Understanding Chemistry: Organic chemistry: Edexcel IGCSE Science Double Award Explained

Organic chemistry is the study of carbon-based compounds. In the Edexcel IGCSE Science Double Award, this section takes you from crude oil in the ground to synthetic polymers in everyday life, connecting structure, naming and reactions along the way.

The edexcel igcse science double award chemistry: organic chemistry section of the 4SD0 specification covers five topics: an introduction to organic chemistry, crude oil, alkanes, alkenes and synthetic polymers. The logical thread that runs through them is straightforward. Crude oil provides the raw materials. Fractional distillation separates them. Alkanes and alkenes are the key hydrocarbon families you need to understand. Polymers show how small molecules join to form the large materials that surround us. These edexcel igcse science double award revision notes walk through each topic with the analytical precision the exam demands.

Introduction to organic chemistry

Organic chemistry is the chemistry of carbon compounds. Carbon atoms form four covalent bonds, which allows them to join together in chains, branched structures and rings. This versatility is why there are millions of organic compounds.

A homologous series is a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties. As the chain length increases, boiling point increases (because the intermolecular forces between larger molecules are stronger) and viscosity increases. The igcse specification expects you to understand this trend and apply it when comparing members of the same series.

Molecular formulae show the actual number of each type of atom in one molecule (e.g. C₂H₆ for ethane). Displayed formulae show every bond between every atom. You need to be comfortable drawing displayed formulae for simple organic molecules, as the exam regularly asks for them.

Crude oil

Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms buried under layers of sedimentary rock. It is a complex mixture of hydrocarbons, most of which are alkanes. Because crude oil is a mixture, the different hydrocarbons can be separated by fractional distillation.

Fractional distillation works because the different hydrocarbons in crude oil have different boiling points. The crude oil is heated until most of it vaporises, and the vapour enters a fractionating column. The column is hot at the bottom and cooler at the top. Hydrocarbons with high boiling points condense near the bottom. Hydrocarbons with low boiling points rise further up the column before condensing. Each fraction collected at a different level contains molecules of a similar chain length.

Fraction	Approximate chain length	Use
Refinery gases	C₁ to C₄	Heating, cooking fuel
Petrol (gasoline)	C₅ to C₁₀	Fuel for cars
Kerosene	C₁₀ to C₁₆	Jet fuel
Diesel	C₁₆ to C₂₀	Fuel for lorries, trains
Fuel oil	C₂₀ to C₄₀	Ships, power stations
Bitumen	C₄₀+	Roads, roofing

The trend is clear: shorter chains mean lower boiling points, lower viscosity, greater flammability and lighter colour. Longer chains mean higher boiling points, higher viscosity and darker colour.

Exam tip: When explaining fractional distillation, do not say the crude oil is "separated by boiling point." The accurate statement is that the crude oil is heated, the vapours rise up the column, and each fraction condenses at the level where the temperature matches its boiling point. The column separates by condensation point, not by boiling different fractions at different times.

Alkanes

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2. "Saturated" means they contain only single C-C bonds. The first four members are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀). You should be able to write the molecular formula and draw the displayed formula for each of these from memory.

The principal reaction of alkanes is combustion. Complete combustion (in a plentiful supply of oxygen) produces carbon dioxide and water:

CH₄(g) + 2O₂(g) -> CO₂(g) + 2H₂O(l)

Incomplete combustion (in a limited supply of oxygen) produces carbon monoxide and/or carbon (soot) alongside water. Carbon monoxide is toxic because it binds to haemoglobin in the blood, preventing oxygen transport. Soot contributes to respiratory problems and global dimming. Both are significant environmental and health concerns.

The edexcel igcse science double award also requires you to understand cracking. There is more demand for short-chain hydrocarbons (like petrol) than for long-chain fractions (like fuel oil). Cracking breaks long-chain alkanes into shorter, more useful molecules. The products include shorter alkanes and alkenes. Cracking can be carried out by passing the hydrocarbon vapour over a hot catalyst (catalytic cracking) or by heating it to a high temperature with steam (steam cracking).

For example, decane can be cracked to produce octane and ethene: C₁₀H₂₂ -> C₈H₁₈ + C₂H₄. Notice that the total number of carbon and hydrogen atoms is conserved. The presence of an alkene in the products is always a sign that cracking has occurred.

