NCERT CLASS 10 SCIENCE CHAPTER 4 CARBON AND ITS COMPOUNDS
NCERT CLASS 10 SCIENCE
CHAPTER 4 – CARBON AND ITS COMPOUNDS
QUESTIONS AND ANSWERS
SECTION 1: IN-TEXT QUESTIONS AND ANSWERS
Q1. What would be the electron-dot structure of carbon dioxide, which has the formula CO₂?
Answer:
Atomic number of carbon = 6.
Electronic configuration of carbon = 2, 4.
Carbon has 4 valence electrons.
Oxygen has 6 valence electrons.
Carbon shares two pairs of electrons with each oxygen atom.
Electron-dot structure:
.. ..
O :: C :: O
.. ..Carbon forms two double covalent bonds with oxygen atoms.
Q2. What would be the electron-dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?
Answer:
A sulphur molecule contains eight sulphur atoms and is represented by S₈.
Each sulphur atom has 6 valence electrons.
Each sulphur atom shares one pair of electrons with each of its two neighbouring sulphur atoms.
The eight sulphur atoms form a ring.
Each sulphur atom completes its octet.
The molecule has a cyclic ring structure containing eight sulphur atoms.
Q3. How many structural isomers can you draw for pentane?
Answer:
Pentane has the molecular formula C₅H₁₂.
It has three structural isomers:
n-Pentane: CH₃–CH₂–CH₂–CH₂–CH₃
Isopentane (2-methylbutane): CH₃–CH(CH₃)–CH₂–CH₃
Neopentane (2,2-dimethylpropane): C(CH₃)₄
Q4. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer:
The two properties of carbon responsible for the formation of a large number of carbon compounds are:
Catenation: Carbon has the ability to form bonds with other carbon atoms, producing long chains, branched chains and rings.
Tetravalency: Carbon has a valency of four and can form four covalent bonds with carbon and other elements.
Q5. What will be the formula and electron-dot structure of cyclopentane?
Answer:
Molecular formula = C₅H₁₀.
Cyclopentane contains five carbon atoms arranged in a ring.
Each carbon atom is connected to two neighbouring carbon atoms and two hydrogen atoms.
All bonds are single covalent bonds.
Q6. Draw the structures for the following compounds:
(a) Ethanoic acid
(b) Bromopentane
(c) Butanone
(d) Hexanal
Answer:
(a) Ethanoic acid: CH₃COOH
(b) Bromopentane: CH₃–CH₂–CH₂–CH₂–CH₂Br
(c) Butanone: CH₃–CO–CH₂–CH₃
(d) Hexanal: CH₃–CH₂–CH₂–CH₂–CH₂–CHO
Q7. How would you name the following compounds?
(a) CH₃–CH₂–Br
(b) CH₃–CH₂–CH₂–OH
(c) CH₃–CH₂–CHO
Answer:
(a) Bromoethane
(b) Propanol
(c) Propanal
Q8. Why is the conversion of ethanol into ethanoic acid an oxidation reaction?
Answer:
The conversion of ethanol into ethanoic acid is an oxidation reaction because oxygen is added to ethanol during the reaction.
Ethanol is oxidised to ethanoic acid in the presence of an oxidising agent such as alkaline potassium permanganate or acidified potassium dichromate.
Reaction:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Q9. 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 produces an incomplete and sooty flame due to insufficient oxygen.
A mixture of ethyne and pure oxygen produces a very hot flame, which is suitable for welding and cutting metals.
SECTION 2: EXERCISE QUESTIONS AND ANSWERS
Q1. What is a homologous series? Explain with an example.
Answer:
A homologous series is a group of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a –CH₂– unit.
Example: Alkane series.
Methane – CH₄
Ethane – C₂H₆
Propane – C₃H₈
Butane – C₄H₁₀
The difference between two successive members is –CH₂–, having a mass difference of 14 u.
Q2. What is the difference between saturated and unsaturated hydrocarbons?
