Q1. Ethane, with the molecular formula C₂H₆ has

(a) 6 covalent bonds
(b) 7 covalent bonds
(c) 8 covalent bonds
(d) 9 covalent bonds

Answer: (b) 7 covalent bonds

Explanation:

Structure of ethane (C₂H₆):

$$C{H}_3-C{H}_3$$

• C–C bond → 1 covalent bond

• Each carbon forms 3 C–H bonds, so 3 + 3 = 6 covalent bonds

Total covalent bonds = 1 + 6 = 7

---

Q2. Butanone is a four-carbon compound with the functional group

(a) carboxylic acid
(b) aldehyde
(c) ketone
(d) alcohol

Answer: (c) ketone

Explanation:

• Butanone (C₄H₈O) has the structure:

$$C{H}_3-CO-C{H}_2-C{H}_3$$

• Contains the CO (carbonyl) group in the middle, which is a ketone functional group.

---

Q3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

(a) the food is not cooked completely
(b) the fuel is not burning completely
(c) the fuel is wet
(d) the fuel is burning completely

Answer: (b) the fuel is not burning completely

Explanation:

Incomplete burning of fuel produces soot (carbon particles).
This soot deposits as a black layer on the vessel’s bottom.

Q4. Explain the nature of the covalent bond using the bond formation in CH₃Cl.

Answer:

CH₃Cl (chloromethane) is formed by covalent bonding, in which atoms share electrons to achieve stable electronic configuration.

Bond Formation Explanation:

• Carbon (C) has 4 valence electrons → needs 4 more to complete its octet.

• Hydrogen (H) has 1 valence electron → needs 1 more.

• Chlorine (Cl) has 7 valence electrons → needs 1 more.

Sharing of electrons:

• Carbon shares one electron each with three hydrogen atoms → forms three C–H single covalent bonds.

• Carbon shares one electron with chlorine → forms one C–Cl single covalent bond.

• All atoms achieve stable octet/duplet configuration.

Electron dot (Lewis) structure:

$$H:C:H$$

(with Cl attached forming shared pair)

Detailed representation:

$$\begin{array}{c}H\\ \mathrm{\mid }\\ H-C-Cl\\ \mathrm{\mid }\\ H\end{array}$$

Nature of bonds: All bonds (C–H and C–Cl) are covalent because they are formed by sharing of electrons, not by electron transfer.

---

Q5. Draw the electron dot structures for:

(a) Ethanoic acid (CH₃COOH)

(b) Hydrogen sulphide (H₂S)

S has 6 valence electrons and forms two single covalent bonds with two H atoms.

$$H:S:H$$

(two lone pairs left on S)

---

(c) Propanone (C₃H₆O)

Structure:

$$C{H}_3-CO-C{H}_3$$

Electron dot structure overview:

 

(O joins central carbon using double bond)

---

(d) Fluorine (F₂)

Each fluorine atom has 7 valence electrons and shares 1 pair forming a single covalent bond.

$$:F\text{—}F:$$

(Each F has 3 lone pairs outside the shared pair)

Q6. What is a homologous series? Explain with an example.

Answer:

A homologous series is a series of organic compounds having the same functional group and similar chemical properties, in which the molecular formula of each successive member differs by –CH₂– (14 u).

Example: Alkanes

All have the functional group — single covalent bonds (C–C, C–H), and differ by CH₂.

---

Q7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Answer:

$$C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2\uparrow$$

---

Q8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?

Answer:

Soap molecules have two ends:

• Hydrophobic tail (repels water, attracts dirt/oil)

• Hydrophilic head (attracts water)

When soap is added to water, hydrophobic ends surround the dirt and hydrophilic ends remain in water, forming micelles.

Micelle diagram explanation:

Dirt is trapped inside a spherical micelle.

Micelles in ethanol?

No micelles will not form in ethanol because ethanol is not polar enough.
Micelle formation requires polar water molecules to keep hydrophilic ends outward.

---

Q9. Why are carbon and its compounds used as fuels for most applications?

Answer:

Carbon and its compounds are used as fuels because:

• They have high calorific value (release large amount of heat on burning)

• Their combustion is controlled and clean

• They produce fewer pollutants when burnt completely

Example:

$$C{H}_4+2O_2\to C{O}_2+2H_{2}O+\text{heat}$$

---

Q10. Explain the formation of scum when hard water is treated with soap.

