Specification Point 4.10:
Know the trend in colour, boiling point and viscosity of the main fractions

Trends of Crude Oil Fractions

Viscosity: Viscosity refers to the ease of flow of a liquid. High viscosity liquids are thick and flow less easily. If the number of carbon atoms increases, the attraction between the hydrocarbon molecules increases, which results in the liquid becoming more viscous – this is because of the increasing length of the hydrocarbon chains. The liquid flows less easily with the increasing molecular mass.

Colour: As carbon chain length increases, liquid colour gets darker, as it becomes thicker and more viscous.

Melting point/boiling point: As the molecules get larger, the intermolecular attraction becomes greater, so more energy in the form of heat is needed to separate the molecules. With increasing molecular size there is an increase in boiling point.

Volatility: Volatility refers to the tendency of a substance to vaporise. As their molecular size increases, hydrocarbon liquids become less volatile

Different fractions obtained from crude oil:

Crude oil fractions, Edexcel IGCSE Chemistry
Crude oil fraction trends, Edexcel IGCSE Chemistry

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.