Introduction
Hydrocarbons is an important chapter of Class 10 Chemistry that introduces organic compounds composed entirely of carbon and hydrogen atoms. This chapter explains the classification of hydrocarbons into alkanes, alkenes, and alkynes, their structures, general formulas, physical and chemical properties, and important chemical reactions.
Students also learn about petroleum and natural gas as major natural sources of hydrocarbons, the process of fractional distillation of petroleum, isomerism, combustion, cracking, hydrogenation, and polymerization. These concepts help explain the importance of hydrocarbons as fuels, industrial raw materials, and the building blocks of many everyday products.
This page contains carefully selected Class 10 Chemistry Chapter 2 Hydrocarbons MCQs with answers and explanations, along with learning outcomes, quick revision notes, important definitions, formulas, exam tips, and related resources to help students prepare effectively for Punjab Board, Sindh Board, FBISE, and other board examinations.
Learning Outcomes
After studying this chapter, students will be able to:
- Define hydrocarbons and classify them.
- Differentiate between alkanes, alkenes, and alkynes.
- Write the general formulas of different hydrocarbon series.
- Explain saturated and unsaturated hydrocarbons.
- Describe important reactions of hydrocarbons.
- Understand isomerism in organic compounds.
- Explain the sources of hydrocarbons.
- Describe fractional distillation of petroleum.
- Explain the industrial uses of hydrocarbons.
Quick Notes – Chapter Summary
- Hydrocarbons contain only carbon and hydrogen atoms.
- Alkanes are saturated hydrocarbons with single covalent bonds.
- Alkenes contain at least one carbon-carbon double bond.
- Alkynes contain at least one carbon-carbon triple bond.
- Alkanes have the general formula CnH₂n+₂.
- Alkenes have the general formula CnH₂n.
- Alkynes have the general formula CnH₂n−₂.
- Petroleum and natural gas are major natural sources of hydrocarbons.
- Fractional distillation separates petroleum into useful fractions.
- Complete combustion produces carbon dioxide, water, and heat.
- Incomplete combustion produces poisonous carbon monoxide.
- Polymerization converts small molecules into polymers used in plastics.
Important Definitions
Hydrocarbon: An organic compound containing only carbon and hydrogen atoms.
Alkane: A saturated hydrocarbon containing only single covalent bonds.
Alkene: An unsaturated hydrocarbon containing at least one double bond.
Alkyne: An unsaturated hydrocarbon containing at least one triple bond.
Isomer: A compound having the same molecular formula but a different structural arrangement.
Polymerization: The process in which many small molecules (monomers) join together to form a large molecule (polymer).
Cracking: The process of breaking large hydrocarbon molecules into smaller and more useful hydrocarbons.
Fractional Distillation: A method used to separate petroleum into different useful fractions based on boiling points.
Important Formulas
General Formula of Alkanes
CnH₂n+₂
General Formula of Alkenes
CnH₂n
General Formula of Alkynes
CnH₂n−₂
Complete Combustion of Hydrocarbons
Hydrocarbon + O₂ → CO₂ + H₂O + Heat
Important Concepts
Combustion of hydrocarbons
Saturated and unsaturated hydrocarbons
Homologous series
Alkanes, alkenes, and alkynes
Isomerism
Hydrogenation
Cracking
Polymerization
Fractional distillation of petroleum
Natural gas
Class 10 Chemistry Chapter 2 – Hydrocarbons MCQs
1. Which elements are present in hydrocarbons?
- A. Carbon, hydrogen, and oxygen
- B. Carbon and hydrogen only ✅
- C. Carbon, hydrogen, and nitrogen
- D. Carbon and oxygen only
Explanation: Hydrocarbons are organic compounds containing only carbon and hydrogen atoms.
2. Which type of hydrocarbon contains only single covalent bonds?
- A. Alkenes
- B. Alkynes
- C. Alkanes ✅
- D. Aromatics
Explanation: Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms.
3. What is the general formula of alkanes?
- A. CnH2n
- B. CnH2n−2
- C. CnH2n+2 ✅
- D. CnHn
Explanation: The general formula of alkanes is CnH2n+2 for straight-chain hydrocarbons.
4. Which hydrocarbon series is known as olefins?
- A. Alkenes ✅
- B. Alkynes
- C. Aromatics
- D. Cycloalkanes
Explanation: Alkenes are unsaturated hydrocarbons with at least one double bond, historically called olefins.
5. What is the general formula for alkenes?
- A. CnH2n+2
- B. CnH2n ✅
- C. CnH2n−2
- D. CnHn
Explanation: Alkenes follow the formula CnH2n for straight-chain compounds.
