IGCSE Chemistry — Organic Chemistry: Notes, Practice Questions & Worked Solutions

IGCSE Chemistry 0620 · Topic 11: Organic Chemistry

Organic Chemistry

Organic chemistry is built around “families” of similar compounds called homologous series — once you know the pattern for one member, you can predict the behaviour of the whole family.

1. Homologous Series

A homologous series is a family of compounds with the same general formula and similar chemical properties, where each successive member differs by a CH₂ unit.

Series	General formula	Functional group / feature
Alkanes	CₙH₂ₙ₊₂	Single bonds only (saturated)
Alkenes	CₙH₂ₙ	Contains a C=C double bond (unsaturated)
Alcohols	CₙH₂ₙ₊₁OH	−OH group
Carboxylic acids	CₙH₂ₙ₊₁COOH	−COOH group

Worked example — testing for unsaturation How would you distinguish an alkene from an alkane using a simple chemical test?
Add bromine water to each. An alkene decolourises orange bromine water (as it reacts by addition across the double bond). An alkane does not react, so the bromine water stays orange.

2. Fractional Distillation of Crude Oil

Crude oil is separated into fractions by boiling point in a fractionating column: refinery gas, petrol (gasoline), naphtha, kerosene, diesel, fuel oil, lubricating oil, and bitumen (from lowest to highest boiling point).

Going down the column (increasing boiling point): chain length increases, viscosity increases, flammability decreases, and volatility decreases.

3. Alcohols, Esters and Polymers

Ethanol can be produced by fermentation (sugar → ethanol + CO₂, using yeast, at ~30°C) or by hydration of ethene (steam + ethene, with a catalyst, at high temperature/pressure) — fermentation is slower but uses renewable resources; hydration is faster and continuous but uses a non-renewable feedstock (ethene from crude oil).

Esterification: alcohol + carboxylic acid → ester + water

Addition vs condensation polymers Addition polymers (e.g. poly(ethene)) form when many unsaturated monomers (alkenes) join together, with no other product formed. Condensation polymers (e.g. nylon, PET) form when monomers join with the loss of a small molecule (usually water) at each link, and typically involve two different types of monomer.

Common mistakes to watch for

Forgetting alkanes are “saturated” (single bonds only) and alkenes are “unsaturated” (contain a double bond) — this vocabulary appears constantly in exam questions.
Mixing up which reactant is the alcohol and which is the acid in esterification.
Assuming all polymers are addition polymers — condensation polymers (proteins, nylon, PET) are just as important in this syllabus.

Key Vocabulary Recap

Saturated — contains only single carbon-carbon bonds

Unsaturated — contains at least one carbon-carbon double bond

Monomer — a small molecule that joins with others to form a polymer

Syllabus Reference

This note covers IGCSE Chemistry 0620 Topic 11, Organic Chemistry: homologous series, functional groups, alkanes, alkenes, fuels and petroleum, fractional distillation, alcohols, carboxylic acids, esters, addition/substitution reactions, and polymers.

Practice questions

IGCSE Chemistry 0620 · Topic 11: Organic Chemistry — Practice Question Bank (no answers)

Organic Chemistry — Practice

Covers: homologous series, alkanes/alkenes, fractional distillation, alcohols, esters, and polymers. 10 questions.

Q1 Core

Define a homologous series.

Q2 Core

State the general formula for alkanes and for alkenes.

Q3 Core

Describe a chemical test that distinguishes an alkene from an alkane.

Q4 Core

Explain the meaning of “saturated” and “unsaturated” hydrocarbons.

Q5 Core

List the fractions obtained from the fractional distillation of crude oil, in order from lowest to highest boiling point.

Q6 Core

State how viscosity and flammability change as you go down the fractionating column (increasing boiling point).

Q7 Extended

Compare the fermentation and hydration methods of producing ethanol, giving one advantage of each.

Q8 Core

Write the general word equation for esterification.

Q9 Extended

Explain the difference between an addition polymer and a condensation polymer.

Q10 Core

Name the monomer used to make poly(ethene).

Worked solutions

Show worked solutions

IGCSE Chemistry 0620 · Topic 11: Organic Chemistry — Solutions

Organic Chemistry — Practice: Solutions

Define homologous series.

✓A homologous series is a family of compounds with the same general formula, similar chemical properties, and a gradual change in physical properties (like boiling point) as chain length increases, with each successive member differing by a CH₂ unit.

General formulas of alkanes/alkenes.

✓Alkanes: CₙH₂ₙ₊₂. Alkenes: CₙH₂ₙ.

Test to distinguish alkene from alkane.

✓Add bromine water. An alkene decolourises the orange bromine water (addition reaction across the C=C double bond). An alkane does not react, so the bromine water stays orange.

Saturated vs unsaturated.

✓Saturated hydrocarbons contain only single carbon-carbon bonds (e.g. alkanes). Unsaturated hydrocarbons contain at least one carbon-carbon double (or triple) bond (e.g. alkenes).

Fractions of crude oil, low to high boiling point.

✓Refinery gas, petrol (gasoline), naphtha, kerosene, diesel, fuel oil, lubricating oil, bitumen.

Viscosity/flammability trend down the column.

✓Going down the column (increasing boiling point/chain length), viscosity increases (fractions become thicker/more syrupy) and flammability decreases.

Fermentation vs hydration for ethanol.

✓Fermentation uses sugar and yeast at around 30°C, is a batch (slow) process, but uses cheap, renewable raw materials. Hydration reacts ethene with steam using a catalyst at high temperature/pressure, is a fast, continuous process producing pure ethanol, but relies on ethene from crude oil, a non-renewable resource.

Esterification word equation.

✓alcohol + carboxylic acid → ester + water

Addition vs condensation polymer.

✓An addition polymer forms when many unsaturated (alkene) monomers join together, with no other product formed. A condensation polymer forms when monomers (often two different types) join together with the loss of a small molecule, usually water, at each new bond.

Q10

Monomer of poly(ethene).

✓Ethene

Book a free trial class

Tell us a little about your requirement.