ICSE Class 9 Chemistry Sample Paper with Solutions Set 05

SECTION-A

(Attempt all questions from this Section)

Question 1

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

(i) In a chemical equation, evolution of gas is indicated by: [1 Mark]
(A) An arrow pointing upwards
(B) An arrow pointing downwards
(C) By writing (aq)
(D) A horizontal arrow

Answer: (A) An arrow pointing upwards

An upward arrow (\(\uparrow\)) right next to the gaseous product indicates that the gas escapes into the atmosphere during a chemical reaction.

Teacher's Note:
a) An upward arrow symbolises the evolution of a gas, whereas a downward arrow (\(\downarrow\)) indicates the precipitation of an insoluble solid.
b) Students must remember that physical states like (s), (l), (g) and (aq) represent solid, liquid, gas and aqueous solutions respectively.

(ii) An electrolytic decomposition reaction can be used: [1 Mark]
(A) To obtain metals from their ores.
(B) In photography.
(C) To produce salts.
(D) For photosynthesis in plants.

Answer: (A) To obtain metals from their ores.

Highly reactive metals (like sodium, magnesium, aluminium) are extracted from their molten ores by electrolytic reduction (electrolytic decomposition).

Teacher's Note:
a) Electrolytic decomposition (electrolysis) requires the passage of electric current through a molten or aqueous electrolyte.
b) Do not confuse this with thermal decomposition, which uses heat energy.

(iii) The homogeneous soilds, arranged symmetrically, directed at definite angles and having a definite geometrical shapes are called: [1 Mark]
(A) Cubes
(B) Crystals
(C) Amorphous solid
(D) Solid

Answer: (B) Crystals

Crystals are homogeneous solids possessing a regular repeating internal arrangement of constituent particles bounded by plane faces meeting at definite angles.

Teacher's Note:
a) The spelling in the question paper 'soilds' is a typographical error for solids.
b) Amorphous solids lack a regular geometric structure and sharp melting points, unlike true crystals.

(iv) How many elements were known when Mendeleev started his work on his periodic table? [1 Mark]
(A) 55
(B) 63
(C) 45
(D) 69

Answer: (B) 63

At the time Mendeleev proposed his periodic table, 63 elements were known to science.

Teacher's Note:
a) Mendeleev examined the relationship between the atomic masses of the 63 elements and their physical and chemical properties.
b) He left vacant gaps in his table for undiscovered elements and predicted their properties accurately.

(v) Assertion (A): All temperatures in the absolute (Kelvin) scale are in positive figures.
Reason (R): Celsius scale includes only positive values. [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.

The Kelvin scale starts at absolute zero (\(0\text{ K}\) or \(-273.15^{\circ}\text{C}\)), so all Kelvin temperatures are positive. However, the Celsius scale frequently includes negative values (temperatures below freezing point of water).

Teacher's Note:
a) Absolute zero is theoretically the lowest possible temperature where molecular motion ceases completely.
b) Reason (R) is clearly incorrect because the Celsius scale has negative temperatures (e.g., \(-5^{\circ}\text{C}\)).

(vi) The proton has a change equal in magnitude but opposite in sign to that of: [1 Mark]
(A) Neutron
(B) Electron
(C) Canal rays
(D) Deuteron

Answer: (B) Electron

A proton carries a unit positive charge (\(+1\)), whereas an electron carries an equal magnitude of unit negative charge (\(-1\)).

Teacher's Note:
a) The typo 'change' in the question refers to 'charge'.
b) Neutrons are neutral subatomic particles with no net electrical charge.

(vii) When water is added to quicklime, rise in temperature is observed. This is an example of: [1 Mark]
(A) Photochemical reaction
(B) Endothermic reaction
(C) Exothermic reaction
(D) Electrochemical reaction

Answer: (C) Exothermic reaction

The reaction between calcium oxide (quicklime) and water forms slaked lime and releases a large amount of heat energy, making it exothermic.

Teacher's Note:
a) Equation: \(\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{Heat}\).
b) Exothermic reactions release heat into the surroundings, resulting in a temperature rise.

