(Attempt all questions from this Section)

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

(i) Valency of magnesium atom is: [1 Mark]
(A) 3
(B) 4
(C) 2
(D) 5

Answer: (C) 2

Magnesium has atomic number 12, with an electronic configuration of \( 2, 8, 2 \). It loses 2 electrons to attain a stable octet structure.

Teacher's Note:
a) Valency represents the combining capacity of an atom, determined by the number of valence electrons or electrons needed to complete an octet.
b) Students often confuse atomic number with valency; ensure you write the electronic configuration first before determining valency.

(ii) The reaction between iron filings and sulphur powder is an example of: [1 Mark]
(A) Combination reaction
(B) Decomposition reaction
(C) Redox reaction
(D) Endothermic reaction

Answer: (A) Combination reaction

Iron and sulphur combine chemically on heating to form iron(II) sulphide (\( \text{FeS} \)), which is a combination reaction.

Teacher's Note:
a) A combination reaction occurs when two or more reactants combine to form a single product.
b) Do not confuse this with physical mixtures; heating causes a true chemical reaction resulting in a new compound.

(iii) Methylated spirit is used as a solvent to dissolve: [1 Mark]
(A) Phosphorus
(B) Chlorophyll
(C) Sulphur
(D) All the above

Answer: (B) Chlorophyll

Methylated spirit (denatured alcohol) is commonly used in laboratory settings to extract and dissolve chlorophyll from green leaves.

Teacher's Note:
a) Different solutes dissolve in different specific solvents based on the principle of like dissolves like.
b) Remember that sulphur dissolves in carbon disulphide, whereas methylated spirit is specifically used for organic pigments like chlorophyll.

(iv) Which is the nearest noble gas of oxygen atom? [1 Mark]
(A) Argon
(B) Xenon
(C) Helium
(D) Neon

Answer: (D) Neon

Oxygen has an atomic number of 8 with electronic configuration \( 2, 6 \). It gains 2 electrons to attain the nearest noble gas configuration of Neon (\( 2, 8 \)).

Teacher's Note:
a) Noble gases possess a completely filled outermost shell (duet or octet).
b) Count the atomic numbers carefully to identify the noble gas with the closest atomic number matching the achieved electron configuration.

(v) Assertion (A): Helium is highly reactive towards atmospheric oxygen.
Reason (R): Group 18 or zero group elements are called noble gases. [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.

Helium is an inert, noble gas with a stable duplet configuration and is extremely unreactive, making Assertion (A) false while Reason (R) is true.

Teacher's Note:
a) Noble gases are chemically inert because their valence shells are completely filled.
b) Always evaluate the truth value of the assertion independently before checking if the reason correctly explains it.

(vi) The atomic number of an element is 20. In modern periodic table, this element is placed in: [1 Mark]
(A) 2^nd period
(B) 4^th period
(C) 1^st period
(D) 3^rd period

Answer: (B) 4^th period

Calcium has atomic number 20 with electronic configuration \( 2, 8, 8, 2 \). Since it has 4 occupied shells, it belongs to the 4^th period.

Teacher's Note:
a) The number of shells in an atom determines its period number in the periodic table.
b) Write out the full shell configuration \( 2, 8, 8, 2 \) to avoid counting shells incorrectly.

(vii) Which of the following metals is used in the laboratory preparation of hydrogen gas? [1 Mark]
(A) Lead
(B) Sodium
(C) Aluminium
(D) Zinc

Answer: (D) Zinc

Granulated zinc reacts safely and steadily with dilute hydrochloric or sulphuric acid in the laboratory preparation of hydrogen gas.

Teacher's Note:
a) Highly reactive metals like sodium react violently with acids, while unreactive metals like lead do not react at all.
b) Zinc is the preferred choice because its reaction rate is moderate and controllable.

(viii) Impurities can be removed from hydrogen by passing it through: [1 Mark]
(A) Lead chloride solution
(B) Lead iodide solution
(C) Silver nitrate solution
(D) Silver chloride solution

Answer: (C) Silver nitrate solution

Passing hydrogen gas through silver nitrate solution removes gaseous impurities such as phosphine and arsine.

Teacher's Note:
a) Impurities arise from traces of metal phosphides and arsenides present in the metal used.
b) Memorize the specific purifying reagents used in gas preparation assemblies, as questions frequently test them.

