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.

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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:

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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.)

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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.

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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.

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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.

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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.

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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.