9th Class Chemistry Chapter # 9 Exercise Solutions – Punjab Board
Group Properties and Elements
(MCQs) with Answers & Explanations
(i) Which element exists in the strongest force of attraction between its atoms?
Options:
(a) N₂
(b) Cl₂
(c) O₂
✅ (d) F₂ ✔️
Answer: ✅ (d) F₂
Explanation:
Among the given options, fluorine atoms (F₂) have the smallest atomic size and the highest electronegativity, resulting in stronger covalent bonding between atoms. The strong attractive forces make F₂ exhibit the strongest force among these diatomic molecules.
(Note: While N≡N triple bond is very strong, the question asks about force of attraction between atoms in a molecule, and fluorine’s small bond length and high electronegativity make this valid in this context. However, in deeper chemical terms, N₂ would have the strongest bond due to the triple bond. If the question refers strictly to bond energy, then (a) N₂ is a stronger answer.)
➡️ If paper wants “strongest bond” — tick (a) N₂
➡️ If paper wants “strongest attraction due to electronegativity/small size” — tick (d) F₂
Safer tick for board exams: ✅ (a) N₂ ✔️
(ii) Which compound you expect to be coloured?
Options:
(a) KCl
(b) BaCl₂
(c) BaCl
✅ (d) NiCl₂ ✔️
Answer: ✅ (d) NiCl₂
Explanation:
NiCl₂ (Nickel chloride) is a compound of a transition metal (Nickel), which has partially filled d-orbitals. These d-electrons can absorb certain wavelengths of visible light and reflect the rest, giving the compound color.
All other compounds (KCl, BaCl₂, BaCl) are from s-block elements and are colorless.
(iii) Elements of which group are coloured?
Options:
(a) Second group
(b) Sixth group
(c) Fifth group
✅ (d) Eighth group ✔️
Answer: ✅ (d) Eighth group
Explanation:
The eighth group contains transition metals like iron (Fe), cobalt (Co), nickel (Ni), etc. These elements have incomplete d-orbitals, which are responsible for colored compounds due to electronic transitions.
Other groups (2, 5, 6) contain s- or p-block elements, which typically form colorless compounds.
(iv) Which of the following is coloured?
Options:
(a) Mg
(b) Ca
✅ (c) S ✔️
(d) Be
Answer: ✅ (c) S (Sulphur)
Explanation:
Sulphur is a non-metal that exists as a yellow solid at room temperature — hence it is colored.
Other options — magnesium, calcium, and beryllium — are silvery-white metals, not naturally colored.
(v) Which halogen acid is unstable at room temperature?
Options:
(a) HBr
(b) HI
(c) HCl
✅ (d) HF ✔️
Answer: ✅ (d) HF
Explanation:
Hydrofluoric acid (HF) is unstable at room temperature because of strong hydrogen bonding between HF molecules. It also tends to form polymeric chains, making it behave unusually.
Also, it is highly reactive and stored in plastic containers rather than glass due to its corrosive nature.
(vi) Which oxide is the most basic oxide?
Options:
✅ (a) Na₂O ✔️
(b) Li₂O
(c) MgO
(d) CO
Answer: ✅ (a) Na₂O
Explanation:
Basic oxides are usually formed by metals, especially Group 1 metals (alkali metals).
Sodium oxide (Na₂O) is the most basic among these — it reacts strongly with water to form sodium hydroxide (NaOH).
CO is a neutral oxide, and MgO is less basic than Na₂O.
(vii) Which group elements are the most reactive elements?
Options:
(a) Transition metal group
✅ (b) First group ✔️
(c) Second group
(d) Third group
Answer: ✅ (b) First group (Alkali Metals)
Explanation:
Group 1 elements (Li, Na, K, Rb, Cs, Fr) are alkali metals and are the most reactive metals.
They easily lose one electron due to low ionization energy and react violently with water.
(viii) The following solutions of a halogen and sodium halide are mixed together. Which solution will turn dark because of reaction?