(ix) Which one of the following pollutants is suspended solids of smoke, dust, and vapour? [1 Mark]
(A) Ozone
(B) Lead
(C) Chlorofluorocarbon
(D) Suspended particulate matter

Answer: (D) Suspended particulate matter

Suspended particulate matter (SPM) consists of microscopic solid or liquid matter suspended in the air, such as dust, smoke, and vapors.

Teacher's Note:
a) SPM poses severe respiratory hazards when inhaled.
b) Distinguish clearly between gaseous pollutants and particulate matter.

(x) On moving down the group the atomic radii: [1 Mark]
(A) Remains constant
(B) Increases
(C) Decreases
(D) First increases then decreases

Answer: (B) increases

As we move down a group, a new principal energy shell is added successively, increasing the distance between the nucleus and the outermost electrons.

Teacher's Note:
a) Atomic size depends on the number of shells and nuclear charge.
b) Do not confuse periodic trends down a group with trends across a period.

(xi) Assertion (A): Percentage of nitrogen in urea is 46.67%.
Reason (R): Percentage composition is the relative proportion of each element present in a compound, expressed as a percentage of the compound's total mass. [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.

Molar mass of urea (\( \text{CO(NH}_2)_2 \)) is \( 60\text{ g/mol} \). Mass of two nitrogen atoms is \( 28\text{ g} \). Percentage of nitrogen = \( (28 / 60) \times 100 = 46.67\% \).

Teacher's Note:
a) Percentage composition is calculated using the formula: (Total mass of the element / Molecular mass of the compound) × 100.
b) Verify calculations carefully during the exam to ensure absolute accuracy.

(xii) Which of the following is a deliquescent salt? [1 Mark]
(A) \( \text{MgCl}_2 \)
(B) \( \text{Na}_2\text{CO}_3 \)
(C) \( \text{MgSO}_4 \)
(D) \( \text{Na}_2\text{SO}_4 \)

Answer: (A) \( \text{MgCl}_2 \)

Magnesium chloride (\( \text{MgCl}_2 \)) absorbs moisture from the atmosphere to form a saturated solution and is a deliquescent substance.

Teacher's Note:
a) Deliquescent substances absorb enough moisture to dissolve completely in the absorbed water.
b) Distinguish clearly between deliquescent, hygroscopic, and efflorescent substances.

(xiii) The graph of variation in volume (V) plotted against pressure (P) at a constant temperature is: [1 Mark]
(A) Hyperbolic
(B) Straight line passing through origin
(C) Straight line parallel to the X-axis
(D) Parabolic

Answer: (A) Hyperbolic

According to Boyle's Law, volume is inversely proportional to pressure at constant temperature, resulting in a rectangular hyperbola curve.

Teacher's Note:
a) Inverse relationships in gas laws always plot as hyperbolas on V versus P graphs.
b) A straight line passing through the origin represents a direct proportionality, such as in Charles's Law (V vs T).

(xiv) The valency of Fe in FeO is: [1 Mark]
(A) +1
(B) +2
(C) +3
(D) +2 and +3

Answer: (B) +2

In iron(II) oxide (\( \text{FeO} \)), oxygen has a valency of -2, so iron exhibits a variable valency of +2.

Teacher's Note:
a) Iron exhibits variable valency, showing +2 in ferrous compounds and +3 in ferric compounds.
b) Always balance the total positive and negative valencies to zero when determining oxidation states or valencies in binary compounds.

(xv) Richa heated a brown substance in the air which turned black forming substance B. What would be substances A and B? [1 Mark]
(A) A = Fe, B = FeO
(B) A = Ca, B = CaO
(C) A = Cu, B = CuO
(D) A = Mn, B = MnO

Answer: (C) A = Cu, B = CuO

Reddish-brown copper metal (A) when heated in air forms black copper(II) oxide (B).

Teacher's Note:
a) This is a classic oxidation reaction where a metal reacts with oxygen upon heating.
b) Note the characteristic color changes: brown copper turns into a black coating of copper oxide.

Question 2

(i) Match the following: [5 Marks]

Answer:
(a) Element short by 1 electron in octet - (v) Halogens
(b) Highly reactive metals - (iii) Alkali metals
(c) Non-reactive elements - (ii) Noble gases
(d) Elements of groups 3 to 12 - (i) Transition elements
(e) Radioactive elements - (vi) Actinides
(f) Elements with 2 electrons in the outermost orbit - (iv) Alkaline earth metals

Teacher's Note:
a) Match the chemical properties and periodic group characteristics accurately.
b) Ensure all sub-parts are correctly cross-referenced before finalizing.