(viii) Efflorescent salts are: [1 Mark]
(A) Salts that lose water of crystallisation when exposed to air
(B) Salts that gain water of crystallisation when exposed to air
(C) Salts that are easily soluble in water
(D) Both a and c

Answer: (A) Salts that lose water of crystallisation when exposed to air

Efflorescent substances lose their water of crystallisation partially or completely when exposed to dry air, turning into an amorphous powder (e.g., washing soda crystals).

Teacher's Note:
a) Common example: Hydrated sodium carbonate (\(\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}\)).
b) Deliquescent substances do the exact opposite by absorbing moisture from the atmosphere.

(ix) The ratio of solute and solvent in a concentrated solution is: [1 Mark]
(A) More solute less solvent
(B) Less solute more solvent
(C) Equal quantities of solute and solvent
(D) All the above

Answer: (A) More solute less solvent

A concentrated solution contains a relatively large amount of dissolved solute in a lesser relative quantity of solvent.

Teacher's Note:
a) Concentration refers to the relative proportion of solute and solvent.
b) A dilute solution contains a relatively small amount of solute in a large amount of solvent.

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

Answer: (B) Noble gases

Noble gases (inert gases) were not known and hence not included in Newland's Law of Octaves, as they were discovered much later.

Teacher's Note:
a) Newland arranged the then known elements in order of increasing atomic masses.
b) The discovery of noble gases disturbed Newland's periodic repetition pattern of every eighth element.

(xi) Assertion (A): Potassium and sodium are used for reaction with dil. Hydrochloric acid in laboratory preparation of hydrogen.
Reason (R): Potassium and sodium reacts violently with dil. \(\text{HCl}\) and dil. \(\text{H}_2\text{SO}_4\). [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 and sodium react so violently and dangerously with dilute acids that the reaction is explosive; hence they are never used in laboratory preparation of hydrogen. Reason (R) is correct, but Assertion (A) is false.

Teacher's Note:
a) Moderately reactive metals like zinc (\(\text{Zn}\)) are preferred for laboratory preparation of hydrogen gas.
b) Highly reactive alkali metals react explosively, making collection unsafe.

(xii) The metal oxide which is reduced by hydrogen is: [1 Mark]
(A) \(\text{Al}_2\text{O}_3$
(B) \(\text{CuO}\)
(C) \(\text{CaO}\)
(D) \(\text{Na}_2\text{O}\)

Answer: (B) \(\text{CuO}\)

Copper(II) oxide (\(\text{CuO}\)) is a metal oxide low in the reactivity series that can be easily reduced to copper metal by passing hydrogen gas over heated \(\text{CuO}\).

Teacher's Note:
a) Equation: \(\text{CuO} + \text{H}_2 \xrightarrow{\text{heat}} \text{Cu} + \text{H}_2\text{O}\).
b) Oxides of highly reactive metals (\(\text{Na}, \text{Ca}, \text{Al}\)) cannot be reduced by hydrogen.

(xiii) In absolute scale, temperature is expressed in terms of: [1 Mark]
(A) Fahrenheit
(B) Celsius
(C) Kelvin
(D) Atmosphere (atm)

Answer: (C) Kelvin

The absolute temperature scale is the Kelvin scale, where zero represents absolute zero.

Teacher's Note:
a) Temperature in Kelvin (\(\text{K}\)) = Temperature in Celsius (\(^{\circ}\text{C}\)) + \(273\).
b) It is termed 'absolute' because it starts from absolute zero where molecular thermal energy is minimal.

(xiv) The mathematical expression for Boyle's law is: [1 Mark]
(A) \(\text{PV} = \text{Constant}\)
(B) \(\text{P} \times \text{Constant} = \text{V}\)
(C) \(\text{V}/\text{P} = \text{Constant}\)
(D) \(\text{V} \times \text{Constant} = \text{P}\)

Answer: (A) \(\text{PV} = \text{Constant}\)

Boyle's Law states that at constant temperature, the product of pressure (\(\text{P}\)) and volume (\(\text{V}\)) of a given mass of dry gas remains constant (\(\text{P}_1\text{V}_1 = \text{P}_2\text{V}_2\)).