Options:
(a) Br₂ and NaCl
(b) Br₂ and NaF
(c) Cl₂ and NaF
✅ (d) Cl₂ and NaI ✔️
Answer: ✅ (d) Cl₂ and NaI
Explanation:
In halogen displacement reactions, a more reactive halogen displaces a less reactive halogen from its salt.
Reactivity order of halogens:
F₂ > Cl₂ > Br₂ > I₂
Cl₂ + NaI → NaCl + I₂ → Iodine is released, solution turns brown/dark.
No reaction in other options because Br₂ cannot displace Cl⁻ or F⁻, and Cl₂ cannot displace F⁻.
(ix) X is a monoatomic gas. Which statement about this is correct?
Options:
(a) X burns in air
(b) X is coloured
✅ (c) X is unreactive ✔️
(d) X will displace iodine from its element
Answer: ✅ (c) X is unreactive
Explanation:
Monoatomic gases are Group 18 noble gases (He, Ne, Ar, etc.) — they exist as individual atoms, not molecules.
They have complete outer electron shells, so they are chemically unreactive/inert.
(x) Which property is correct for Group 18 elements?
Options:
✅ (a) Low catalytic activity ✔️
(b) High density
(c) Low electrical conductivity
(d) High melting point
Answer: ✅ (a) Low catalytic activity
Explanation:
Group 18 elements (noble gases) are inert because their outermost shells are full.
Due to their stability, they:
Don’t easily react (low catalytic activity)
Are gases at room temperature (low density, low melting point)
Do not conduct electricity well
So, (a) is correct.
Short Answer Questions
i. Why does it become easier to cut an alkali metal when we move from top to bottom in a group?
Answer:
As we move down Group 1 (alkali metals), the atomic size increases and the metallic bonding becomes weaker due to increased distance between nuclei and valence electrons.
This makes the metal softer, so it becomes easier to cut (e.g., sodium is softer than lithium, potassium is softer than sodium).
ii. Predict the reactivity of potassium towards halogens.
Answer:
Potassium is a very reactive metal, and halogens are highly reactive non-metals.
When potassium reacts with a halogen (like Cl₂), it readily loses one electron to form K⁺, while the halogen gains an electron to form X⁻.
So, potassium shows vigorous reaction with halogens and forms ionic salts like KCl.
iii. In the following reaction, chlorine acts as an oxidising agent. Which is the reducing agent?
Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)
Answer:
Chlorine (Cl₂) gains electrons and gets reduced, so it is the oxidising agent.
Br⁻ ions (from NaBr) lose electrons and get oxidised to Br₂, so Br⁻ (bromide ion) is the reducing agent.
✅ Reducing agent: Br⁻ (from NaBr)
iv. Why does iodine exist in the solid state at room temperature?
Answer:
Iodine molecules (I₂) are larger and heavier compared to other halogens.
Due to their large size, they have stronger Van der Waals forces (intermolecular forces), which hold the molecules close together in a solid form at room temperature.
v. How does Ni catalyse the reaction involving hydrogenation of oil?
Answer:
In hydrogenation of oil, Nickel (Ni) acts as a solid catalyst.
It provides a surface where unsaturated oil molecules and hydrogen gas can come together.
Ni helps break the double bonds in unsaturated fatty acids and adds hydrogen atoms, converting oil into solid fat (e.g., margarine).
✅ Reaction becomes faster and more efficient due to the Ni catalyst.
Constructed Response Questions
1. Which noble gas should have the lowest boiling point and why?
Answer:
Among the noble gases, helium (He) has the lowest boiling point.
Explanation:
Noble gases are monoatomic and held together by weak Van der Waals (London dispersion) forces.
Helium has the smallest atomic size and lowest atomic mass among all noble gases.
Due to this, the intermolecular forces between helium atoms are extremely weak.
As a result, it requires very little energy to convert helium from liquid to gas.
Boiling Point of He ≈ –269°C, which is the lowest boiling point of any element.
2. Compare the reactions of alkali metals with chlorine.
Answer:
Alkali metals (Group 1: Li, Na, K, Rb, Cs) react vigorously with chlorine gas to form ionic salts.
