CBSE Class 10 Science HOTs Carbon And its Compounds Set 09

This is Set 09 of higher-order thinking (HOTS) questions for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds, with step-by-step answers and a Teacher's Note for each question. The questions test identification of unknown organic compounds from clues, homologous series and isomers, unsaturated versus saturated hydrocarbons, bromine water and flame tests, and the structure and bonding of carbon compounds.

Very Short Answer Type HOTS Questions

Question 1. A neutral compound 'A' (C₃H₆O₂) is formed by the reaction of an acid 'B', which is present in an ant's sting and an alcohol 'C', which is used in the preparation of tincture of iodine. Identify the compounds B and C.
Answer:
Step 1: The acid present in an ant's sting is methanoic acid (formic acid), HCOOH. So B = HCOOH.
Step 2: The alcohol used to make tincture of iodine is ethanol, C₂H₅OH. So C = C₂H₅OH.
Check: HCOOH + C₂H₅OH → HCOOC₂H₅ + H₂O. The neutral, sweet-smelling ester A is ethyl methanoate, C₃H₆O₂, which matches the given formula.
Final answer: B is methanoic acid and C is ethanol.
In simple words: An acid and an alcohol join to give a neutral ester; the formula C₃H₆O₂ fits formic acid plus ethanol.

Teacher's Note:
1. Add the carbon atoms of the acid (1) and the alcohol (2) to check the ester formula.
2. Esters are neutral and sweet smelling.

Question 2. Manasi wrote the names of four compounds as the first members of their respective homologous series. (1) Methanol (2) Methanal (3) Methanone (4) Methanoic acid. Answer the following questions: (A) Which name has she written incorrectly? Justify your answer. (B) What name should she have written instead?
Answer:
(A) The name methanone is wrong. A ketone has the group >C=O joined to two carbon atoms on either side, so the smallest ketone must have three carbon atoms. A one-carbon ketone cannot exist.
(B) She should have written propanone (acetone), CH₃–CO–CH₃, the first member of the ketone series.
The other three are correct: methanol (CH₃OH, alcohol), methanal (HCHO, aldehyde) and methanoic acid (HCOOH, carboxylic acid).
In simple words: The first ketone is propanone, because a ketone group needs a carbon on both sides.

Teacher's Note:
1. Check that the carbon chain is long enough for the functional group.
2. Aldehydes can have one carbon, ketones need at least three.

Question 3. Why are unsaturated hydrocarbons more reactive than saturated hydrocarbons ?
Answer:
Step 1: Unsaturated hydrocarbons contain a carbon–carbon double or triple bond. One of these bonds is weaker and is easily broken.
Step 2: The carbon atoms can then add other atoms (for example H₂, Br₂) to become saturated, which is called an addition reaction.
Step 3: Saturated hydrocarbons have only strong C–C single bonds with all valencies satisfied, so they do not add anything easily.
In simple words: Multiple bonds can open up and add atoms, but single bonds cannot.

Teacher's Note:
1. Link reactivity with the presence of a double or triple bond.
2. Hydrogenation and the bromine water test are examples of addition.

Question 4. How many covalent bonds are there in a molecule of cyclohexane?
Answer:
Step 1: The molecular formula of cyclohexane is C₆H₁₂. Its six carbon atoms form a ring.
Step 2: C–C bonds: six (one between each pair of neighbouring carbon atoms in the ring).
Step 3: C–H bonds: twelve (one for each hydrogen atom).
Total = 6 + 12 = 18 covalent bonds.
In simple words: Count the six ring bonds and the twelve C–H bonds to get 18.

Teacher's Note:
1. In a ring there are as many C–C bonds as carbon atoms.
2. Check with valency: each carbon has 2 C–C and 2 C–H bonds, which makes 4.