6. Which hydrocarbon series is known as acetylenes?
- A. Alkenes
- B. Alkynes ✅
- C. Aromatics
- D. Cycloalkanes
Explanation: Alkynes are unsaturated hydrocarbons with at least one triple bond, historically called acetylenes.
7. Which reaction converts ethene to ethane?
- A. Substitution
- B. Hydrogenation ✅
- C. Combustion
- D. Polymerization
Explanation: Hydrogenation adds hydrogen to an alkene, converting it into an alkane.
8. Which reaction produces polymers from hydrocarbons?
- A. Hydrogenation
- B. Cracking
- C. Polymerization ✅
- D. Isomerization
Explanation: Polymerization links monomer units to form large molecules called polymers.
9. What is isomerism in hydrocarbons?
- A. Same formula, same structure
- B. Same molecular formula but different structural arrangement ✅
- C. Different formulas and structures
- D. Different formulas, same properties
Explanation: Isomers have the same molecular formula but differ in the arrangement of atoms.
10. Which fraction from petroleum contains the smallest hydrocarbons?
- A. Kerosene
- B. Petroleum gas ✅
- C. Diesel
- D. Fuel oil
Explanation: Petroleum gas contains the lightest hydrocarbons and is used as a fuel.
11. Which process breaks large hydrocarbon molecules into smaller ones?
- A. Polymerization
- B. Cracking ✅
- C. Hydrogenation
- D. Distillation
Explanation: Cracking decomposes heavy hydrocarbons into lighter, more useful products.
12. Which hydrocarbon is the main component of natural gas?
- A. Ethane
- B. Methane ✅
- C. Propane
- D. Butane
Explanation: Methane (CH4) is the major component of natural gas.
13. What is the product of complete combustion of hydrocarbons?
- A. Carbon dioxide and water ✅
- B. Carbon monoxide and water
- C. Carbon and water
- D. Carbon dioxide only
Explanation: Complete combustion produces CO2 and H2O with release of energy.
14. Which environmental problem is caused by incomplete combustion of hydrocarbons?
- A. Ozone layer depletion
- B. Carbon monoxide poisoning ✅
- C. Acid rain
- D. Global cooling
Explanation: Incomplete combustion forms CO, a toxic gas that binds with hemoglobin.
15. Which polymer is produced from ethene?
- A. PVC
- B. Polyethylene ✅
- C. Polystyrene
- D. Bakelite
Explanation: Polyethylene is formed by polymerizing ethene molecules.
16. A homologous series is a group of compounds having:
- A. Different functional groups
- B. The same functional group and similar chemical properties ✅
- C. Different molecular formulas and different properties
- D. The same molecular mass only
Explanation: Members of a homologous series have the same functional group, similar chemical properties, and successive members differ by a –CH₂ unit.
17. Which hydrocarbon decolourizes bromine water?
- A. Methane
- B. Ethane
- C. Ethene ✅
- D. Propane
Explanation: Unsaturated hydrocarbons such as ethene react with bromine water, causing its brown colour to disappear.
18. Which petroleum fraction is commonly used as fuel for jet aircraft?
- A. Petroleum gas
- B. Kerosene ✅
- C. Diesel oil
- D. Bitumen
Explanation: Kerosene is an important fraction of petroleum and is widely used as aviation fuel.
19. Successive members of a homologous series differ by:
- A. CH
- B. CH₂ ✅
- C. CH₃
- D. C₂H₂
Explanation: Each successive member of a homologous series differs from the previous one by one methylene (–CH₂–) group.
20. Which process is used in petroleum refineries to separate crude oil into different useful fractions?
- A. Cracking
- B. Polymerization
- C. Fractional distillation ✅
- D. Hydrogenation
Explanation: Fractional distillation separates crude oil into different fractions according to their boiling points.
Exam Tips
- Memorize the general formulas of alkanes, alkenes, and alkynes.
- Learn the difference between saturated and unsaturated hydrocarbons.
- Understand the major reactions such as combustion, hydrogenation, cracking, and polymerization.
- Revise petroleum fractions and their common uses.
- Practice identifying hydrocarbons from their molecular formulas.
Common Mistakes
❌ Confusing the formulas of alkanes, alkenes, and alkynes.
❌ Thinking alkenes are saturated hydrocarbons.
❌ Mixing hydrogenation with polymerization.
❌ Forgetting that methane is the main component of natural gas.
❌ Confusing complete combustion with incomplete combustion.
Conclusion
Hydrocarbons form the foundation of organic chemistry and are among the most important compounds used as fuels and industrial raw materials. Understanding their classification, properties, reactions, and natural sources helps students build a strong foundation for advanced chemistry topics. Regular practice of Hydrocarbon MCQs strengthens concepts and improves board examination preparation.