CBSE Class 10 Science Carbon and its compound Sure Shot Questions Set 05

This is Sure Shot Set 05 for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds: board-style questions that are asked again and again, each with its marks, a step-by-step answer and a Teacher's Note. The questions cover homologous series, properties of carbon, identification of compounds from clues, reactions of ethanol and ethanoic acid, esterification, isomers, and soaps and detergents.

Short Answer Questions (1 to 3 Marks)

Question 1. What is a homologous series of carbon compounds? Give an example and list its three characteristics. [3 marks]
Answer:
A homologous series is a series of carbon compounds that have the same functional group and similar chemical properties, with successive members differing by a –CH₂– group.
Example: the alkanes (CH₄, C₂H₆, C₃H₈, ...) with the general formula C_nH_2n+2.
Characteristics: (i) all members have the same general formula and the same functional group; (ii) two adjacent members differ by one carbon and two hydrogen atoms (a CH₂ unit), which is 14 u in molecular mass; (iii) their chemical properties are similar and their physical properties change gradually (for example, boiling point rises) with the increase in molecular mass.
In simple words: Members of a homologous series share a functional group and differ by CH₂ (14 u).

Teacher's Note:
1. Quote the general formula in your example.
2. Three characteristics are the usual marking points.

Question 2. List two properties of carbon which lead to the huge number of carbon compounds we see around us, giving reason for each. [2 marks]
Answer:
1. Catenation: carbon atoms can link with each other through strong covalent bonds (because carbon is small) to form long chains, branched chains and rings.
2. Tetravalency: carbon has four valence electrons, so each carbon atom can form four bonds with other carbon atoms or with atoms such as H, O, N, S and Cl.
In simple words: Catenation gives chains and rings; tetravalency lets each carbon bond to four atoms.

Teacher's Note:
1. Give a reason with each property.
2. Carbon also forms single, double and triple bonds.

Question 3. A compound A (C₂H₄O₂) reacts with Na metal to form a compound 'B' and evolves a gas which burns with a pop sound. Compound 'A' on treatment with an alcohol 'C' in presence of an acid forms a sweet smelling compound 'D' (C₄H₈O₂). On addition of NaOH to 'D' gives back B and C. Identify A, B, C and D. Write the reactions involved. [3 marks]
Answer:
A = CH₃COOH (ethanoic acid); with sodium it gives B = CH₃COONa (sodium ethanoate) and hydrogen gas (burns with a pop).
A with alcohol C in the presence of an acid gives an ester D of formula C₄H₈O₂, so C = C₂H₅OH (ethanol) and D = CH₃COOC₂H₅ (ethyl ethanoate).
2CH₃COOH + 2Na → 2CH₃COONa + H₂
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (acid catalyst)
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
In simple words: A is ethanoic acid, B its sodium salt, C ethanol and D the ester ethyl ethanoate.

Teacher's Note:
1. The pop sound shows hydrogen gas.
2. Ester + NaOH gives back the alcohol and the sodium salt of the acid.

Question 4. (a) Draw the structures for (i) ethanol, (ii) ethanoic acid. (b) Why is the conversion of ethanol to ethanoic acid considered an oxidation reaction? Write the oxidising agent used in the reaction involved. [3 marks]
Answer:
(a) (i) Ethanol: CH₃–CH₂–OH, that is, H–C(H)(H)–C(H)(H)–O–H. (ii) Ethanoic acid: CH₃–C(=O)–OH, that is, a methyl group joined to the carboxyl group –COOH.
(b) Ethanol gains oxygen (–CH₂OH becomes –COOH), so the change is an oxidation reaction. The oxidising agent is alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇), with heating.
CH₃CH₂OH → CH₃COOH
In simple words: Adding oxygen to ethanol gives ethanoic acid; KMnO₄ or K₂Cr₂O₇ supplies the oxygen.

Teacher's Note:
1. Show every bond of carbon and oxygen in structural formulae.
2. Oxidising agents are not part of the final formula.