Answer:
Saturated hydrocarbons contain only single bonds between carbon atoms, whereas unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms.
Examples:
Saturated hydrocarbon: Ethane (C₂H₆)
Unsaturated hydrocarbon: Ethene (C₂H₄)
Unsaturated hydrocarbon: Ethyne (C₂H₂)
Q3. Why are carbon and its compounds used as fuels for most applications?
Answer:
Carbon and its compounds are widely used as fuels because:
They have high calorific values.
They produce a large amount of heat on combustion.
Many of them are easily available.
They can be stored and transported conveniently.
Q4. Explain the formation of covalent bonds in methane.
Answer:
Methane has the molecular formula CH₄.
Carbon has four valence electrons.
Each hydrogen atom has one valence electron.
Carbon shares one electron with each of the four hydrogen atoms.
Four single covalent bonds are formed.
Thus, carbon completes its octet and each hydrogen completes its duplet.
Q5. What are the properties of covalent compounds?
Answer:
The important properties of covalent compounds are:
They generally have low melting and boiling points.
They are generally poor conductors of electricity.
They are formed by sharing electrons.
They usually exist as molecules.
Many covalent compounds are insoluble in water but soluble in organic solvents.
Q6. What is catenation? Why does carbon show catenation?
Answer:
Catenation is the property of an element to form bonds with atoms of the same element, resulting in chains or rings.
Carbon shows catenation because:
Carbon has a small atomic size.
Carbon-carbon bonds are strong.
Carbon can form long chains, branched chains and ring structures.
Q7. What is tetravalency of carbon?
Answer:
Carbon has four electrons in its outermost shell. It needs four more electrons to complete its octet.
Therefore, carbon has a valency of four and is called tetravalent.
It forms four covalent bonds with other atoms.
Q8. What is a functional group? Give examples.
Answer:
A functional group is an atom or group of atoms that determines the characteristic chemical properties of an organic compound.
Examples:
Alcohol – OH
Aldehyde – CHO
Ketone – CO
Carboxylic acid – COOH
Halo group – Cl, Br, I
Q9. What happens when ethanol reacts with sodium?
Answer:
When ethanol reacts with sodium, sodium ethoxide and hydrogen gas are produced.
Chemical equation:
2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
Hydrogen gas is released during the reaction.
Q10. What happens when ethanoic acid reacts with sodium carbonate?
Answer:
Ethanoic acid reacts with sodium carbonate to form sodium ethanoate, carbon dioxide and water.
Chemical equation:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
Carbon dioxide gas is released during the reaction.
Q11. What is esterification? Write its chemical equation.
Answer:
The reaction between a carboxylic acid and an alcohol in the presence of concentrated sulphuric acid to form an ester is called esterification.
Example:
Ethanoic acid reacts with ethanol to form ethyl ethanoate and water.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Esters generally have a pleasant fruity smell.
Q12. What is saponification?
Answer:
The reaction in which an ester reacts with an alkali to produce an alcohol and the salt of a carboxylic acid is called saponification.
This reaction is used in the manufacture of soap.
Q13. Explain the cleansing action of soap.
Answer:
Soap molecules have two parts:
Hydrophilic end: This part is attracted towards water.
Hydrophobic end: This part is attracted towards oil and grease.
When soap is added to water containing dirt and grease, soap molecules arrange themselves into structures called micelles.
The hydrophobic ends attach to grease, while the hydrophilic ends remain in water.
The grease and dirt are surrounded by soap molecules and are washed away with water.
Q14. Why are soaps ineffective in hard water?
Answer:
Soaps are ineffective in hard water because they react with calcium and magnesium salts present in hard water to form an insoluble substance called scum.
This reduces the cleansing action of soap.
Q15. What is the difference between soaps and detergents?