Teacher's Note:
a) Mathematically, \(\text{P} \propto 1/\text{V}\), which implies \(\text{PV} = k\) (constant).
b) This law shows the inverse relationship between pressure and volume of a gas.

(xv) Taj Mahal has recently shown discolouration and damage. The most probable reason for this is: [1 Mark]
(A) Acid rain
(B) Storm
(C) Old age of the monument
(D) Ultraviolet rays damage

Answer: (A) Acid rain

Sulphur dioxide and nitrogen oxides released from surrounding industries combine with rainwater to form acid rain, which corrodes and yellows the white marble (\(\text{CaCO}_3\)) of the Taj Mahal.

Teacher's Note:
a) Acid rain reacts with calcium carbonate in marble to form soluble calcium sulphate, washing it away ('marble cancer').
b) Controlling industrial emissions is crucial to preserving historical monuments from acid rain.

Question 2

(i) Elements of the periodic table with atomic numbers from 3 to 18 are shown in the table below. Some elements are shown by letters even though the letters are not the usual symbols of the elements. [5 Marks]

Which of these is
(a) A noble gas
(b) A halogen
(c) An alkali metal
(d) An element with valency 4
(e) An element with valency 2

Answer:
(a) H and P (Atomic numbers 10 and 18, Neon and Argon)
(b) G and O (Atomic numbers 9 and 17, Fluorine and Chlorine)
(c) A and I (Atomic numbers 3 and 11, Lithium and Sodium)
(d) D and L (Atomic numbers 6 and 14, Carbon and Silicon)
(e) B and J (Atomic numbers 4 and 12, Beryllium and Magnesium)

Teacher's Note:
a) Map the given atomic numbers to their respective positions in the Modern Periodic Table (Group 1 for alkali metals, Group 17 for halogens, Group 18 for noble gases).
b) Valency is determined by the number of valence electrons or combining capacity (e.g., Group 2 elements have valency 2).

(ii) Complete the following table which refers to the action of heat on three substances named in the first column. [5 Marks]

Answer:

Teacher's Note:
a) Thermal decomposition of metal carbonates and nitrates produces characteristic metal oxides, gases, and colour changes.
b) Students must memorise specific colour changes during thermal dissociation experiments.

(iii) Fill in the blanks: [5 Marks]
(a) Pollutants such as \(\text{NO}_2\), \(\text{SO}_2\) and \(\text{SO}_3\) 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) Acid rain is primarily caused by oxides of sulphur and nitrogen dissolving in atmospheric moisture.
b) Chlorofluorocarbons (CFCs) release free chlorine radicals under UV light, which catalytically destroy ozone molecules in the stratosphere.

(iv) In the given figure:
Name the shells denoted by A, B, and C. Which shell has least energy?
(a) Name X and state the charge on it.
(b) The above sketch is of ............ model of an atom. [5 Marks]

[Figure: Bohr model of an atom showing a central nucleus X surrounded by concentric circular shells labelled K (innermost), L (middle), and M (outermost), with shells also marked A, B, and C respectively from inside out, containing electrons.]

Answer:
Shell A = K shell (Innermost shell).
Shell B = L shell (Middle shell).
Shell C = M shell (Outermost shell).
The shell having the least energy is Shell A (K shell, closest to the nucleus).
(a) X is the nucleus; it is positively charged.
(b) The above sketch is of the Bohr model of an atom.

Teacher's Note:
a) Energy levels of electron shells increase with distance from the nucleus; hence K shell has the lowest energy.
b) The nucleus contains positively charged protons and neutral neutrons.

