Intermolecular forces
Read our general notes on Risk Assessment
Why can water and petrol not mix?
Ulex replies ...
It's all a question of intermolecular forces. Water molecules are strongly attracted to each other by all three of the types of intermolecular force, especially the hydrogen bonding. Only molecules or ions that can interact strongly with water molecules can mix with them freely.
Hydrocarbon molecules are attracted to each other by van der Waals forces. If hydrocarbon and water molecules come into contact, the only forces of attraction between them are relatively weak van der Waals forces and the forces caused by the water molecules inducing polarity in what would otherwise be non-polar molecules. These forces are no match for the strong forces, especially the hydrogen bonding, which have to be broken if the molecules were to mix.
To put it bluntly, the mixing, if it occurred would be so endothermic as to be virtually impossible.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 1 ratings so far
, rated at 
updated: 12 February 2004