Alkenes

Alkenes are unsaturated hydrocarbons with the general formula C_nH_2n. "Unsaturated" means they contain at least one C=C double bond. The first two members are ethene (C₂H₄) and propene (C₃H₆). The double bond is what makes alkenes more reactive than alkanes.

The test to distinguish an alkene from an alkane is the bromine water test. When you add bromine water (which is orange) to an alkene, the bromine water is decolorised (turns colourless). With an alkane, no colour change occurs. This is because the alkene undergoes an addition reaction with bromine across the double bond.

Addition reactions are the characteristic reactions of alkenes. The double bond opens up and new atoms add across it. Key examples include:

Hydrogenation: Adding hydrogen across the double bond in the presence of a nickel catalyst converts an alkene into an alkane. This is used industrially to harden vegetable oils into margarine.
Hydration: Adding steam across the double bond in the presence of an acid catalyst (phosphoric acid) converts an alkene into an alcohol. Ethene + steam -> ethanol.
Bromination: Adding bromine across the double bond (the basis of the bromine water test).

Common mistake: Students sometimes write that alkenes undergo "substitution" reactions. Alkenes undergo addition reactions, where atoms add across the double bond. Substitution reactions are characteristic of alkanes (where a hydrogen atom is replaced), not alkenes. Mixing up these two reaction types is a reliable way to lose marks.

Synthetic polymers

Polymers are very large molecules made by joining together many small molecules called monomers. The igcse 4sd0 chemistry: organic chemistry specification focuses on addition polymerisation, where unsaturated monomers (alkenes) join together by opening their double bonds.

The simplest example is polyethene (polythene), made from ethene monomers. Many ethene molecules open their C=C double bonds and link together to form a long chain of -CH₂-CH₂- repeating units. The reaction requires high temperature and high pressure. No other product is formed besides the polymer itself.

Other common addition polymers include polypropene (from propene) and poly(chloroethene), also known as PVC (from chloroethene). Given the monomer, you should be able to draw the repeating unit of the polymer, and given the polymer, you should be able to identify the monomer.

The edexcel igcse science double award explained assessment expects you to discuss the environmental issues associated with polymers. Most addition polymers are non-biodegradable, meaning they do not decompose naturally. This leads to problems with waste disposal: landfill sites fill up, and plastic pollution accumulates in oceans and ecosystems. Solutions include recycling, developing biodegradable polymers, and reducing plastic use. Burning waste polymers can generate energy but also produces toxic gases if the polymer contains chlorine (as PVC does).

Self-check: Can you write the general formula for both alkanes and alkenes? Can you draw the displayed formula for propene and show how it polymerises? Can you balance the equation for the complete combustion of propane? Can you explain why cracking is economically important? If you can answer all four confidently, you have a strong grasp of the organic chemistry required for this qualification.

Connecting the topics

The five chemistry: organic chemistry edexcel igcse topics form a logical chain. Crude oil provides the starting material. Fractional distillation separates it into usable fractions. The fractions contain alkanes, which can be cracked to produce alkenes. Alkenes undergo addition reactions, including addition polymerisation to make polymers. Exam questions in the edexcel igcse science double award practice questions section often span multiple topics within this chain, so understanding the connections is as important as knowing each topic individually.

When you study these edexcel igcse science double award notes, pay particular attention to displayed formulae. Organic chemistry questions almost always require you to draw structures, and a misplaced bond or missing hydrogen atom is enough to lose a mark. Practice drawing the structures of methane, ethane, propane, butane, ethene and propene until they come naturally.

Self-check questions

State two differences between the structure of a plant cell and an animal cell.
Describe the process of osmosis and explain why it is important in living organisms.
Explain the difference between an exothermic and an endothermic reaction, giving one example of each.
A car accelerates from rest to 20 m/s in 5 seconds. Calculate its acceleration and the resultant force if the car has a mass of 1200 kg.
Describe the structure of an atom, naming each subatomic particle and stating its relative charge and mass.

Use the Green Bridge CBT platform for edexcel igcse science double award practice questions on organic chemistry, and revisit the edexcel igcse science double award revision notes to reinforce the logical connections between crude oil, hydrocarbons, reactions and polymers. Analytical thinking pays dividends in this section: if you understand why each reaction happens, the specific details become much easier to remember.