Variation of Boiling Point of Hydrocarbons With Number of Carbon Atoms

Hydrocarbon molecules contain only carbon and hydrogen atoms. The carbon atoms form a chain, sometimes split into branches, even rings, and the hydrogen atoms form bonds with the carbon atoms in the carbon chain.

The length of the carbon chain is not fixed. There can be many carbon atoms in a hydrocarbon. There is a correlation between the number of carbon atoms in a hydrocarbon and the melting points and the boiling points. Most of the melting points are below room temperature, and most of the boiling points are above room temperature, so we are concerned mostly with boiling points. The table below shows boiling points for various numbers of carbon atoms.

Number of Carbon Atoms in Molecule

Boilng Point ° C

1-4

Up to 25

5-7

40-100

8-11

100-150

11-14

150-250

15-19

220-340

20-30

Over 350

30-40

Over 400

The increase in boiling point as the number of carbon atoms – and the lengths of the carbon chains - is due to an increased amount of cross linking and tangling up of the chains.

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