(v)
(a) Deduce the molecular formula of the following compounds:
1. Ammonium dichromate
2. Lead nitrate
3. Ferrous chloride
(b) What is the valency of the underlined element in the following compounds?
1. \(\text{Mg}_3\underline{\text{N}}_2$
2. \(\text{Mn}\underline{\text{O}}_2$ [5 Marks]

Answer:
(a)
1. Ammonium dichromate:
Radicals: \(\text{NH}_4^{+}\) and \(\text{Cr}_2\text{O}_7^{2-}\)
Criss-cross valencies: \((\text{NH}_4)_2\text{Cr}_2\text{O}_7$
2. Lead nitrate:
Radicals: \(\text{Pb}^{2+}\) and \(\text{NO}_3^{-}\)
Criss-cross valencies: \(\text{Pb(NO}_3)_2$
3. Ferrous chloride:
Radicals: \(\text{Fe}^{2+}\) and \(\text{Cl}^{-}\)
Criss-cross valencies: \(\text{FeCl}_2$
(b)
1. In \(\text{Mg}_3\text{N}_2\), valency of Nitrogen (\(\text{N}\)) is \(3\).
2. In \(\text{MnO}_2\), valency of Manganese (\(\text{Mn}\)) is \(4\).

Teacher's Note:
a) To find the molecular formula, write symbols and valencies of the constituent radicals, then criss-cross the numbers.
b) In binary compounds, the valency of an element can be determined by balancing it against the fixed valency of the other element (oxygen is divalent, magnesium is divalent).

SECTION-B

(Attempt any four questions)

Question 3

(i) Give reason:
(i) Why do clothes dry up faster in an open ground than in a closed room?
(ii) Why does a desert cooler cool better on a hot dry day? [2 Marks]

Answer:
(i) In an open ground, the high speed of wind and larger exposed surface area increase the rate of evaporation; hence, wet clothes dry up faster.
(ii) On a hot dry day, the temperature is high and the humidity of air is very low. This greatly increases the rate of evaporation of water inside the desert cooler, causing greater cooling.

Teacher's Note:
a) Evaporation causes cooling because particles absorb latent heat from their surroundings to change phase.
b) Low humidity and high wind speed accelerate the rate of evaporation.

(ii) Explain with an example the following characteristics of a chemical reaction.
(a) Evolution of gas
(b) Change of colour [2 Marks]

Answer:
(a) Evolution of gas: In some chemical reactions, a gas is evolved as one of the products. Example:
\(\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$
(b) Change of colour: Some chemical reactions are characterised by a distinct change in the colour of reactants. Example:
When an iron nail is dipped in copper sulphate solution, the blue colour changes to light green due to formation of ferrous sulphate:
\(\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}\)

Teacher's Note:
a) Characteristics of chemical reactions include evolution of gas, change of colour, change of state, and change in temperature.
b) Always include balanced chemical equations when asked to explain with an example.

(iii) Classify below salts as deliquescence, hygroscopic, and desiccants.
\(\text{CuSO}_4\), Glauber salt, \(\text{FeCl}_2\), \(\text{CuO}\), \(\text{MgCl}_2\), Washing soda [3 Marks]

Answer:
Efflorescent / Hydrated salts: Washing soda, Glauber's salt.
Deliquescent: \(\text{FeCl}_2\), \(\text{MgCl}_2$
Hygroscopic / Desiccants: \(\text{CuO}\), \(\text{CuSO}_4$ (anhydrous)

Teacher's Note:
a) Deliquescent salts absorb moisture from the atmosphere and dissolve in it to form a solution.
b) Hygroscopic substances absorb moisture without dissolving, and strong ones are used as drying agents (desiccants).

(iv) Discuss any three pollutants based on the following points:
(a) Origin
(b) Harmful effect [3 Marks]

Teacher's Note:
a) Air pollutants originate mostly from vehicular emissions, industrial combustion, and domestic sources.
b) Carbon monoxide is poisonous because it binds strongly to haemoglobin in preference to oxygen.

Question 4

(i) Explain closed physical contact with an example: [2 Marks]

Answer:
Closed physical contact (mixing): A chemical reaction occurs when two substances are mixed intimately in a solid state, often requiring grinding or friction.
Example: When solid iodine and solid phosphorus are brought into close contact and mixed, they react explosively. Similarly, sodium metal reacts violently when it comes into contact with water.