Answer:

Hard water contains calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
Soap reacts with these ions to form insoluble salts called scum.

$$2C_{17}{H}_{35}COONa+C{a}^{2+}\to (C_{17}{H}_{35}COO{)}_{2}Ca+2N{a}^{+}$$

Scum wastes soap and prevents lather formation.

---

Q11. What change will you observe if you test soap with litmus paper (red and blue)?

Answer:

Soap solution is basic in nature.

---

Q12. What is hydrogenation? What is its industrial application?

Answer:

Hydrogenation is a process in which hydrogen is added to unsaturated compounds in the presence of a palladium, nickel or platinum catalyst.

Example:

$$C_2{H}_4+H_2\stackrel{Ni}{\to }{C}_2{H}_6$$

Industrial application:

Used for converting vegetable oils into solid fats like margarine / vanaspati ghee.

---

Q13. Which of the following hydrocarbons undergo addition reactions:

C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄?

Answer:

Unsaturated hydrocarbons undergo addition reaction.
Unsaturated compounds contain double or triple bonds.

Correct answer:

$$C_3{H}_6\text{(propene)},C_2{H}_2\text{(ethyne)}$$

---

Q14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Answer:

Bromine water test

Experiment:

Add bromine water (orange) to the sample.

---

Q15. Explain the mechanism of cleaning action of soaps.

Answer:

Soap molecules contain:

• Hydrophobic tail (binds with grease and dirt)

• Hydrophilic head (binds with water)

Steps:

1. Hydrophobic tails attach to oil/dirt particles.

2. Hydrophilic heads stay in water.

3. Micelles are formed trapping dirt inside.

4. Agitation lifts micelles away and washes them off.

Diagram:

(Dirt particle in the centre surrounded by soap molecules in a spherical micelle)

 

 

PAGE NO. 61 – QUESTIONS & ANSWERS

Q1. What would be the electron dot structure of carbon dioxide which has the formula CO₂?

Answer:

Carbon dioxide has one carbon atom and two oxygen atoms.
Carbon has 4 valence electrons and each oxygen has 6 valence electrons.

Carbon shares two electrons with each oxygen, forming two double bonds.

Electron dot structure:

$$\cdot \cdot O::C::O\cdot \cdot$$

Or

O = C = O

Explanation:

Carbon needs 4 electrons to complete its octet, and oxygen needs 2 electrons.
By sharing electrons, all atoms achieve stable configuration.

---

Q2. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?

(Hint: The eight atoms of sulphur are joined together in the form of a ring.)

Answer:

Sulphur (atomic number 16) has 6 valence electrons.
In S₈, eight sulphur atoms form a ring structure, each sharing 2 electrons with adjacent sulphur atoms.

Electron dot structure (simplified ring form):

$$S-S-S-S-S-S-S-S$$

Each S atom has 6 electrons around it (2 shared, 4 unshared).

Explanation:

Sulphur does not form double or triple bonds like oxygen; instead it forms a ring structure by single covalent bonds.

PAGE NO. 68 – QUESTIONS

Q1. How many structural isomers can you draw for pentane?

Answer:

Pentane (C₅H₁₂) has three structural isomers:

---

Q2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Answer:

1. Catenation – Ability of carbon atoms to bond with other carbon atoms forming long chains, branched chains or rings.

2. Tetravalency – Carbon forms 4 covalent bonds with other atoms (H, O, N, Cl etc.)

---

Q3. What will be the formula and electron dot structure of cyclopentane?

Answer:

Formula of cyclopentane = C₅H₁₀

Electron dot structure:

Five carbon atoms form a closed ring, each carbon bound to two other carbons and two hydrogens.

Ring form (simplified):

$$C-C-C-C-C(\text{closed ring})$$

Q4. Draw the structures for the following compounds

(i) Ethanoic acid
(ii) Bromopentane
(iii) Butanone
(iv) Hexanal

Answer:

Q5. How would you name the following compounds?

Answer:

(i) Bromoethane
(ii) Propanone
(iii) Hexanol

PAGE 71 – QUESTIONS & ANSWERS

1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Answer:

The conversion of ethanol to ethanoic acid is an oxidation reaction because oxygen is added to ethanol during the process.

Reaction:

$$C{H}_{3}C{H}_{2}OH\stackrel{(O)}{\to }C{H}_{3}COOH$$

Explanation:

• Ethanol (CH₃CH₂OH) gets oxidised to ethanoic acid (CH₃COOH) when oxidising agents such as alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇) are used.

