[Byrne]

Okay, so I'm in the process of doing a project on the Van der Waals Equation and I've come across a statement from one of his biographies that I don't quite understand.  It refers to the thesis he published in 1873 that earned him his doctor's degree:

"In this thesis he put forward an "Equation of State" embracing both the gaseous and liquid state; he could demonstrate that these two states of aggregation not only merge into each other in a continuous manner, but they are in fact of the same nature."

ALSO

"The assumption that there is no force of attraction between gas particles cannot be true.  If it was, gases would never condense to form liquids."

I really have no clue what this means and I'd apprecaite it if someone could perhaps elobrate or try to put it in plain old English for me.  Thanks.

EDIT: Also, having come across van der Waals constants, I'd like to know how they are determined.  I'm not having very much luck with my internet research right now.

[Donaldson Tan]

"The assumption that there is no force of attraction between gas particles cannot be true.  If it was, gases would never condense to form liquids."

(P + a/V²)(V - b) = RT

the VDW equation is a modified form of the perfect gas equation. To take in account of attractive and repulsive forces between gas particles, Van Der Waal's introduces the constants a and b.

Constant a describes the intermolecular attraction/repulsion, whereas P refers to the forces due to momentum transfer when gas particles collide. Considering a volume of gas, it can be seen that the surface molecules seem to be attracted inwards. This diminishing of pressure is taken in account in the constant a.

Constant b describes the total volume of the gas particules, and V describes the volume occupied by the gas.

Hence, a is called the attraction perimeter and b is called the repulsion perimeter.