Teacher's Note:
a) Many chemical reactions require close physical contact because reactant particles must approach each other within interatomic distances for a reaction to occur.
b) Surface area and solid-state mixing greatly influence reaction rates.

(ii) Draw a neat and labelled diagram of Bohr's model of an atom. [2 Marks]

Answer:

[Figure: Diagram of Niels Bohr's Atomic Model showing a central positively charged nucleus containing protons and neutrons, surrounded by concentric circular orbits labelled K, L, M representing energy levels, with electrons shown revolving in these shells.]

Teacher's Note:
a) Ensure the central nucleus is labelled clearly as positively charged and the shells are labelled K, L, M.
b) Electrons must be shown distributed in specific shells according to the \(2n^2\) rule.

(iii) Rutuja went to see a paint factory along with her father who resided beside a nallah. She was in pain to see the dirty colour of water and enquired about it to her father. Her father explained to her that it was because of the pollutants flushed into the nallah. Can you write about different causes of water pollution? [3 Marks]

Answer:
Causes of Water Pollution:
1. Household detergents: Chemical cleaning agents and organic substances discharged into water bodies.
2. Industrial waste: Effluents from factories containing toxic chemicals, heavy metals, and acids.
3. Domestic sewage: Untreated wastewater from households containing organic matter and pathogens.
4. Offshore oil drilling: Spills and leaks from oil tankers and rigs polluting marine waters.
5. Agricultural wastes: Runoff containing chemical fertilisers, pesticides, and weedicides.
6. Thermal pollution: Discharge of hot water from industries, which lowers dissolved oxygen levels and harms aquatic life.

Teacher's Note:
a) Water pollution disrupts aquatic ecosystems and makes water unfit for human consumption.
b) Effluents must be treated in sewage treatment plants before being discharged into natural water bodies.

(iv) Write whether the given statement is True/False.
(a) The constant temperature at which a gas changes into a liquid by giving out heat energy, is called the freezing point of that liquid.
(b) Ice at \(0\text{ K}\) is more effective in cooling a substance than water at \(0\text{ K}\).
(c) A liquid flows easily because its particles are able to slide over one another. [3 Marks]

Answer:
(a) False. (It is called the liquefaction or condensation point, not the freezing point).
(b) False. (\(0\text{ K}\) is absolute zero; ice does not exist as liquid/solid water at \(0\text{ K}\) in normal contexts, and latent heat concepts apply to \(0^{\circ}\text{C}\)).
(c) True.

Teacher's Note:
a) Condensation is the process where a gas turns into a liquid by releasing latent heat.
b) Liquids can flow because intermolecular forces are weaker than in solids, allowing particles to slide past each other.

Question 5

(i) Draw an orbital diagram of bonding between two oxygen atoms. [2 Marks]

Answer:

[Figure: Orbital/Lewis dot diagram showing two oxygen atoms sharing two pairs of electrons between them, forming a double covalent bond (\(\text{O}=\text{O}\)), with each oxygen atom contributing two electrons to the shared pool.]

Teacher's Note:
a) An oxygen atom has 6 valence electrons (\(2, 6\)) and requires 2 more electrons to complete its octet.
b) Two oxygen atoms share two pairs of electrons to form a stable double covalent molecule (\(\text{O}_2\)).

(ii) Write an example with a balanced reaction of:
(a) Thermal decomposition
(b) Electrical decomposition [2 Marks]

Answer:
(a) Thermal decomposition: A decomposition reaction brought about by heat.
\(2\text{KClO}_3 \xrightarrow{\text{Heat}} 2\text{KCl} + 3\text{O}_2\uparrow$
(b) Electrical decomposition: On passing an electric current through acidulated water, water decomposes into hydrogen and oxygen.
\(2\text{H}_2\text{O}_{(l)} \xrightarrow{\text{Electric Current}} 2\text{H}_{2(g)} + \text{O}_{2(g)}\)

Teacher's Note:
a) Decomposition reactions involve breaking down a single reactant into two or more simpler products using energy.
b) Ensure all stoichiometry coefficients are balanced correctly.