Answer:
| Soaps | Detergents |
|---|---|
| Sodium or potassium salts of long-chain fatty acids | Generally sodium salts of sulphonic acids or sulphates |
| Form scum in hard water | Do not form scum easily in hard water |
| Less effective in hard water | Effective in hard water |
| Generally biodegradable | Some detergents may be non-biodegradable |
Q16. What is combustion? Write the chemical equation for the combustion of methane.
Answer:
Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and usually light.
Chemical equation:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Methane burns in oxygen to produce carbon dioxide, water and heat.
Q17. What is addition reaction? Give an example.
Answer:
An addition reaction is a reaction in which atoms are added to an unsaturated hydrocarbon to form a saturated compound.
Example:
CH₂=CH₂ + H₂ → CH₃–CH₃
This reaction takes place in the presence of a catalyst such as nickel or palladium.
Q18. What is substitution reaction? Give an example.
Answer:
A substitution reaction is a reaction in which one atom or group of atoms in a compound is replaced by another atom or group.
Example:
CH₄ + Cl₂ → CH₃Cl + HCl
This reaction takes place in the presence of sunlight.
Q19. Why does carbon form a large number of compounds?
Answer:
Carbon forms a large number of compounds because of two important properties:
Catenation: Carbon forms bonds with other carbon atoms to make long chains and rings.
Tetravalency: Carbon can form four covalent bonds with other atoms.
Q20. What is the difference between ethanol and ethanoic acid?
Answer:
| Ethanol | Ethanoic Acid |
|---|---|
| Formula is C₂H₅OH | Formula is CH₃COOH |
| It is an alcohol | It is a carboxylic acid |
| It does not turn blue litmus red | It turns blue litmus red |
| It is neutral in nature | It is acidic in nature |
| Used as a solvent and fuel | Used in vinegar and chemical industries |
SECTION 3: IMPORTANT ADDITIONAL QUESTIONS AND ANSWERS
Q1. What is carbon?
Answer: Carbon is a non-metallic element with atomic number 6. It is an important element found in living organisms and many organic compounds.
Q2. What is the atomic number of carbon?
Answer: The atomic number of carbon is 6.
Q3. What is the electronic configuration of carbon?
Answer: The electronic configuration of carbon is 2, 4.
Q4. What is the valency of carbon?
Answer: The valency of carbon is 4.
Q5. What are hydrocarbons?
Answer: Compounds made up of only carbon and hydrogen atoms are called hydrocarbons.
Q6. What are alkanes?
Answer: Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms.
General formula: CₙH₂ₙ₊₂.
Q7. What are alkenes?
Answer: Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond.
General formula for open-chain alkenes with one double bond: CₙH₂ₙ.
Q8. What are alkynes?
Answer: Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond.
General formula for open-chain alkynes with one triple bond: CₙH₂ₙ₋₂.
Q9. What is a covalent bond?
Answer: A covalent bond is a chemical bond formed by the mutual sharing of electron pairs between atoms.
Q10. What is a single covalent bond?
Answer: A single covalent bond is formed when two atoms share one pair of electrons.
Example: H–H.
Q11. What is a double covalent bond?
Answer: A double covalent bond is formed when two atoms share two pairs of electrons.
Example: O=O.
Q12. What is a triple covalent bond?
Answer: A triple covalent bond is formed when two atoms share three pairs of electrons.
Example: N≡N.
Q13. What is an isomer?
Answer: Isomers are compounds having the same molecular formula but different structural arrangements of atoms.
Q14. What is a homologous series?
Answer: A homologous series is a family of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a –CH₂– unit.
Q15. What is the molecular formula of methane?
Answer: CH₄.
Q16. What is the molecular formula of ethane?
Answer: C₂H₆.
Q17. What is the molecular formula of ethene?
Answer: C₂H₄.
Q18. What is the molecular formula of ethyne?
Answer: C₂H₂.
Q19. What is the molecular formula of ethanol?
Answer: C₂H₅OH.
Q20. What is the molecular formula of ethanoic acid?
Answer: CH₃COOH.
Q21. What is the common name of ethanoic acid?