(ii) (a) Deduce the molecular formula of the following salts:
A. Potassium bisulphide
B. Sodium nitrite
C. Calcium carbonate [3 Marks]
(b) What is the valency of the underlined element in the following compounds?
A. \(\text{Na}\underline{\text{Al}}\text{O}_2\)
B. \(\text{Fe}(\text{NO}_3)_2\) [2 Marks]

Answer:
(a)
A. Potassium bisulphide: Potassium (\( \text{K}^+ \)), Bisulphide (\( \text{HS}^- \)) → \( \text{KHS} \)
B. Sodium nitrite: Sodium (\( \text{Na}^+ \)), Nitrite (\( \text{NO}_2^- \)) → \( \text{NaNO}_2 \)
C. Calcium carbonate: Calcium (\( \text{Ca}^{2+} \)), Carbonate (\( \text{CO}_3^{2-} \)) → \( \text{CaCO}_3 \)
(b)
A. In \( \text{NaAlO}_2 \), Aluminium has a valency of \( +3 \).
B. In \( \text{Fe(NO}_3)_2 \), Iron has a valency of \( +2 \).

Teacher's Note:
a) Use the criss-cross method of valencies to derive correct chemical formulas.
b) Clearly determine radical charges before writing valencies for complex ions.

(iii) Fill in the blanks: [5 Marks]
(a) Sulphur trioxide reacts with water vapour to form __________.
(b) Ozone is found in the upper layer of the atmosphere called __________.
(c) The full form of CFC is __________.
(d) Ozone absorbs the harmful __________ rays coming from the Sun.
(e) Excess accumulation of greenhouse gases is causing further warming of the Earth which results in __________

Answer:
(a) Sulphuric acid
(b) Stratosphere
(c) Chlorofluorocarbons
(d) Ultraviolet
(e) Global warming

Teacher's Note:
a) Fill in the blanks with precise scientific terminology.
b) Check spelling for terms like chlorofluorocarbons and stratosphere.

(iv) The description of atomic particles of two elements X and Y is given below: [5 Marks]

(a) What is the atomic number of Y?
(b) What is the mass number of X?
(c) What is the relation between X and Y?
(d) Which element/elements do they represent?
(e) Write the electronic configuration of X?

Answer:
(a) Atomic number of Y = number of protons = 8.
(b) Mass number of X = protons + neutrons = 8 + 8 = 16.
(c) X and Y are isotopes because they have the same atomic number but different mass numbers.
(d) X and Y represent oxygen because oxygen has an atomic number of 8.
(e) Electronic configuration of X = 2, 6.

Teacher's Note:
a) Atomic number equals the number of protons, and mass number is the sum of protons and neutrons.
b) Isotopes share identical chemical properties due to the same number of electrons.

(v) State the type of reaction: [5 Marks]
(a) \(\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + \text{Cl}_2 + 2\text{H}_2\text{O}\)
(b) \(\text{Zn(NO}_3)_2 + 2\text{NaOH} \rightarrow \text{Zn(OH)}_2 + 2\text{NaNO}_3$
(c) \((\text{NH}_4)_2\text{Cr}_2\text{O}_7 \rightarrow \text{N}_2 + 4\text{H}_2\text{O} + \text{Cr}_2\text{O}_3$
(d) \(2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}\)
(e) \(2\text{AgNO}_3 + \text{Cu} \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag}\)

Answer:
(a) Redox reaction
(b) Double displacement / Precipitation reaction
(c) Thermal decomposition reaction
(d) Combination / Synthesis reaction
(e) Displacement reaction

Teacher's Note:
a) Identify reaction types by observing reactant and product structural changes.
b) Pay close attention to electron transfers in redox and single displacement reactions.

SECTION-B

(Attempt any four questions)

Question 3

(i) Write a note on greenhouse effect. [2 Marks]

Answer:
1. The greenhouse effect is the natural process by which the Earth's atmosphere traps infrared radiations reflected from the Earth's surface using greenhouse gases like carbon dioxide, water vapour, and methane.
2. These gases act as a thermal blanket, keeping the planet warm enough to sustain life, though excessive accumulation leads to accelerated global warming.

Teacher's Note:
a) Explain both the natural benefits and environmental hazards of the greenhouse effect clearly.
b) Mention primary greenhouse gases such as carbon dioxide and water vapour.

(ii) What are displacement reactions? Explain with an example. [2 Marks]

Answer:
1. A displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound.
2. Example: \(\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}\), where zinc displaces copper from copper sulphate solution.