(iii) Hydrogen may be prepared in the laboratory by the action of a metal on an acid. [3 Marks]
(a) Which of the metals copper, zinc, magnesium or sodium would be the most suitable?
(b) Which of the acids dilute sulphuric, concentrated sulphuric, dilute nitric acid and concentrated nitric acid would you choose? Explain why you would not use the acids you reject.
(c) How would you modify your apparatus to collect dry hydrogen? Which drying agent would you employ for this purpose?

Answer:
(a) Zinc (\(\text{Zn}\)) is the most preferred and suitable metal for laboratory preparation of hydrogen.
(b) Dilute sulphuric acid (\(\text{dil. H}_2\text{SO}_4\)) is chosen. Concentrated sulphuric acid is not used because it acts as a strong oxidising agent and produces sulphur dioxide (\(\text{SO}_2\)) instead of hydrogen. Nitric acid (dilute or concentrated) is not used because it is a powerful oxidising agent and oxidises hydrogen to water (\(\text{H}_2\text{O}\)).
(c) To collect dry hydrogen, the gas is passed through a drying tower containing a drying agent. The drying agents employed are fused calcium chloride (\(\text{CaCl}_2\)), caustic potash stick (\(\text{KOH}\)), or phosphorus pentoxide (\(\text{P}_2\text{O}_5\)).

Teacher's Note:
a) Zinc reacts at a moderate, controllable rate with dilute acids without being explosive.
b) Oxidising acids like \(\text{HNO}_3\) fail to yield hydrogen gas because they produce water upon reacting with metals.

(iv) Complete the following two reactions. Write similarity and difference regarding the type of reaction between below two reactions. [3 Marks]
(a) \(\text{NaOH} + \text{HCl} \rightarrow$
(b) \(\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow$

Answer:
(a) \(\text{NaOH}_{(aq)} + \text{HCl}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_2\text{O}_{(l)}\)
(b) \(\text{Na}_2\text{SO}_{4(aq)} + \text{BaCl}_{2(aq)} \rightarrow \text{BaSO}_{4(s)}\downarrow + 2\text{NaCl}_{(aq)}\)
Similarity: Both reactions are double displacement reactions where the reactants exchange radicals to form two new compounds.
Difference: Reaction (a) is a neutralisation reaction with no precipitate formed. Reaction (b) is a precipitation reaction where an insoluble white solid (\(\text{BaSO}_4\)) precipitates out.

Teacher's Note:
a) Double displacement reactions generally occur in aqueous solutions between ionic compounds.
b) Identifying precipitation versus neutralisation helps classify double displacement reactions accurately.

Question 6

i. State the valency and formula of the following ions: [2 Marks]
(a) Ammonium ion
(b) Cupric ion

Answer:
(a) Ammonium ion: Valency = \(+1\), Formula = \(\text{NH}_4^{+}\)
(b) Cupric ion: Valency = \(+2\), Formula = \(\text{Cu}^{2+}\)

Teacher's Note:
a) An ammonium ion is a polyatomic basic radical carrying a single positive charge.
b) Cupric denotes copper in its higher valency state (\(+2\)).

ii. Write two impacts of acid rain. [2 Marks]

Answer:
1. Changes the acidity of soil: Acid rain increases soil acidity, leaching essential nutrients like calcium and potassium, thus reducing soil fertility.
2. Affects water bodies and marine organisms: The water of lakes and rivers becomes acidic, making them toxic and unable to support aquatic life.