General Reaction:
2M+Cl2→2MCl
(Where M = alkali metal like Na, K)
Explanation:
Alkali metals have one electron in their outermost shell, which they easily lose to form a +1 ion (M⁺).
Chlorine is a halogen with 7 valence electrons; it needs one electron to complete its octet.
Chlorine gains one electron to form Cl⁻.
The reaction produces white crystalline salts like NaCl, KCl.
Trend:
Reactivity increases down the group (Li < Na < K < Rb < Cs)
Because atomic size increases and ionization energy decreases, making electron loss easier.
3. Why are almost all the metals solids while non-metals generally exist as gases and solids?
Answer:
Metals:
Have strong metallic bonds formed by the attraction between positive metal ions and a ‘sea of delocalized electrons’.
These bonds are very strong and require high energy to break, so most metals are solids at room temperature.
Exception: Mercury (Hg) is a metal but exists as liquid due to weak bonding and small size.
Non-Metals:
Do not have metallic bonds.
Exist mostly as small molecules (e.g., O₂, N₂, Cl₂) with weak Van der Waals forces between them.
Hence, many non-metals are gases (like oxygen, nitrogen) or brittle solids (like sulphur, iodine).
Conclusion:
Metallic bonding gives metals their solid nature, while weak intermolecular forces cause non-metals to exist as gases or soft solids.
4. Name any three elements in the periodic table which exist as liquids.
Answer:
Three elements that are liquid at or near room temperature are:
Mercury (Hg) – A metal, liquid at room temperature (25°C).
➤ Used in thermometers, barometers.Bromine (Br₂) – A non-metal, exists as reddish-brown liquid.
➤ Has strong intermolecular forces compared to other halogens.Gallium (Ga) – A metal, melts at ~29.8°C, just above room temperature.
➤ Can melt in hand due to low melting point.
These are rare exceptions — most elements are either solid or gas at room conditions.
5. Why are transition elements different from normal elements?
Answer:
Transition elements (d-block elements) are unique and show properties different from normal s- and p-block elements.
Key Differences:
Variable Oxidation States:
Transition metals can exhibit more than one oxidation state (e.g., Fe²⁺, Fe³⁺) due to similar energy of 3d and 4s orbitals.Colored Compounds:
They form colored salts and solutions due to d-d electron transitions.
Example: NiCl₂ is green, CuSO₄ is blue.Catalytic Properties:
Transition metals and their compounds act as catalysts in many reactions.
Example: Fe in Haber Process, Ni in hydrogenation.Formation of Complexes:
Transition metals easily form complex ions due to availability of empty d-orbitals.Magnetic Properties:
Some show paramagnetism due to unpaired d-electrons.
These features are not commonly found in normal (main group) elements, which makes transition elements chemically versatile and industrially important.
6. Compare the reactivity of chlorine and bromine as an oxidising agent.
Answer:
Both chlorine (Cl₂) and bromine (Br₂) are halogens and act as oxidising agents (they gain electrons).
Comparison:
| Property | Chlorine | Bromine |
|---|---|---|
| Atomic Size | Smaller | Larger |
| Electronegativity | Higher | Lower |
| Oxidising Strength | Stronger | Weaker |
| Reactivity | Higher | Lower |
Reaction Example:
Cl2+2NaBr→2NaCl+Br2
Chlorine displaces bromine from bromide salt, showing it is a stronger oxidising agent.
Reason:
Chlorine gains electrons more easily due to smaller size and higher electron affinity.
7. Which element is the most reactive and which is the least reactive among halogens? Give two reasons to explain your answer.
Answer:
Most reactive halogen: Fluorine (F₂)
Least reactive halogen: Iodine (I₂)
Reasons:
Electronegativity:
Fluorine has the highest electronegativity of all elements, so it strongly attracts electrons.Atomic Size:
Fluorine has the smallest atomic radius, so the nucleus exerts greater pull on electrons.
Iodine has larger atoms with weaker attraction, making it less reactive.
Additional Info:
Fluorine can oxidize all other halide ions (Cl⁻, Br⁻, I⁻), but iodine cannot displace any halogen from its salt.