Question 5. A student adds a spoon full of powdered sodium hydrogen carbonate to a flask containing ethanoic acid. List two main observations, he must note in his note book, about the reaction that takes places. Also write chemical equation for the reaction. [3 marks]
Answer:
Observations: (i) A colourless, odourless gas is evolved with brisk effervescence. (ii) The gas turns lime water milky and extinguishes a burning splinter brought near it (it is carbon dioxide).
Equation: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
In simple words: Ethanoic acid reacts with baking soda to give fizzing carbon dioxide, sodium ethanoate and water.

Teacher's Note:
1. Carbon dioxide turns lime water milky.
2. Mention the products by name as well.

Question 6. On dropping a small piece of sodium in a test tube containing carbon compounds X with molecular formula, C₂H₆O, a brisk effervescence is observed and a gas Y is produced. On burning a burning splinter at the mouth of the test tube the gas evolved burns with a pop sound. Identify X and Y. Also write the chemical equation for the reaction. Write the name and structure of the product formed when you heat X with excess conc. sulphuric acid. [3 marks]
Answer:
X is ethanol, C₂H₅OH, and Y is hydrogen gas, H₂ (it burns with a pop sound).
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
When X is heated with excess conc. H₂SO₄ at 443 K, it is dehydrated to ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O. Structure of ethene: H₂C=CH₂, with a double bond between the two carbon atoms.
In simple words: Ethanol releases hydrogen with sodium and forms ethene when strongly heated with conc. H₂SO₄.

Teacher's Note:
1. Molecular formula C₂H₆O can be ethanol or an ether, but the reaction with sodium points to ethanol.
2. Conc. H₂SO₄ acts as a dehydrating agent.

Question 7. (a) What is saponification ? (b) Write differences between soaps and detergents under the following heads : (i) Chemical composition. (ii) Action with hard water. [3 marks]
Answer:
(a) Saponification is the reaction of an ester with an alkali (sodium hydroxide) to give the sodium salt of the carboxylic acid and an alcohol: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. Soap is made by the saponification of fats and oils.
(b)(i) Chemical composition: Soaps are sodium or potassium salts of long-chain carboxylic acids. Detergents are generally sodium salts of long-chain sulphonic acids (or ammonium salts).
(ii) Action with hard water: Soaps form insoluble scum with the calcium and magnesium salts in hard water, while detergents do not form scum and keep cleaning in hard water.
In simple words: Soap is made by saponification; soaps give scum in hard water, detergents do not.

Teacher's Note:
1. Saponification is the alkaline hydrolysis of an ester.
2. Detergents have sulphonate heads, not carboxylate.

Question 8. An organic compound A having molecular formula C₂H₄O₂ reacts with sodium metal and evolves a gas B which readily catches fire. A also reacts with ethanol in the presence of concentrated sulphuric acid to form sweet smelling substance C used in making perfumes. (a) Identify the compounds A, B and C. (b) Write balanced chemical equations to represent the conversion of: (i) compound A into compound B. (ii) compound A into compound C. [3 marks]
Answer:
(a) A is ethanoic acid (CH₃COOH), gas B is hydrogen (H₂), and C is ethyl ethanoate (CH₃COOC₂H₅).
(b)(i) 2CH₃COOH + 2Na → 2CH₃COONa + H₂
(ii) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)
In simple words: The acid releases hydrogen with sodium and forms a sweet-smelling ester with ethanol.

Teacher's Note:
1. A gas that catches fire is hydrogen.
2. The ester has a fruity smell and is used in perfumes.

Question 9. An organic compound A of molecular formula C₂H₆O on oxidation gives an acid B with the same number of carbon atoms in the molecule as A. Compound A is often used for sterilization of skin by doctors. Name the compounds A and B. Write the chemical equation involved in the formation of B from A. [3 marks]
Answer:
A is ethanol (ethyl alcohol, C₂H₅OH), which doctors use to sterilise the skin before an injection.
B is ethanoic acid (CH₃COOH), which has the same two carbon atoms as A.
CH₃CH₂OH → CH₃COOH + H₂O (oxidation with alkaline KMnO₄ or acidified K₂Cr₂O₇, heat); ethanol gains one oxygen atom and loses two hydrogen atoms to form ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
In simple words: Ethanol is oxidised to ethanoic acid.