Teacher's Note:
a) Reactivity series is essential for predicting whether a displacement reaction will occur.
b) Include both the general definition and a balanced chemical equation.

(iii) Define the following:
(a) Solution
(b) Crystallisation
(c) Hard water [3 Marks]

Answer:
(a) Solution: A homogeneous mixture of two or more chemically non-reacting substances whose composition can be varied within certain limits.
(b) Crystallisation: The process by which crystals of a substance separate out on cooling its hot saturated solution.
(c) Hard water: Water that does not readily form lather with soap.

Teacher's Note:
a) Definitions must include key scientific keywords such as homogeneous, saturated, and lather.
b) Avoid vague descriptions; stick to precise standard textbook definitions.

(iv) State whether the given statements are true or false.
(a) In the long form of the periodic table, the elements are arranged in the ascending order of atomic mass number.
(b) The number of shells is equal to the group number from which the element belongs.
(c) Alkali metals are present in Group 1 of the periodic table. [3 Marks]

Answer:
(a) False. (Correct statement: Elements are arranged in ascending order of atomic number.)
(b) False. (Correct statement: The number of shells is equal to the period number.)
(c) True.

Teacher's Note:
a) Read statements carefully to identify subtle periodic table rule violations.
b) Providing corrected statements for false assertions earns full credit even if not explicitly asked.

Question 4

(i) Explain below two characteristics of chemical reaction with example:
(a) Formation of precipitate
(b) Change of state [2 Marks]

Answer:
(a) Formation of precipitate: Some chemical reactions form an insoluble solid substance called a precipitate.
(b) Change of state: Some reactions involve a transition of physical states, such as \(\text{NH}_3(\text{g}) + \text{HCl}(\text{g}) \rightleftharpoons \text{NH}_4\text{Cl}(\text{s})\).

Teacher's Note:
a) Characteristics of chemical reactions provide observable evidence that a reaction has occurred.
b) Always accompany explanations with a relevant chemical equation.

(ii) Write differences between Dalton's Atomic Theory and Modern Atomic Theory. [2 Marks]

Answer:

Teacher's Note:
a) Contrast atomic indivisibility with the discovery of sub-atomic particles.
b) Present differences in a clear comparative table format.

(iii) Explain why:
(a) Water is an excellent liquid to use in cooling systems.
(b) A solution is always clear and transparent.
(c) Lakes and rivers do not suddenly freeze in the winters. [3 Marks]

Answer:
(a) Water has a very high specific heat capacity, allowing it to absorb large quantities of heat without a significant rise in temperature.
(b) Solute particles in a solution are extremely small (less than 1 nm) and do not scatter light.
(c) Water has a high latent heat of fusion, requiring a large amount of heat to be removed before it turns into ice.

Teacher's Note:
a) Relate thermal properties of water to its hydrogen bonding and high specific heat.
b) Mention particle size constraints when explaining optical clarity in solutions.

(iv) What is meant by kinetic theory of gases? Write two postulates. [3 Marks]

Answer:
1. The kinetic theory of gases states that matter is made up of tiny particles in constant random motion.
2. Postulate 1: Matter is composed of small particles called atoms, molecules, or ions.
3. Postulate 2: Particles have inter-particle spaces between them, known as intermolecular spaces.

Teacher's Note:
a) Kinetic theory explains macroscopic gas properties through microscopic particle motion.
b) Clearly state at least two distinct postulates as requested.

Question 5

(i) Deduce the molecular formula of the following:
(a) Calcium nitrate
(b) Sodium chloride [2 Marks]

Answer:
(a) Calcium nitrate: \(\text{Ca}^{2+}\) and \(\text{NO}_3^-\) → \(\text{Ca(NO}_3)_2$
(b) Sodium chloride: \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) → \(\text{NaCl}\)

Teacher's Note:
a) Apply the criss-cross method correctly by balancing ionic charges.
b) Enclose polyatomic radicals like nitrate in parentheses when adding subscripts.

(ii) Give reason: In the manufacture of ammonia molybdenum is added. [2 Marks]

Answer:
1. Molybdenum acts as a promoter which enhances the catalytic efficiency of iron in Haber's process.
2. Equation: \(\text{N}_2 + 3\text{H}_2 \xrightarrow[\text{molybdenum}]{\text{Iron}} 2\text{NH}_3\)

Teacher's Note:
a) Distinguish between a catalyst and a promoter: promoters improve catalyst efficiency.
b) Mention both the catalyst (iron) and promoter (molybdenum) for complete marks.