Answer: The common name of ethanoic acid is acetic acid.
Q22. What is vinegar?
Answer: Vinegar is a dilute solution of ethanoic acid in water. It generally contains about 5–8% ethanoic acid.
Q23. What is glacial acetic acid?
Answer: Pure ethanoic acid freezes at approximately 16.6°C and forms ice-like crystals. Therefore, it is called glacial acetic acid.
Q24. What is oxidation?
Answer: Oxidation is a chemical process in which oxygen is added to a substance or hydrogen is removed from it.
Q25. Name two oxidising agents used in the chapter.
Answer:
Alkaline potassium permanganate (KMnO₄).
Acidified potassium dichromate (K₂Cr₂O₇).
Q26. What is denatured alcohol?
Answer: Ethanol mixed with poisonous substances or other chemicals to make it unfit for drinking is called denatured alcohol.
Q27. What is absolute alcohol?
Answer: Pure ethanol containing almost no water is called absolute alcohol.
Q28. What is micelle formation?
Answer: The arrangement of soap molecules in a spherical structure around oil or grease particles in water is called micelle formation.
Q29. Why does soap form scum in hard water?
Answer: Soap reacts with calcium and magnesium ions present in hard water to form insoluble salts called scum.
Q30. Why are detergents better than soaps for washing clothes in hard water?
Answer: Detergents do not form insoluble precipitates with calcium and magnesium ions. Therefore, they remain effective in hard water.
Q31. What is the functional group of alcohol?
Answer: The functional group of alcohol is –OH (hydroxyl group).
Q32. What is the functional group of aldehydes?
Answer: The functional group of aldehydes is –CHO.
Q33. What is the functional group of ketones?
Answer: The functional group of ketones is >C=O.
Q34. What is the functional group of carboxylic acids?
Answer: The functional group of carboxylic acids is –COOH.
Q35. What is the functional group of halo compounds?
Answer: The functional group of halo compounds is a halogen atom such as fluorine (F), chlorine (Cl), bromine (Br) or iodine (I).
SECTION 4: IMPORTANT DIFFERENCES
Q1. Differentiate between saturated and unsaturated hydrocarbons.
Answer:
| Saturated Hydrocarbons | Unsaturated Hydrocarbons |
|---|---|
| Contain only single bonds between carbon atoms | Contain double or triple bonds |
| Generally less reactive | Generally more reactive |
| Undergo substitution reactions | Undergo addition reactions |
| Example: Ethane | Example: Ethene and ethyne |
Q2. Differentiate between alkanes, alkenes and alkynes.
Answer:
| Alkanes | Alkenes | Alkynes |
|---|---|---|
| Saturated hydrocarbons | Unsaturated hydrocarbons | Unsaturated hydrocarbons |
| Have single bonds | Have at least one double bond | Have at least one triple bond |
| General formula CₙH₂ₙ₊₂ | General formula CₙH₂ₙ | General formula CₙH₂ₙ₋₂ |
| Example: Ethane | Example: Ethene | Example: Ethyne |
Q3. Differentiate between ethanol and ethanoic acid.
Answer:
| Ethanol | Ethanoic Acid |
|---|---|
| C₂H₅OH | CH₃COOH |
| Alcohol | Carboxylic acid |
| Neutral in nature | Acidic in nature |
| Does not turn blue litmus red | Turns blue litmus red |
| Used as a solvent and fuel | Used in vinegar |
Q4. Differentiate between soap and detergent.
Answer:
| Soap | Detergent |
|---|---|
| Sodium or potassium salts of long-chain fatty acids | Generally sodium salts of sulphonic acids or sulphates |
| Forms scum in hard water | Does not form scum easily |
| Less effective in hard water | Effective in hard water |
| Generally biodegradable | Some detergents are non-biodegradable |
Q5. Differentiate between addition and substitution reactions.