• In this reaction, the ethanol molecule loses hydrogen atoms and gains oxygen, which is the definition of oxidation.

$$\text{Oxidation}=\text{addition of oxygen or removal of hydrogen}$$

Therefore, the conversion is considered an oxidation reaction.

---

2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Answer:

A mixture of ethyne and air is not used for welding because it does not produce sufficient heat.

Explanation:

• Burning ethyne in air gives a yellow, smoky flame and produces a temperature of about 1500°C, which is not hot enough to melt metals for welding.

• However, when ethyne is burnt in oxygen, it gives a hot, clean blue flame with a temperature of about 3000°C, which is suitable for welding metals.

Reactions:

$$C_2{H}_2+O_2\to C{O}_2+H_{2}O+\text{heat (lower)}$$
$$C_2{H}_2+2.5O_2\to 2C{O}_2+H_{2}O+\text{very high heat}$$

Q3. How would you distinguish experimentally between ethanol and ethanoic acid?

Answer:

Ethanol and ethanoic acid can be distinguished by the sodium bicarbonate test (baking soda test).

Experiment:

• Take a small amount of ethanol in one test tube and ethanoic acid in another.

• Add a pinch of sodium bicarbonate (NaHCO₃) or baking soda solution to both test tubes.

Observation:

Reason / Reaction:

Ethanoic acid reacts with sodium bicarbonate to produce carbon dioxide gas:

$$C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2\uparrow$$

Ethanol does not react with NaHCO₃, therefore no gas is evolved.

---

Q4. What are oxidising agents?

Answer:

Oxidising agents are chemical substances that add oxygen or remove hydrogen from other substances in a chemical reaction.

Example:

• Alkaline potassium permanganate (KMnO₄)

• Acidified potassium dichromate (K₂Cr₂O₇)

Explanation:

Oxidising agents help in converting:

$$\text{Ethanol}\to \text{Ethanoic acid}$$

by supplying oxygen.

PAGE 74 – QUESTIONS & ANSWERS

1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer:

An alcohol and a carboxylic acid can be distinguished using the sodium bicarbonate (baking soda) test.

Experiment:

• Take a small amount of the unknown liquid in two different test tubes.

• Add a pinch of sodium bicarbonate (NaHCO₃) or baking soda solution to each.

Observation:

Reason / Reaction:

Carboxylic acids react with NaHCO₃ to produce carbon dioxide gas (CO₂), which causes bubbling:

$$C{H}_{3}COOH+NaHC{O}_3\to C{H}_{3}COONa+H_{2}O+C{O}_2\uparrow$$

Alcohols do not react with sodium bicarbonate, so no gas is evolved.

---

2. What are oxidising agents?

Answer:

Oxidising agents are substances that add oxygen or remove hydrogen from other substances during a chemical reaction.

Examples of oxidising agents:

• Alkaline potassium permanganate (KMnO₄)

• Acidified potassium dichromate (K₂Cr₂O₇)

Explanation:

Oxidising agents are used in reactions such as:

$$C{H}_{3}C{H}_{2}OH\stackrel{[O]}{\to }C{H}_{3}COOH$$

Here, ethanol is oxidised to ethanoic acid because oxygen is supplied by the oxidising agent.

PAGE 76 – QUESTIONS & ANSWERS

Q1. Would you be able to check if water is hard by using a detergent?

Answer:

No, we would not be able to check if water is hard by using a detergent.

Explanation:

Detergents form lather easily in both soft and hard water because they do not react with calcium (Ca²⁺) and magnesium (Mg²⁺) ions present in hard water.
Therefore, detergents cannot indicate whether water is hard or soft.

---

Q2. People use a variety of methods to wash clothes. Usually, after adding the soap, they ‘beat’ the clothes on a stone, or beat them with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Answer:

Agitation is necessary because it helps soap micelles remove dirt from the fabric.

Explanation:

• Soap molecules form micelles around dirt particles.

• The hydrophobic ends of soap molecules attach to dirt and oil.

• The hydrophilic ends remain in water.

• Agitation (scrubbing, beating or washing machine movement) helps pull the dirt-filled micelles away from the fabric surface so the dirt can be washed away with water.

Conclusion:

Without agitation, micelles cannot detach dirt particles, and clothes will not become completely clean.