Teacher's Note:
a) Acid rain is corrosive to marble structures, metals, and limestone buildings.
b) Aquatic ecosystems collapse when pH levels drop below tolerance limits.

iii. Explain similarities of hydrogen each with group 1 and group 17 elements based on following points: [3 Marks]
(a) Electronic configuration
(b) Ion formation
(c) Valency

Teacher's Note:
a) Hydrogen occupies a unique position in the periodic table due to its dual resemblance with alkali metals and halogens.
b) Like alkali metals, it forms univalent positive ions, and like halogens, it needs one electron to complete its duplet.

iv. A gas is enclosed in a vessel at standard temperature. At what temperature will the volume of a gas enclosed be \(\frac{1}{6}\text{th}\) of its initial volume at constant pressure? [3 Marks]

Answer:
Let the initial volume of gas (\(\text{V}_1\)) = \(x$
Initial temperature of gas (\(\text{T}_1\)) = \(0^{\circ}\text{C} = 0 + 273 = 273\text{ K}$
Final volume (\(\text{V}_2\)) = \(\frac{x}{6}$
Final temperature (\(\text{T}_2\)) = ?
Using Charles's Law: \(\frac{\text{V}_1}{\text{T}_1} = \frac{\text{V}_2}{\text{T}_2}\)
\(\frac{x}{273} = \frac{\frac{x}{6}}{\text{T}_2}\)
\(\text{T}_2 = \frac{273 \times x}{6 \times x} = 45.5\text{ K}$
In Celsius: \(45.5 - 273 = -227.5^{\circ}\text{C}$

Teacher's Note:
a) Always convert Celsius temperatures to Kelvin (\(\text{K} = ^{\circ}\text{C} + 273\)) before applying gas laws.
b) Charles's Law states that volume is directly proportional to absolute temperature at constant pressure (\(\text{V} \propto \text{T}\)).

Question 7

i. Deduce the molecular formula of the following conversions:
(a) Potassium dichromate
(b) Lead chromate
(c) Calcium carbonate [3 Marks]

Answer:
(a) Potassium dichromate:
Radicals: \(\text{K}^{+}\) and \(\text{Cr}_2\text{O}_7^{2-}\)
Formula: \(\text{K}_2\text{Cr}_2\text{O}_7$
(b) Lead chromate:
Radicals: \(\text{Pb}^{2+}\) and \(\text{CrO}_4^{2-}\)
Formula: \(\text{PbCrO}_4$
(c) Calcium carbonate:
Radicals: \(\text{Ca}^{2+}\) and \(\text{CO}_3^{2-}\)
Formula: \(\text{CaCO}_3\)

Teacher's Note:
a) Cross-multiply the valencies of the respective cations and anions to get the simplest neutral formula.
b) Simplify common stoichiometric ratios where applicable.

ii. Define solubility. Explain factors affecting solubility? [3 Marks]

Answer:
Solubility of a solute in a particular solvent at a particular temperature is the maximum amount of solute in grams that can be dissolved in \(100\text{ grams}\) of a solvent to form a saturated solution at that temperature.
Factors Affecting Solubility:
1. Size of solute particles: The smaller the size of solute particles, the greater will be the solubility.
2. Stirring: It brings more of the solvent in contact with the solute, thus increasing the rate of dissolution.
3. Temperature: The solubility of a solid solute generally increases with an increase in temperature, whereas gas solubility decreases with temperature and increases with pressure.

Teacher's Note:
a) Solubility is always specified at a definite temperature because it changes with thermal conditions.
b) Mention both solid and gas solute behaviours for a complete explanation.

iii. Observe the following graph and answer the following questions: [4 Marks]
(a) Name and state the law expressed by the given graph.
(b) Derive the mathematical expression for it.
(c) Give one application of the above law.

[Figure: Line graph of Volume (\(\text{mL}\)) on Y-axis against Temperature (\(^{\circ}\text{C}\)) on X-axis, showing a straight line starting from \(0.5\) at \(0^{\circ}\text{C}\) and extending from \(-300^{\circ}\text{C}\) to \(300^{\circ}\text{C}\).]