Teacher's Note:
1. Use [O] for the oxygen supplied by the oxidising agent.
2. Both compounds have two carbon atoms.

Question 10. A compound 'X' on heating with excess conc. sulphuric acid at 443 K gives an unsaturated compound 'Y'. 'X' also reacts with sodium metal to evolve a colourless gas 'Z'. Identify 'X', 'Y' and 'Z'. Write the equation of the chemical reaction of formation of 'Y' and also write the role of sulphuric acid in the reaction. [3 marks]
Answer:
X = ethanol (CH₃CH₂OH), Y = ethene (CH₂=CH₂) and Z = hydrogen gas (H₂).
CH₃CH₂OH → CH₂=CH₂ + H₂O (excess conc. H₂SO₄, 443 K)
CH₃CH₂OH + Na → CH₃CH₂ONa + H₂ (balanced: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂)
Role of H₂SO₄: it acts as a dehydrating agent, removing water from ethanol.
In simple words: Heating ethanol with conc. H₂SO₄ gives ethene; sodium gives hydrogen.

Teacher's Note:
1. Y is unsaturated, so it has a double bond.
2. Use excess acid and 443 K for ethene.

Question 11. When ethanol reacts with ethanoic acid in the presence of conc. H₂SO₄, a substance with fruity smell is produced. Answer the following: (i) State the class of compounds to which the fruity smelling compound belong. Write the chemical equation for the reaction and write the chemical name of the product formed. (ii) State the role of conc. H₂SO₄. [3 marks]
Answer:
(i) The fruity smelling compound is an ester. The reaction is esterification: CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O. The product is ethyl ethanoate (ethyl acetate).
(ii) Conc. H₂SO₄ acts as a catalyst and as a dehydrating agent, which removes the water formed and helps the reaction go forward.
In simple words: Acid + alcohol with conc. H₂SO₄ gives a fruity ester and water.

Teacher's Note:
1. Esters are named from the alcohol part first.
2. Esters are used in perfumes and as flavouring agents.

Question 12. (a) Write chemical equation of the reactions of ethanoic acid with (i) sodium, (ii) sodium carbonate, (iii) ethanol in the presence of conc. H₂SO₄. (b) State the role of concentrated sulphuric acid in the esterification reaction. (c) Write one use of ethanoic acid. [3 marks]
Answer:
(a)(i) 2CH₃COOH + 2Na → 2CH₃COONa + H₂
(ii) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
(iii) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)
(b) Concentrated sulphuric acid is a dehydrating agent and catalyst. It removes water from the reaction mixture, so the reaction goes in the forward direction to form the ester.
(c) Ethanoic acid is used to make vinegar, which preserves pickles.
In simple words: Ethanoic acid reacts with sodium, with carbonates and with alcohols; conc. H₂SO₄ removes the water in esterification.

Teacher's Note:
1. The second product of sodium carbonate reaction is water and carbon dioxide, not hydrogen.
2. Esterification is reversible; removing water helps.

Question 13. (a) Amongst soaps and detergents which can work effectively in hard water and why ? (b) Name the salts which cause hardness of water. (c) What change will you observe if you test soap with litmus paper (red and blue). [3 marks]
Answer:
(a) Detergents work effectively in hard water, because they are salts of sulphonic acids whose calcium and magnesium salts are soluble in water, so no insoluble scum is formed. Soaps form scum.
(b) Hardness is caused by the dissolved salts (chlorides, sulphates and hydrogencarbonates) of calcium and magnesium.
(c) A soap solution is slightly alkaline (it is the salt of a strong base, NaOH, and a weak acid). It turns red litmus paper blue and has no effect on blue litmus paper.
In simple words: Detergents work in hard water; Ca and Mg salts cause hardness; soap turns red litmus blue.