Descriptive Questions
1. Explain the role of catalytic converter in an automobile.
Answer:
A catalytic converter is a device installed in the exhaust system of automobiles to reduce harmful gases emitted from the engine.
Role:
It contains platinum (Pt), palladium (Pd), or rhodium (Rh) as catalysts.
It helps convert harmful gases into less toxic substances through redox reactions.
Main reactions in catalytic converter:
Carbon monoxide (CO) → converted to carbon dioxide (CO₂)
2CO+O2→2CO2Unburnt hydrocarbons → converted to CO₂ and H₂O
Nitrogen oxides (NOx) → converted to nitrogen gas (N₂)
Conclusion:
Catalytic converters help in reducing air pollution and protect the environment by controlling the emission of toxic gases.
2. Why do the chemical reactivities of alkali metals increase down the group whereas they decrease down the group in case of halogens?
Answer:
(a) Alkali Metals (Group 1):
Reactivity increases down the group (Li < Na < K < Rb < Cs).
Atomic size increases → outer electron is farther from nucleus.
Ionization energy decreases → easier to lose electron.
Therefore, more reactive as we go down.
(b) Halogens (Group 17):
Reactivity decreases down the group (F > Cl > Br > I).
Atomic size increases → attraction for incoming electron decreases.
Electron affinity decreases.
So, halogens become less reactive down the group.
✅ Conclusion:
Alkali metals react by losing electrons, and halogens react by gaining electrons. These opposite trends explain their respective reactivity patterns.
3. Why are metals generally tough and strong whereas non-metals are neither tough nor strong?
Answer:
Metals:
Have strong metallic bonding — attraction between positive metal ions and delocalized electrons.
Layers of atoms are closely packed, and they can slide without breaking.
This makes metals tough, strong, and ductile.
Non-metals:
Lack metallic bonding.
Exist as molecules or weak lattices.
Held by weak Van der Waals or covalent bonds.
Hence, they are generally brittle, non-ductile, and not strong.
✅ Result:
Metallic bonding gives strength and flexibility to metals, while non-metals lack such bonding.
4. Both alkali metals and halogens are very reactive elements with roles opposite to each other. Explain.
Answer:
Alkali Metals (Group 1):
Have 1 valence electron.
Lose 1 electron easily to form +1 cations (M⁺).
Highly electropositive and reactive metals.
Halogens (Group 17):
Have 7 valence electrons.
Gain 1 electron easily to form –1 anions (X⁻).
Highly electronegative and reactive non-metals.
Opposite Roles:
Alkali metals are electron donors (reducing agents).
Halogens are electron acceptors (oxidizing agents).
They readily form ionic salts, e.g., Na + Cl → NaCl.
✅ Their reactivity complements each other, leading to stable compounds.
5. Why hydrogen bromide is thermally unstable as compared to hydrogen chloride?
Answer:
Hydrogen bromide (HBr) is less thermally stable than hydrogen chloride (HCl).
Reasons:
Bond Strength:
H–Br bond is weaker than H–Cl bond.
Less energy is needed to break the bond in HBr, making it easier to decompose.
Atomic Size:
Bromine atom is larger than chlorine.
Larger atomic radius results in longer bond, which is weaker and less stable.
✅ Therefore, HBr decomposes more easily upon heating than HCl.
6. Compare the properties of metals and non-metals.
| Property | Metals | Non-Metals |
|---|---|---|
| State | Mostly solids | Solids, gases (few liquids) |
| Appearance | Shiny (lustrous) | Dull |
| Malleability | Malleable | Brittle (if solid) |
| Ductility | Ductile | Non-ductile |
| Conductivity | Good conductor of heat & electricity | Poor conductor (except graphite) |
| Density | Generally high | Generally low |
| Reactivity | Lose electrons (form cations) | Gain/share electrons (form anions/covalent bonds) |
| Bonding | Metallic bonding | Covalent or Van der Waals forces |
| Examples | Iron, Copper, Sodium | Sulphur, Oxygen, Chlorine |
✅ This table helps in quick understanding and revision.
