Boiling Point

  • The hydrocarbons are grouped together into homologous series according to their mass and chain length.
  • Gradation in the physical properties of a homologous series can be seen in the trend in boiling points of the alkanes.
  • Each alkane has a boiling point that is higher than the one before it.
  • As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them.
  • This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point.

Boiling Point Table, IGCSE & GCSE Chemistry revision notes

  • This is more easily seen on a graph.

Alkanes - Boiling Point Graph, IGCSE & GCSE Chemistry revision notes

Graph showing gradation in the boiling points of the first 8 alkanes


  • Viscosity refers to the ease of flow of a liquid.
  • High viscosity liquids are thick and flow less easily.
  • Viscosity also increases with increasing chain length.


  • Molecular size again influences the ease of ignition.
  • Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules.

Combustion of Alkanes

  • These compounds undergo complete and incomplete combustion.
  • Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

CH4+ 2O2→CO2+ 2H2O

  • Incomplete combustion occurs when there is insufficient oxygen to burn so either carbon monoxide and water or carbon and water form e.g:

2CH4+ 3O2→ 2CO + 4H2O

CH4+ O2→ C + 2H2O

Toxicity of Carbon Monoxide

Incomplete combustion occurs when there is insufficient oxygen to burn.
The products of these reactions are unburnt fuel, carbon monoxide and water.
Methane for example, undergoes incomplete combustion in an oxygen poor environment:

2CH4+ 3O2→ 2CO + 4H2O

CH4+ O2→ C + 2H2O

  • Carbon monoxide is an extremely poisonous gas as it combines with hemoglobin in blood and prevents it from carrying oxygen.
  • It is particularly malevolent as it is colourless, odourless and tasteless, making it difficult to detect.
  • A lack of oxygen supply to the brain can lead to fainting, coma or in worst case scenarios, even death.
  • Incomplete combustion occurs in some appliances such as boilers and stoves as well as in internal combustion engines, where space is cramped.
  • Incomplete combustion of gasoline in car engines is a major source of CO and C:

C8H18 + 8.5O2 → 8CO + 9H2O

C8H18 + 4.5O2 → 8C + 9H2O

  • The carbon particles released clump together to form soot which gradually falls back to the ground.
  • Soot causes respiratory problems and covers buildings and statues, making them look unclean.

Soot Statue, Edexcel GCSE Chemistry

Statues and monuments in densely populated areas become blackened over time from soot

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.