Answer:
(a) Charles's Law. Statement: Pressure remaining constant, the volume of a given mass of a dry gas increases or decreases by \(\frac{1}{273}\) of its volume at \(0^{\circ}\text{C}\) for each \(1^{\circ}\text{C}\) increase or decrease in temperature respectively.
(b) Mathematical expression: Let \(\text{V}_0\) be the volume of a fixed mass of a gas at \(0^{\circ}\text{C}\), and \(\text{V}\) be its volume at temperature \(t^{\circ}\text{C}\) at constant pressure.
\(\text{V} = \text{V}_0 + \frac{\text{V}_0}{273}t = \text{V}_0\left(1 + \frac{t}{273}\right) = \text{V}_0\left(\frac{273 + t}{273}\right)\)
Since \(\frac{\text{V}_0}{273} = k\) (constant), \(\text{V} = k \times \text{T}\) or \(\frac{\text{V}}{\text{T}} = k$ (where \(\text{T} = 273 + t\)).
(c) Application: Hot air is less dense than cold air; hence hot air is filled into meteorological balloons for weather forecasting.

Teacher's Note:
a) Charles's Law establishes the direct proportionality between volume and absolute temperature of a gas.
b) The graph intercepts the temperature axis at absolute zero (\(-273^{\circ}\text{C}\)) where theoretical gas volume becomes zero.

Question 8

i. The following questions are related to the long form of the periodic table. [2 Marks]
(a) State the modern periodic law.
(b) In which group are halogens placed in the long form of the periodic table?

Answer:
(a) Modern Periodic Law: The physical and chemical properties of elements are the periodic function of their atomic numbers.
(b) Halogens are placed in Group 17 of the long form of the periodic table.

Teacher's Note:
a) Henry Moseley modified Mendeleev's periodic law by using atomic number instead of atomic mass as the fundamental property.
b) Halogens (Fluorine, Chlorine, Bromine, Iodine) are reactive non-metals located in Group 17.

ii.
(a) \(1\text{ atmosphere} = \underline{\quad\quad}\text{ cm. Hg} = \underline{\quad\quad}\text{ mm Hg}$
(b) A \(\underline{\quad\quad}\) in the pressure at a constant temperature increases the volume of a gas. [2 Marks]

Answer:
(a) \(1\text{ atmosphere} = 76\text{ cm. Hg} = 760\text{ mm Hg}$
(b) A decrease in the pressure at a constant temperature increases the volume of a gas.

Teacher's Note:
a) Standard atmospheric pressure is defined as the pressure exerted by a mercury column of \(76\text{ cm}\) (\(760\text{ mm}\)) height.
b) According to Boyle's law, pressure and volume are inversely proportional.

iii. Explain with balanced reaction the action of cold water on: [3 Marks]
(a) Potassium
(b) Sodium
(c) Calcium

Teacher's Note:
a) Alkali metals react vigorously with cold water, producing metal hydroxides and hydrogen gas.
b) Calcium reacts steadily, and the metal floats as hydrogen bubbles stick to its surface.

iv. What is the effect of chlorofluorocarbon on ozone layer? Explain in detail. [3 Marks]

Answer:
Excessive use of chlorofluorocarbons (\(\text{CFCs}\)) is one of the main reasons for ozone layer depletion. They are released by refrigerators and air conditioning systems.
The chlorofluorocarbons are decomposed by ultraviolet rays to highly reactive chlorine which is produced in the atomic form:
\(\text{CF}_2\text{Cl}_2 \xrightarrow{\text{Ultraviolet rays}} \text{CF}_2\text{Cl} + \text{Cl}\text{ (Free radical)}$
The free radical \([\text{Cl}]\) reacts with ozone to form chlorine monoxide:
\(\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2\text{ (Chlorine monoxide)}$
Chlorine monoxide then reacts with atomic oxygen to produce more chlorine free radicals:
\(\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2\text{ (Free radical)}$
This free radical destroys ozone, and the process continues depleting the ozone layer.

Teacher's Note:
a) A single chlorine free radical can catalytically destroy thousands of ozone molecules before it is deactivated.
b) Ozone layer depletion allows harmful UV-B rays to reach Earth, increasing the risk of skin cancer and cataracts.