Teacher's Note:
1. Soaps are made with alkali, so they are basic.
2. Scum is the insoluble Ca or Mg soap.

Question 14. (a) Draw and write the names of any two structural isomers of C₅H₁₂. (b) Discuss the drawback of using soaps over detergents for cleansing process. [3 marks]
Answer:
(a) (i) Pentane: CH₃–CH₂–CH₂–CH₂–CH₃. (ii) 2-Methylbutane: CH₃–CH(CH₃)–CH₂–CH₃. (A third isomer is 2,2-dimethylpropane, C(CH₃)₄.)
(b) Soaps form an insoluble scum with the calcium and magnesium salts in hard water. A large amount of soap is therefore wasted and the clothes do not become clean, whereas detergents give lather and clean even in hard water.
In simple words: Soap wastes itself as scum in hard water, so detergents are better there.

Teacher's Note:
1. Isomers need the same formula, C₅H₁₂.
2. Scum also sticks on the clothes.

Long Answer Questions (5 Marks)

Question 15. A compound A with a molecular formula of C₂H₄O₂ reacts with a base to give salt and water. Identify A, state its nature and the name of the functional group it possesses. Write chemical equation for the reaction involved. (ii) When the above stated compound A reacts with another compound B having molecular formula C₂H₆O in the presence of an acid, a sweet smelling compound C is formed. (1) Identify B and C. (2) State the role of acid in this reaction. (3) Write chemical equation for the reaction involved. [5 marks]
Answer:
(i) A is ethanoic acid, CH₃COOH. Its nature is acidic, and the functional group is the carboxylic acid group, –COOH.
CH₃COOH + NaOH → CH₃COONa + H₂O
(ii)(1) B (C₂H₆O, which forms an ester with the acid) is ethanol, C₂H₅OH, and C is the ester ethyl ethanoate, CH₃COOC₂H₅.
(2) The acid (conc. H₂SO₄) acts as a catalyst, which speeds up the reaction, and it also absorbs the water formed so that the ester is obtained.
(3) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
In simple words: Ethanoic acid is acidic and forms an ester with ethanol in the presence of an acid catalyst.

Teacher's Note:
1. C₂H₆O reacting with an acid points to an alcohol.
2. The formula in part (i) is C₂H₄O₂, as it must give a salt with a base.

Question 16. (i) Name the compound formed when ethanol is heated at 443 K in the presence of conc. H₂SO₄ and draw its electron dot structure. State the role of conc. H₂SO₄ in this reaction. (ii) What is hydrogenation? Explain it with the help of a chemical equation. State the role of this reaction in industry. [5 marks]
Answer:
(i) The compound formed is ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, 443 K). Electron dot structure of ethene:

Conc. H₂SO₄ acts as a dehydrating agent; it removes a water molecule from ethanol.
(ii) Hydrogenation is the addition of hydrogen to an unsaturated compound in the presence of a catalyst (nickel or palladium) to form a saturated compound: CH₂=CH₂ + H₂ → CH₃–CH₃
Industrial role: It is used to convert vegetable oils (which have unsaturated carbon chains) into solid vegetable ghee (saturated fats).
In simple words: Conc. H₂SO₄ dehydrates ethanol to ethene; nickel and hydrogen turn unsaturated oils into saturated fats.

Teacher's Note:
1. Ethene has a double bond with two shared pairs.
2. The catalyst is not used up in the reaction.

Question 17. Soaps and detergents are both types of salts. State the difference between the two. Write the mechanism of the cleaning action of soap. Why do soaps not form lather with hard water? Mention any two problems that arise due to the use of detergents instead of soaps. [5 marks]
Answer:
Difference: Soaps are sodium or potassium salts of long-chain carboxylic acids. Detergents are generally sodium salts of long-chain sulphonic acids (or ammonium salts with chloride or bromide ions).
Cleaning action of soap: A soap molecule has a hydrophilic ionic head and a hydrophobic hydrocarbon tail. In water the soap molecules form micelles with the tails inside and the heads outside. The oily dirt is trapped in the centre of the micelle. On agitation the dirt is dislodged and held in suspension as micelles, which are then rinsed away.
Hard water: The calcium and magnesium ions of hard water react with soap and form insoluble calcium and magnesium salts of the fatty acid (scum), so soap does not form lather.
Problems with detergents: (i) Many detergents are non-biodegradable and cause water pollution. (ii) They accumulate in water bodies, cause excess foam and harm aquatic life (and may enter the food chain).
In simple words: Soap micelles trap oily dirt; hard water ions turn soap into scum; non-biodegradable detergents pollute water.