Question 5. Which of the following C₂H₆, C₂H₅OH, CH₃COOH is a hydrocarbon?
Answer:
A hydrocarbon contains only carbon and hydrogen.
C₂H₅OH and CH₃COOH also contain oxygen, so they are not hydrocarbons.
C₂H₆ (ethane) contains only carbon and hydrogen, so it is the hydrocarbon.
In simple words: Only the compound made of C and H alone is a hydrocarbon.

Teacher's Note:
1. Look at the elements present, not at the number of carbon atoms.
2. C₂H₅OH is an alcohol and CH₃COOH is a carboxylic acid.

Question 6. An alkane has molecular weight 86. Write its molecular formula. What will be its physical state?
Answer:
Step 1: The general formula of alkanes is C_nH_2n+2, so mass = 12n + (2n + 2) = 14n + 2.
Step 2: 14n + 2 = 86, so 14n = 84 and n = 6.
Molecular formula: C₆H₁₄ (hexane).
Physical state: Hexane is a liquid at room temperature (boiling point about 69 °C). Alkanes from C₅ to about C₁₆ are liquids.
In simple words: 86 u fits the six-carbon alkane C₆H₁₄, which is a liquid.

Teacher's Note:
1. Use 14n + 2 for the mass of an alkane.
2. The first four alkanes are gases, the next ones are liquids.

Question 7. An organic compound burns with a sooty flame. Is it a saturated or an unsaturated compound?
Answer:
It is an unsaturated compound.
Unsaturated hydrocarbons have a higher percentage of carbon and burn incompletely in air. The unburnt carbon particles glow yellow and form soot.
Saturated hydrocarbons burn with a clean blue flame in a good supply of air.
In simple words: A sooty yellow flame shows an unsaturated compound.

Teacher's Note:
1. Flame colour is a quick test for unsaturation.
2. Soot is unburnt carbon.

Question 8. The molecular formulae of alkenes X and Y are CxH₈ and C₁₀Hy respectively. What are the relative molecular masses of X and Y?
Answer:
Step 1: The general formula of alkenes is C_nH_2n.
For X: 2x = 8, so x = 4. X is C₄H₈ and its relative molecular mass = 4 × 12 + 8 × 1 = 56 u.
For Y: y = 2 × 10 = 20. Y is C₁₀H₂₀ and its relative molecular mass = 10 × 12 + 20 × 1 = 140 u.
In simple words: Use C_nH_2n to find the missing number, then add the atomic masses.

Teacher's Note:
1. Atomic masses used: C = 12 u and H = 1 u.
2. The two alkenes differ by six CH₂ units: 6 × 14 = 84 u.

Short Answer Type HOTS Questions

Question 9. Raghav used to fall sick during weather change. Every year, he would have bouts of cough and chest congestion. The doctor would prescribe certain antibiotics along with cough syrup. His friend remarked him that one of the ingredients of cough syrup is the compound A which is the active ingredient of all alcoholic drinks and also used in medicines such as tincture of iodine. Identify the chemical substance A. What happens when A reacts with sodium?
Answer:
Step 1: The active ingredient of all alcoholic drinks, also used in tincture of iodine and cough syrups, is ethanol (C₂H₅OH). So A is ethanol.
Step 2: Ethanol reacts with sodium to give sodium ethoxide and hydrogen gas, with brisk bubbling.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑
The hydrogen gas burns with a pop sound when a burning splinter is brought near.
In simple words: A is ethanol; sodium displaces hydrogen from it and gives sodium ethoxide.

Teacher's Note:
1. Ethanol is a good solvent, which is why it is used in medicines.
2. Dry sodium and dry ethanol should be used.

Question 10. Alkanes are saturated compounds of carbon and hydrogen that can be represented by the general formula C_nH_2n+2 where 'n' is the number of carbon atoms. An example of such a compound is ethane, C₂H₆. Maya has a compound of carbon and hydrogen whose formula is C₃H₄. (A) What is true about the type of flame this compound will give on combustion? (B) Name this hydrocarbon and draw its electron dot structure.
Answer:
Step 1: For C₃H₄, C_nH_2n−2 with n = 3 gives H = 4, so it is an alkyne (unsaturated).
(A) Unsaturated hydrocarbons burn in air with a yellow, sooty (smoky) flame because of incomplete combustion.
(B) The compound is propyne, CH₃–C≡C–H. Its electron dot structure:

In simple words: C₃H₄ is propyne, an alkyne that burns with a sooty flame.