7. V₂O₅ catalyst is preferred over platinum in the oxidation of sulphur dioxide. Give reasons.
Answer:
Vanadium pentoxide (V₂O₅) is used as a catalyst in the Contact Process for producing sulphur trioxide (SO₃) from sulphur dioxide (SO₂).
2SO2+O2→V2O52SO3
Reasons why V₂O₅ is preferred over platinum:
Cost-effective:
V₂O₅ is much cheaper than platinum, which is an expensive noble metal.Less Poisoning:
V₂O₅ is less affected by impurities like arsenic.
Platinum can be poisoned by impurities, reducing its efficiency.Effective at High Temperature:
V₂O₅ remains effective at industrial temperatures (~450°C), making it ideal for continuous production.
✅ Hence, V₂O₅ is widely used as a practical and efficient catalyst in SO₂ oxidation.
Investigative Questions
1. Explain the role of sodium as heat transfer agent in the atomic nuclear power plant. Which property of sodium is utilized in this role?
Answer:
In nuclear power plants, especially in fast breeder reactors, molten sodium is used as a heat transfer agent (coolant).
Role of Sodium:
When nuclear fission takes place, a large amount of heat is produced.
Molten sodium absorbs this heat from the reactor core and transfers it to water in a heat exchanger to produce steam.
The steam drives turbines to generate electricity.
Why Sodium is used:
High Thermal Conductivity:
Sodium conducts heat very efficiently, making it ideal for rapid heat transfer.Low Melting Point (98°C):
It melts easily and can be pumped like a liquid.High Boiling Point (~883°C):
It remains liquid even at high reactor temperatures, preventing vaporization.Chemically Stable (in inert environment):
Though reactive with water and air, in controlled reactor environment, it’s safe.
✅ Conclusion:
Sodium’s excellent thermal conductivity and wide liquid temperature range make it a reliable heat transfer agent in nuclear reactors.
2. Why and how does lithium behave differently from the rest of the alkali metals?
Answer:
Although lithium (Li) is an alkali metal (Group 1), it shows different chemical behavior compared to other members like Na, K, Rb, and Cs.
Reasons for Different Behavior:
Small Atomic and Ionic Size:
Lithium has the smallest size in the group, leading to higher charge density and stronger bonds.High Polarizing Power:
Li⁺ ion has strong polarizing ability → more covalent character in its compounds (e.g., LiI, Li₂CO₃), unlike other alkali metals that form more ionic compounds.Formation of Oxide Only:
Lithium forms only lithium oxide (Li₂O) when burned in air, while sodium and others form peroxides or superoxides.Solubility & Hydrolysis:
Lithium salts like LiF, LiOH are less soluble in water and often undergo hydrolysis, unlike the highly soluble salts of other alkali metals.Diagonal Relationship with Magnesium:
Lithium resembles magnesium (Group 2) in properties (similar ionic size, reactions, etc.), causing anomalous behavior.
✅ Conclusion:
Lithium’s small size, high charge density, and unique bonding characteristics make its chemical behavior different from other alkali metals.
3. Why aluminum metal is used in the manufacture of cooking utensils whereas magnesium is not considered useful for this purpose?
Answer:
✅ Aluminum (Al) is preferred for cooking utensils due to:
Good Thermal Conductivity:
It quickly conducts heat, allowing uniform cooking.Corrosion Resistance:
A protective oxide layer (Al₂O₃) forms on the surface, preventing rust or further corrosion.Light Weight and Malleability:
Easy to shape and light to handle.Non-toxic and Economical:
Safe for food and cheaper than many metals.
❌ Magnesium (Mg) is not suitable because:
Highly Reactive:
Reacts with acids in food (e.g., tomatoes), forming magnesium salts which can be harmful or spoil taste.Easily Oxidized:
Forms oxide layer that is not as stable as aluminum’s and can degrade over time.Low Melting Point (650°C):
Not suitable for high-heat cooking as it may warp or degrade under continuous use.Mechanical Weakness:
Magnesium is less strong and less durable, making it unsuitable for daily cookware.
✅ Conclusion:
Aluminum is used because it is durable, safe, corrosion-resistant, and conducts heat efficiently, while magnesium lacks these qualities.