(iii) How to prepare hydrogen using acid and metals. Write the reaction of acids with below metals.
(a) Mg
(b) Al
(c) Zn [3 Marks]

Answer:
1. Metals above hydrogen in the reactivity series react with dilute acids to displace hydrogen gas.
(a) Magnesium: \(\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$
(b) Aluminium: \(2\text{Al} + 3\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2\uparrow$
(c) Zinc: \(\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow\)

Teacher's Note:
a) Ensure all chemical equations are fully balanced.
b) Include upward arrows to indicate gas evolution.

(iv) Complete the following reaction with balanced equation. Write type of the reaction.
(a) \(\text{CuCO}_3 \xrightarrow{\Delta}\)
(b) \(\text{Zn(NO}_3)_2 \xrightarrow{\Delta}\)
(c) \(\text{AgOH} \xrightarrow{\Delta}\) [3 Marks]

Answer:
(a) \(\text{CuCO}_3 \xrightarrow{\Delta} \text{CuO} + \text{CO}_2\uparrow\) (Thermal decomposition of metal carbonate)
(b) \(2\text{Zn(NO}_3)_2 \xrightarrow{\Delta} 2\text{ZnO} + 4\text{NO}_2\uparrow + \text{O}_2\uparrow\) (Thermal decomposition of metal nitrate)
(c) \(2\text{AgOH} \xrightarrow{\Delta} \text{Ag}_2\text{O} + \text{H}_2\text{O}\) (Thermal decomposition)

Teacher's Note:
a) Thermal decomposition reactions require heat energy to break down compounds into simpler products.
b) Balance equations carefully, especially when multiple gaseous products are formed.

Question 6

(i) What are isobars? Explain with an example. [2 Marks]

Answer:
1. Isobars are atoms of different elements having the same mass number but different atomic numbers.
2. Example: \({}_{20}^{40}\text{Ca}\) and \({}_{18}^{40}\text{Ar}\) have different atomic numbers but the same mass number of 40.

Teacher's Note:
a) Contrast isobars with isotopes: isobars have different atomic numbers and elements, whereas isotopes have the same element.
b) Provide clear numerical mass and atomic number annotations.

(ii) Tanvi was feeling dull and cold. Her Father told her to eat something so that she would feel energetic and warm. Tanvi wondered how the digestion of food would make her feel warm from the inside. Could you explain? [2 Marks]

Answer:
1. Digestion breaks down complex food substances into simpler molecules like glucose through decomposition reactions.
2. Glucose undergoes biological oxidation in cells, releasing thermal energy that keeps the body warm and energetic.

Teacher's Note:
a) Connect digestion with cellular respiration and exothermic oxidation.
b) Frame answers clearly, addressing daily life applications of chemical concepts.

(iii) The volume occupied by a certain gas was found to be \(5.6\text{ dm}^3\) at \(2\text{ atmospheric}\) pressure. If the pressure is increased by \(20\%\), find the new volume of the gas. [3 Marks]

Answer:
Initial volume (\(V_1\)) = \(5.6\text{ dm}^3\)
Initial pressure (\(P_1\)) = \(2\text{ atm}\)
Increase in pressure = \(20\%\) of \(2 = 0.4\text{ atm}\)
New pressure (\(P_2\)) = \(2 + 0.4 = 2.4\text{ atm}\)
Using Boyle's Law: \(P_1 V_1 = P_2 V_2\)
\(2 \times 5.6 = 2.4 \times V_2\)
\(V_2 = \frac{2 \times 5.6}{2.4} = 4.67\text{ dm}^3\)

Teacher's Note:
a) Apply Boyle's Law formula \(P_1V_1 = P_2V_2\) for constant temperature processes.
b) Double-check percentage calculations for pressure increments.

(iv) Why is the position of Hydrogen in the periodic table anomalous? [3 Marks]

Answer:
1. Hydrogen has one valence electron, placing it in Group 1 with alkali metals, but it also resembles halogens by requiring one electron to complete its duplet.
2. Due to these dual properties sharing characteristics with both alkali metals and halogens, its exact position remains anomalous in the periodic table.

Teacher's Note:
a) Highlight similarities with both alkali metals and halogens.
b) Explain why hydrogen cannot be definitively classified into a single group.

Question 7

(i) Give the valency and the formulae of the following radicals:
(a) Nitride
(b) Bicarbonate
(c) Sulphate [3 Marks]

Answer:

Teacher's Note:
a) Memorize common radical formulas and charges accurately.
b) Present answers in a clear tabular format.