Teacher's Note:
1. Compare the formula with C_nH_2n+2: two hydrogen atoms fewer than C₃H₈ for each extra bond.
2. The triple bond is shown by three shared pairs between two carbon atoms.

Question 11. Compounds with identical formula but different structures are called structural isomers. (A) In the case of saturated hydrocarbons, what is the minimum number of carbon atoms needed in a molecule for it to have a structural isomer? (B) Draw the structural isomers of the saturated hydrocarbon having the minimum number of carbon atoms mentioned in (A). (C) Is there any similarity between C₃H₇OH and C₄H₉OH? What is the common difference between these two?
Answer:
(A) The minimum number of carbon atoms is 4. Methane, ethane and propane cannot be arranged in more than one way, because no branching is possible with fewer than four carbon atoms.
(B) The saturated hydrocarbon with four carbon atoms is C₄H₁₀. Its two isomers are:
(i) Butane: CH₃–CH₂–CH₂–CH₃
(ii) 2-Methylpropane (isobutane): CH₃–CH(CH₃)–CH₃
(C) Yes. Both are alcohols with the same functional group (–OH), the same general formula C_nH_2n+1OH and similar chemical properties, so they belong to the same homologous series. They differ by one –CH₂– unit, that is 14 u in molecular mass.
In simple words: Branching starts with four carbon atoms; the two alcohols are neighbours in a homologous series differing by CH₂.

Teacher's Note:
1. Isomers have the same molecular formula but different structures.
2. Neighbouring homologues always differ by CH₂ (14 u).

Question 12. Prasad has a saturated alcohol X of chemical formula C₄H₉OH. (A) Write the chemical formula of a member Y that comes two places after X in the homologous series and state by how much will its molecular mass differ from that of X. (B) How do the chemical properties of X compare with those of Y? Give reason for your answer. (C) Write the chemical formula of the product Z formed by heating Y with acidified potassium dichromate. Write the general formula for compounds in the homologous series that Z belongs to.
Answer:
(A) Two places after C₄H₉OH (butanol) we add two CH₂ units: Y = C₆H₁₃OH (hexanol). Mass of X = 4 × 12 + 9 + 16 + 1 = 74 u and mass of Y = 6 × 12 + 13 + 16 + 1 = 102 u. The difference is 2 × 14 = 28 u.
(B) X and Y have similar chemical properties because both contain the same functional group, –OH, which decides the chemical properties.
(C) Acidified K₂Cr₂O₇ oxidises the alcohol to the carboxylic acid: C₆H₁₃OH → C₅H₁₁COOH (Z, hexanoic acid). Z belongs to the carboxylic acids, with general formula C_nH_2n+1COOH (or C_nH_2nO₂).
In simple words: Two more CH₂ groups add 28 u; the alcohol group keeps the properties the same; oxidation gives a carboxylic acid.

Teacher's Note:
1. Each CH₂ adds 14 u.
2. Oxidation of an alcohol keeps the carbon number and converts –CH₂OH into –COOH.

