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

Organic chemistry is the study of carbon compounds, and it connects directly to the fuels, plastics and materials you encounter every day

The organic chemistry section of the Pearson Edexcel IGCSE Science Single Award (4SS0) introduces you to hydrocarbons, crude oil, and the polymers that shape modern life. It is the final chemistry section in the specification, and it draws on the bonding concepts from the principles of chemistry section while pointing outward to real-world applications. For a single award qualification that spans biology, chemistry and physics, organic chemistry is where the chemistry content feels most tangible.

This article provides edexcel igcse science single award revision notes for every specification point in the organic chemistry section. The treatment below is systematic and precise, following the specification order: introduction to organic molecules, crude oil and its fractions, alkanes, alkenes, and synthetic polymers. Use these edexcel igcse science single award notes as a reference, and test your understanding with the self-check questions at the end.

Introduction to organic chemistry

A hydrocarbon is a compound that contains only hydrogen and carbon atoms. This definition is tested directly in exams, and the word "only" is load-bearing. A molecule that also contains oxygen or nitrogen is not a hydrocarbon, even if it is primarily made of carbon and hydrogen.

The specification requires you to represent organic molecules using four types of formula:

At this level, the displayed formula is the one examiners ask you to draw most often. Every bond must be shown as a line, and every atom must be labelled. Partial or ambiguous structures lose marks.

Crude oil

Crude oil is a mixture of hydrocarbons. It is formed over millions of years from the remains of ancient marine organisms buried under layers of sediment. Because it is a mixture, its components can be separated by physical methods, specifically fractional distillation.

Fractions of crude oil

The specification requires you to know the names and uses of the main fractions obtained from crude oil. These fractions are separated in a fractionating column, where the crude oil is heated and the vapours rise. Fractions with lower boiling points rise higher in the column before condensing.

The trends through these fractions are consistent: as the molecules get larger (more carbon atoms), the colour darkens, the boiling point increases, and the viscosity (thickness) increases. Refinery gases are colourless and flow freely; bitumen is black and extremely viscous.

Combustion of hydrocarbons

A fuel is a substance that, when burned, releases heat energy. Hydrocarbons are widely used as fuels, but the products of their combustion depend on the oxygen supply.

Complete combustion occurs when there is plenty of oxygen. The products are carbon dioxide and water:

hydrocarbon + oxygen -> carbon dioxide + water

Incomplete combustion occurs when the oxygen supply is limited. The products may include carbon monoxide (a toxic gas) and carbon (soot), in addition to water:

hydrocarbon + limited oxygen -> carbon monoxide + carbon + water

Carbon monoxide is poisonous because it binds to haemoglobin in red blood cells more strongly than oxygen does, reducing the blood's capacity to carry oxygen. This can lead to oxygen starvation of tissues, which is potentially fatal.

Pollutants from combustion

In car engines, the temperature is high enough for nitrogen and oxygen from the air to react, forming oxides of nitrogen (NO_x). Some hydrocarbon fuels also contain sulfur impurities. When these burn, they produce sulfur dioxide (SO₂). Both sulfur dioxide and oxides of nitrogen dissolve in rainwater to form acids, contributing to acid rain. Acid rain damages buildings, kills aquatic life in lakes and rivers, and harms forests.

Alkanes

Alkanes are the simplest homologous series of hydrocarbons. Their general formula is C_nH_2n+2. They are described as saturated hydrocarbons because all the carbon-carbon bonds are single bonds; there are no double bonds. Every carbon atom is bonded to the maximum possible number of other atoms.

The specification requires you to draw structural and displayed formulae for alkanes with up to five carbon atoms and to name the unbranched-chain isomers:

Alkenes

Alkenes contain at least one carbon-carbon double bond (C=C). This double bond is their functional group. Their general formula is C_nH_2n. They are described as unsaturated hydrocarbons because the double bond means they have fewer hydrogen atoms than the corresponding alkane.

The specification requires you to draw structural and displayed formulae for alkenes with up to four carbon atoms:

The test to distinguish between an alkane and an alkene uses bromine water. Bromine water is orange-brown. When shaken with an alkene, it is decolourised (turns colourless) because the bromine reacts with the double bond. When shaken with an alkane, the bromine water stays orange-brown because alkanes do not react with bromine water under normal conditions.

Synthetic polymers

An addition polymer is formed when many small molecules called monomers join together. The monomer must contain a C=C double bond, which opens up during the reaction so that the molecules can link end to end. No other product is formed; the polymer is the only product.

The simplest example is poly(ethene), formed from ethene monomers. In the repeat unit of poly(ethene), the double bond has opened to form two single bonds that connect to neighbouring monomer units, with each carbon still carrying its two hydrogen atoms.

You need to be able to:

• Draw the repeat unit of poly(ethene) from the monomer ethene.
• Deduce the structure of a monomer from the repeat unit of an addition polymer (reverse the process: close the single bonds back into a double bond and detach the unit from the chain).
• Deduce the repeat unit from the monomer (open the double bond into two single bonds and show the continuation bonds on each side).

Disposal problems

Addition polymers present environmental challenges because they are chemically inert and do not biodegrade. This means they persist in landfill sites for hundreds of years. Burning polymers can produce toxic gases, including hydrogen chloride from PVC and carbon monoxide from incomplete combustion. The specification expects you to explain these problems clearly.

Self-check questions

1. Define the term "hydrocarbon."
2. Name the six fractions of crude oil in order of increasing boiling point and give one use for each.
3. Write the word equation for the complete combustion of a hydrocarbon.
4. Explain why carbon monoxide is dangerous, with reference to haemoglobin.
5. State the general formula for alkanes and draw the displayed formula for propane.
6. State the general formula for alkenes and explain why they are described as unsaturated.
7. Describe the bromine water test and state the result you would see with an alkene and with an alkane.
8. Explain two problems associated with the disposal of addition polymers.

Placing organic chemistry in the wider specification

The chemistry: organic chemistry edexcel igcse section wraps up the chemistry content of the 4SS0 specification. It is a section where the language matters: "hydrocarbon" means only carbon and hydrogen, "saturated" means no double bonds, "unsaturated" means at least one double bond, and "decolourised" is the correct word for the bromine water test. These terms are precise, and the examiner expects them to be used precisely.

For your igcse 4ss0 chemistry: organic chemistry revision, focus on drawing displayed formulae accurately, knowing the crude oil fractions and their trends, and explaining the combustion products and their environmental effects. These are the areas that carry the most marks in the edexcel igcse exam.

Self-check questions

1. State two differences between the structure of a plant cell and an animal cell.
2. Describe the process of osmosis and explain why it is important in living organisms.
3. Explain the difference between an exothermic and an endothermic reaction, giving one example of each.
4. 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.
5. Describe the structure of an atom, naming each subatomic particle and stating its relative charge and mass.

Build on these edexcel igcse science single award notes by working through edexcel igcse science single award practice questions. The edexcel igcse science single award explained approach here gives you the content; past paper practice gives you the exam technique. The Green Bridge CBT platform provides organic chemistry questions organised by specification point, allowing you to target your revision and measure your progress before the exam.