Properties of Hydrocarbons

Hydrocarbons are compounds made up of only hydrogen and carbon atoms. The number of atoms within a compound can affect its structure and properties.

Understanding the basics of hydrocarbons is important for organic chemistry, as they are a fundamental component of crude oil.

Hydrocarbons are an incredibly diverse group, so categorising them beyond a few basic properties can be quite complex. However, some of the key features of hydrocarbons include:

  • Boiling point
  • Viscosity
  • Flammability

Boiling point

The boiling point is the temperature at which a substance changes from a liquid to a gas.

As hydrocarbon molecules increase in size, their boiling points also increase due to stronger intermolecular forces. This is due to an increase in the number of electrons in the molecule and a greater surface area. Therefore, it requires more heat energy to break these forces of attraction.

  • For example, ethane (C2H6) has a boiling point of -89 °C, while heptane (C7H16) has a boiling point of 98 °C


Viscosity is a measure of a liquid’s resistance to flow, often referred to as its ‘thickness’. A highly viscous liquid tends to be thick and does not flow easily, whereas a liquid with low viscosity is runny, so it flows more easily.

As the size of the hydrocarbon increases, its viscosity also increases. This is due to the increasing intermolecular forces of attraction between the molecules.


Flammability indicates how easily a hydrocarbon can combust. Smaller hydrocarbons are much more flammable than larger hydrocarbons, so they are very effective as fuels, releasing large amounts of energy.

As hydrocarbons increase in size, they become less flammable.