State that petroleum is a mixture of hydrocarbons

Organic Chemistry – Fuels (Cambridge IGCSE Chemistry 0620)

Learning Objectives

• Identify the three main fossil fuels.
• State that hydrocarbons contain only carbon and hydrogen.
• State that petroleum is a mixture of hydrocarbons and that it is the only fossil fuel that is liquid at room temperature.
• Describe the fractional‑distillation process, including the pre‑heating of crude oil, and explain how the physical properties of the fractions change down the column.
• Recall the main uses of each petroleum fraction.
• Recognise the basic environmental impact of burning hydrocarbon fuels, including the role of sulphur.

Fossil Fuels

• Coal – solid fuel, mainly carbon.
• Natural gas – gaseous mixture, dominated by methane (CH₄).
• Petroleum (crude oil) – liquid mixture of hydrocarbons; the only fossil fuel that is liquid at ambient temperature, making it the primary source of transport fuels.

What is Petroleum?

Petroleum, also called crude oil, is a naturally occurring liquid found in underground reservoirs. It originates from the remains of ancient marine organisms that, over millions of years, have been subjected to heat and pressure. The result is a complex mixture of hydrocarbons – molecules that contain only carbon (C) and hydrogen (H).

Hydrocarbon Families Present in Petroleum

Each petroleum fraction typically contains a mixture of these families: lighter fractions (e.g., petrol) are rich in alkanes and cycloalkanes, while heavier fractions contain an increasing proportion of aromatic molecules.

Preparation of Crude Oil for Distillation

• Crude oil is first pumped from the reservoir and stored in a tank.
• Before entering the distillation column it is pre‑heated (≈ 350 °C) to vaporise the hydrocarbons.
• Water, salts and solid impurities are removed in a “desalting” step to prevent corrosion and fouling of the column.

Fractional Distillation of Petroleum

In a fractional‑distillation column the heated vapour rises through a temperature gradient. Components with lower boiling points travel higher up the column and are collected as lighter fractions; those with higher boiling points condense lower down and form heavier fractions.

How Physical Properties Change Down the Column

Main Uses of the Fractions

• Gas: domestic cooking and heating; feedstock for chemicals such as ammonia and methanol.
• Petrol (gasoline): fuel for cars, motorcycles and small engines.
• Naphtha: petrochemical feedstock for making plastics, synthetic fibres and solvents.
• Kerosene: jet fuel, heating, lamps and some types of portable stoves.
• Diesel: trucks, buses, trains, some cars and marine engines.
• Fuel oil: heating for large buildings, power‑station boilers.
• Lubricating oil: engine oils, hydraulic fluids and machinery lubricants.
• Bitumen (asphalt): road surfacing, roofing and waterproofing.

Environmental Impact of Burning Hydrocarbon Fuels

• Carbon dioxide (CO₂) – greenhouse gas produced in complete combustion.
• Carbon monoxide (CO) and soot (particulate carbon) – products of incomplete combustion; harmful to health.
• Nitrogen oxides (NO_x) – formed when atmospheric nitrogen reacts with oxygen at high temperatures; contribute to smog and acid rain.
• Sulphur oxides (SO_x) – arise from sulphur compounds naturally present in crude oil; also cause acid rain.
• Understanding that petroleum is a mixture of hydrocarbons explains why these pollutants are formed and why cleaner alternatives are being developed.