Van der Waals forces

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Question:

What are van der Waals forces?

Answer:

Van der Waals forces are the weak attractions between molecules that allow droplets to condense and provide surface tension. They also play a role in allowing crystals (for example, snowflakes) to form.

The simplest model of a gas is the "ideal gas" model, in which the atoms or molecules that make up the gas do not interact with one another. In the late 1800's Johannes Diderik van der Waals proposed a revision to the ideal gas equation of state to account for phenomena related to liquid-vapor transformations, that could not be explained by the ideal gas law. His insight was to include corrections to account for the volumes and the mutual attraction of the molecules of the gas, and the revised equation is now called the van der Waals equation of state.

We now understand that atoms and molecules have an "excluded volume" --- that is, a size --- because their electrons cannot overlap. We also understand that a weak attraction results from distortions of the cloud of electrons around an atom or molecule induced by that of another, and vice versa. This is called the London dispersion force. The phrase "van der Waals forces" is used rather loosely, and can include both of these contributions.

Water is another matter. In addition to the forces discussed above, water has a dipole moment (a plus and minus pair of charges), and is very polarizable (meaning that its distribution of charge responds readily to other charges). Water also loves to form hydrogen bonds --- the hydrogen in water would like a little bit of electron density, and the oxygen of another water can donate a little spare electron density. Water may well be the most complex of the "simple" liquids!

-Ed Lyman