Question 13. Bromine water is a reddish-brown solution of bromine in water. Niti was given three solutions in three test tubes containing butane, butene and butyne respectively. (A) Out of the three hydrocarbons, which one can she identify using bromine water and why? (B) Illustrate your answer by writing the balanced chemical equations involved in the hydrocarbons. (C) What type of reaction would she observe and where can it be used?
Answer:
(A) Unsaturated hydrocarbons decolourise bromine water, saturated ones do not. Butene and butyne turn the reddish-brown colour colourless, while butane leaves the colour unchanged, so butane is the one she can identify as the saturated hydrocarbon. (Butene and butyne are both unsaturated; butyne will use up twice as much bromine.)
(B) C₄H₈ + Br₂ → C₄H₈Br₂ (butene gives dibromobutane)
C₄H₆ + 2Br₂ → C₄H₆Br₄ (butyne gives tetrabromobutane)
Butane: C₄H₁₀ + Br₂ → no reaction in the dark.
(C) An addition reaction. It is used as a simple test to distinguish saturated from unsaturated hydrocarbons.
In simple words: Bromine water loses its colour with butene and butyne (addition) but not with butane.

Teacher's Note:
1. Decolourisation of bromine water means unsaturation is present.
2. In butyne, two Br₂ molecules are added because of the triple bond.

Question 14. A child heated ethanol C₂H₅OH with potassium permanganate and product 'X' is formed but when she heated ethanol with concentrated sulphuric acid she obtained 'Y' product. (A) How many carbon atoms will compound 'X' and 'Y' contain? Write the chemical equation involved. (B) (i) Which one of the compounds 'X' or 'Y' will react with ethanol in the presence of an acid catalyst to give a compound 'Z'? Illustrate with a chemical equation. (ii) What type of compound is formed in the above reaction with respect to the functional group it contains? (iii) State one characteristic property and use of this type of compound.
Answer:
(A) With alkaline KMnO₄ ethanol is oxidised to ethanoic acid, so X = CH₃COOH. With excess conc. H₂SO₄ at 443 K ethanol is dehydrated, so Y = CH₂=CH₂ (ethene). Both X and Y contain 2 carbon atoms.
CH₃CH₂OH → CH₃COOH (alkaline KMnO₄, heat)
CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, 443 K)
(B)(i) X, ethanoic acid, reacts with ethanol: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (acid catalyst). Z is ethyl ethanoate.
(ii) Z is an ester (functional group –COO–).
(iii) Esters have a sweet, fruity smell; they are used in perfumes and as flavouring agents.
In simple words: Oxidation gives ethanoic acid, dehydration gives ethene, and the acid with ethanol forms a sweet-smelling ester.

Teacher's Note:
1. Remember the two uses of ethanol with a reagent: oxidation and dehydration.
2. Esterification needs an acid and an alcohol with an acid catalyst.

Question 15. The first member of the unsaturated hydrocarbon having double bond is ethene. Polythene is made from ethene. When ethene is subjected to high temperature and pressure, ethene molecules react with each other to form plastic. Monu writes the formula of some hydrocarbons: C₃H₆, C₄H₈, C₅H₁₀. (A) Can you deduce the general formula for such hydrocarbons? Do you find any relation between the number of carbon and hydrogen atoms in these compounds? (B) State and define the property of carbon atoms which helps to form large molecules forming the plastic. Also, write the electron dot structure of ethene molecule.
Answer:
(A) The general formula is C_nH_2n (alkenes). The number of hydrogen atoms is always twice the number of carbon atoms.
(B) The property is catenation: the ability of carbon atoms to link with other carbon atoms by covalent bonds to form long chains. Because of it, a very large number of ethene molecules can join into the long chain of polythene.
Electron dot structure of ethene (C₂H₄), with a double bond between the carbon atoms:

In simple words: Alkenes follow C_nH_2n; carbon's catenation lets small molecules join into long plastic chains.

Teacher's Note:
1. Check the formula with the three given examples: 3 and 6, 4 and 8, 5 and 10.
2. In the dot structure each carbon shows four shared pairs in total.

Question 16. An allotrope of carbon has a molecular mass of 720. Discuss its structure.
Answer:
Step 1: Number of carbon atoms = 720 ÷ 12 = 60. The allotrope is fullerene C₆₀ (Buckminsterfullerene).
Step 2: Its 60 carbon atoms are joined in a hollow, football-shaped cage made of 20 hexagonal and 12 pentagonal rings.
Step 3: Each carbon atom is bonded to three others, with a mixture of single and double bonds. The shape resembles the geodesic dome designed by the architect Buckminster Fuller.
In simple words: 720 ÷ 12 = 60, so it is the C₆₀ football-shaped fullerene.