(ii) Explain the physical properties of water based on the below points.
(a) Latent heat of fusion of ice
(b) Latent heat of vapourisation of water [3 Marks]

Answer:
(a) Latent heat of fusion of ice: The amount of heat energy required by ice to change into water without temperature change is \(336\text{ J/g}\) (or \(80\text{ cal/g}\)).
(b) Latent heat of vapourisation of water: The energy required to change water into its vapour at its boiling point without temperature change is \(2260\text{ J/g}\) (or \(540\text{ cal/g}\)).

Teacher's Note:
a) Define latent heat clearly as heat absorbed or released during state changes without temperature variation.
b) Include standard numerical values in both joules and calories.

(iii) At \(17^\circ\text{C}\), a certain amount of a petrol occupies a capacity of \(0.4\text{ liter}\). What temperature should it be heated to so that its volume is (a) doubled, (b) reduced to half with the pressure remaining constant? [4 Marks]

Answer:
Initial volume (\(V_1\)) = \(0.4\text{ L}\)
Initial temperature (\(T_1\)) = \(17^\circ\text{C} + 273 = 290\text{ K}\)
(a) When volume is doubled (\(V_2 = 0.8\text{ L}\)):
Using Charles's Law: \(\frac{V_1}{T_1} = \frac{V_2}{T_2}\)
\(\frac{0.4}{290} = \frac{0.8}{T_2}\) → \(T_2 = 580\text{ K} = 307^\circ\text{C}\)
(b) When volume is reduced to half (\(V_2 = 0.2\text{ L}\)):
\(\frac{0.4}{290} = \frac{0.2}{T_2}\) → \(T_2 = 145\text{ K} = -128^\circ\text{C}\)

Teacher's Note:
a) Always convert temperatures from Celsius to Kelvin before applying gas laws.
b) Apply Charles's Law (\(V_1/T_1 = V_2/T_2\)) for constant pressure conditions.

Question 8

(i) (a) Name the incomplete period.
(b) What common feature is seen at the end of the 2^nd and 3^rd period? [2 Marks]

Answer:
(a) The seventh period is an incomplete period.
(b) Both periods end with an inert (noble) gas having a stable valence shell configuration.

Teacher's Note:
a) Note that the 7^th period is still being filled with newly discovered elements.
b) Noble gases conclude periods by establishing stable octets.

(ii) \(100\text{ cm}^3\) of a gas at \(27^\circ\text{C}\) is cooled to \(20^\circ\text{C}\) at constant pressure. Calculate the volume of gas at \(20^\circ\text{C}\). [2 Marks]

Answer:
\(V_1 = 100\text{ cm}^3\)
\(T_1 = 27 + 273 = 300\text{ K}\)
\(T_2 = 20 + 273 = 293\text{ K}\)
Using Charles's Law: \(\frac{V_1}{T_1} = \frac{V_2}{T_2}\)
\(\frac{100}{300} = \frac{V_2}{293}\)
\(V_2 = \frac{100 \times 293}{300} = 97.66\text{ cm}^3\)

Teacher's Note:
a) Always convert Celsius temperatures to Kelvin absolute temperatures.
b) Double-check cross-multiplication arithmetic.

(iii) Explain with balanced chemical reaction of reaction of hydrogen with:
(a) Chlorine
(b) Potassium
(c) \(\text{Fe}_2\text{O}_3\) [3 Marks]

Answer:
(a) Chlorine: Hydrogen reacts with chlorine in diffused sunlight to form hydrogen chloride. \(\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\)
(b) Potassium: Hydrogen reacts with heated potassium to form potassium hydride. \(2\text{K} + \text{H}_2 \rightarrow 2\text{KH}\)
(c) \(\text{Fe}_2\text{O}_3\): Hydrogen reduces ferric oxide to iron metal. \(\text{Fe}_2\text{O}_3 + 3\text{H}_2 \rightarrow 2\text{Fe} + 3\text{H}_2\text{O}\)

Teacher's Note:
a) Highlight hydrogen's dual nature acting as a non-metal with halogens and a reducing agent with metal oxides.
b) Verify that all reaction equations are properly balanced.

(iv) Write effects of global warming. [3 Marks]

Answer:
1. Melting of polar ice caps leads to a rise in sea levels, causing coastal flooding.
2. Changes in rainfall patterns disrupt agricultural crop zones globally.
3. Extreme weather events and ecosystem disruptions threaten biodiversity.

Teacher's Note:
a) List clear environmental consequences of global temperature increases.
b) Mention sea level rise and climate disruption as core impacts.