Teacher's Note:
1. Draw a labelled micelle diagram if asked.
2. Remember scum is calcium or magnesium soap.

Question 18. An organic compound A is widely used as a preservative in pickles and has a molecular formula C₂H₄O₂. This compound reacts with ethanol to form a sweet smelling compound B. (a) Identify the compound A. (b) Write the chemical equation for the reaction with ethanol to form compound B. (c) How can we get compound A from B? (d) Name the process and write corresponding chemical equation. (e) Which gas is produced when compound A reacts with washing soda? Write the chemical equation. [5 marks]
Answer:
(a) A is ethanoic acid (acetic acid), CH₃COOH. Its 5–8% solution, vinegar, is used to preserve pickles.
(b) CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄); B is ethyl ethanoate.
(c) A can be obtained back from B by treating the ester with an alkali (NaOH) followed by acidification, or by heating with an acid.
(d) The process with alkali is saponification: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (the sodium ethanoate formed gives ethanoic acid on adding acid).
(e) With washing soda (sodium carbonate), carbon dioxide gas is produced: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
In simple words: A is ethanoic acid; the ester B returns the acid after saponification, and washing soda gives carbon dioxide.

Teacher's Note:
1. Pickle preservation is the clue for ethanoic acid.
2. Alkaline hydrolysis first gives the sodium salt of the acid.

Question 19. (a) Write chemical equations for the following: (i) Conversion of ethanol to ethene. (ii) Reaction of ethanol with ethanoic acid in presence of conc. sulphuric acid. (iii) Reaction of ethanol with sodium metal. (b) What are isomers? Write any two possible structures for the formula C₅H₁₂. [5 marks]
Answer:
(a)(i) CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, 443 K)
(ii) CH₃CH₂OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)
(iii) 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
(b) Isomers are compounds with the same molecular formula but different structural formulae. Two isomers of C₅H₁₂: (1) pentane, CH₃–CH₂–CH₂–CH₂–CH₃; (2) 2-methylbutane, CH₃–CH(CH₃)–CH₂–CH₃. (A third isomer is 2,2-dimethylpropane, CH₃–C(CH₃)₂–CH₃.)
In simple words: Three equations for ethanol, and isomers have one formula but different structures.

Teacher's Note:
1. Pentane has three isomers in all.
2. Name the product in each equation.

Question 20. (a) Write the molecular formula of an organic compound having its name suffixed with '-ol' and having two carbon atoms in the molecule. With the help of balanced chemical equation indicate what happens when it is heated with excess of concentrated H₂SO₄. (b) Write names of the following compounds: (i) HCOOH (ii) CH₃COCH₂CH₃. (c) Explain why carbon generally forms compounds by covalent bonds. [5 marks]
Answer:
(a) The compound is ethanol, C₂H₅OH. Heated with excess conc. H₂SO₄ at 443 K it loses water and forms ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O
(b) (i) HCOOH is methanoic acid; (ii) CH₃COCH₂CH₃ is butanone.
(c) Carbon has four valence electrons. To get a noble gas configuration it would have to lose or gain four electrons, which needs a very large amount of energy and gives unstable ions. So carbon shares its electrons with other atoms and forms covalent bonds.
In simple words: Ethanol gives ethene with conc. H₂SO₄; carbon shares electrons instead of losing or gaining four.

Teacher's Note:
1. The ending -ol means alcohol.
2. -oic acid ends carboxylic acids and -one ketones.