Answer:
| Addition Reaction | Substitution Reaction |
|---|---|
| Atoms are added to an unsaturated compound | An atom or group is replaced by another atom or group |
| Usually occurs in unsaturated hydrocarbons | Commonly occurs in saturated hydrocarbons |
| Converts unsaturated compounds into saturated compounds | Forms a new substituted compound |
| Example: Hydrogenation of ethene | Example: Chlorination of methane |
SECTION 5: IMPORTANT CHEMICAL REACTIONS
Q1. Combustion of methane.
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Q2. Oxidation of ethanol.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Q3. Reaction of ethanol with sodium.
2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
Q4. Reaction of ethanoic acid with sodium hydroxide.
CH₃COOH + NaOH → CH₃COONa + H₂O
Q5. Reaction of ethanoic acid with sodium carbonate.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
Q6. Reaction of ethanoic acid with sodium hydrogen carbonate.
CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O
Q7. Esterification reaction.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Q8. Addition reaction of ethene with hydrogen.
CH₂=CH₂ + H₂ → CH₃–CH₃
Q9. Addition reaction of ethyne with hydrogen.
HC≡CH + 2H₂ → CH₃–CH₃
Q10. Substitution reaction of methane with chlorine.
CH₄ + Cl₂ → CH₃Cl + HCl
This reaction takes place in the presence of sunlight.
SECTION 6: IMPORTANT FORMULAE AND FUNCTIONAL GROUPS
Methane = CH₄
Ethane = C₂H₆
Propane = C₃H₈
Butane = C₄H₁₀
Pentane = C₅H₁₂
Ethene = C₂H₄
Propene = C₃H₆
Ethyne = C₂H₂
Propyne = C₃H₄
Ethanol = C₂H₅OH
Methanol = CH₃OH
Ethanoic acid = CH₃COOH
Important Functional Groups:
Alcohol = –OH
Aldehyde = –CHO
Ketone = >C=O
Carboxylic acid = –COOH
Halo group = –Cl, –Br, –I, –F
Important General Formulae:
Alkanes = CₙH₂ₙ₊₂
Alkenes = CₙH₂ₙ
Alkynes = CₙH₂ₙ₋₂
SECTION 7: IMPORTANT ONE-MARK QUESTIONS
Q1. What is the atomic number of carbon?
Answer: 6.
Q2. What is the valency of carbon?
Answer: 4.
Q3. What is the electronic configuration of carbon?
Answer: 2, 4.
Q4. Which type of bond is generally formed by carbon?
Answer: Covalent bond.
Q5. What is catenation?
Answer: The ability of carbon to form bonds with other carbon atoms.
Q6. What is the functional group of alcohol?
Answer: –OH.
Q7. What is the functional group of carboxylic acid?
Answer: –COOH.
Q8. What is the molecular formula of methane?
Answer: CH₄.
Q9. What is the molecular formula of ethene?
Answer: C₂H₄.
Q10. What is the molecular formula of ethyne?
Answer: C₂H₂.
Q11. What is the common name of ethanoic acid?
Answer: Acetic acid.
Q12. Which acid is present in vinegar?
Answer: Ethanoic acid.
Q13. What is the product formed when ethanol is oxidised?
Answer: Ethanoic acid.
Q14. Which gas is released when ethanoic acid reacts with sodium carbonate?
Answer: Carbon dioxide.
Q15. Which catalyst is commonly used in hydrogenation of vegetable oils?
Answer: Nickel.
Q16. What is the chemical name of baking soda?
Answer: Sodium hydrogen carbonate (NaHCO₃).
Q17. What is the chemical formula of washing soda?
Answer: Na₂CO₃·10H₂O.
Q18. Which type of reaction is shown by saturated hydrocarbons with chlorine in sunlight?
Answer: Substitution reaction.
Q19. Which type of reaction is shown by unsaturated hydrocarbons with hydrogen?
Answer: Addition reaction.
Q20. What is the difference between two successive members of a homologous series?
Answer: –CH₂–.
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