Teacher's Note:
1. Divide the mass by 12 to get the number of carbon atoms.
2. Fullerene is the third crystalline allotrope after diamond and graphite.

Question 17. Two non-metals A and B combine with each other by sharing of electrons to form compound C. (a) What is the nature of compound C ? (b) Will it dissolve in water or organic solvents? (c) Will it be a good or bad conductor of electricity? (d) Will it have high or low melting/boiling point ?
Answer:
(a) Sharing of electrons forms covalent bonds, so C is a covalent compound.
(b) Covalent compounds are generally insoluble or only slightly soluble in water, but dissolve in organic solvents such as benzene or ether.
(c) It will be a bad conductor of electricity, because it has no ions or free electrons.
(d) It will have low melting and boiling points, because the forces of attraction between its molecules are weak.
In simple words: Two non-metals sharing electrons give a covalent compound with poor conductivity and low melting point.

Teacher's Note:
1. Contrast with ionic compounds, which have high melting points and conduct when molten.
2. Some covalent compounds such as sugar do dissolve in water.

Teacher's Note:
1. Test each formula against C_nH_2n+2.
2. A gradual change in physical properties is a feature of every homologous series.

Long Answer Type HOTS Questions

Question 19. (i) What is the type of reaction taking place between ethane and chlorine to form monochloroethane? (ii) The reaction between ethene and chlorine forms only one product. Name the type of this reaction. (iii) (a) Draw the structural formula of ethene. (b) What is the feature of the ethene structure which allows ethene to react with chlorine?
Answer:
(i) A substitution reaction (in the presence of sunlight): C₂H₆ + Cl₂ → C₂H₅Cl + HCl. One hydrogen atom is replaced by a chlorine atom and a second product, HCl, is formed.
(ii) An addition reaction: CH₂=CH₂ + Cl₂ → CH₂Cl–CH₂Cl (1,2-dichloroethane). Only one product is formed.
(iii)(a) Structural formula of ethene: H₂C=CH₂, that is, each carbon is joined to two hydrogen atoms and to the other carbon by a double bond.
(b) The carbon–carbon double bond (unsaturation) allows ethene to add chlorine.
In simple words: Saturated ethane substitutes; unsaturated ethene adds.

Teacher's Note:
1. Substitution gives two products, addition gives one.
2. Sunlight is needed for the chlorine substitution of alkanes.

Question 20. The molecules of alkene family are represented by a general formula CnH₂n. Now answer the following : (i) What do n and 2n signify? (ii) What is the name of alkene when n = 4? (iii) What is the molecular formula of alkene when n = 6? (iv) What is the molecular formula of the alkene if there are six H atoms in it? (v) What is the molecular formula and structural formula of the first member of the alkene family? (vi) Write the molecular formulae of lower and higher homologues of an alkene which contains four carbon atoms.
Answer:
(i) n is the number of carbon atoms and 2n is the number of hydrogen atoms in the molecule.
(ii) For n = 4 the alkene is butene (C₄H₈).
(iii) For n = 6 the formula is C₆H₁₂ (hexene).
(iv) 2n = 6 gives n = 3, so the alkene is C₃H₆ (propene).
(v) The first member is ethene, C₂H₄, with structural formula H₂C=CH₂.
(vi) The alkene with four carbon atoms is C₄H₈. The lower homologue is C₃H₆ and the higher homologue is C₅H₁₀.
In simple words: Put n into C_nH_2n for each part; neighbours differ by one carbon and two hydrogens.

Teacher's Note:
1. Alkenes start from n = 2 because one carbon cannot form a double bond.
2. Neighbouring members differ by CH₂.