This web site gives a derivation of the Van der Waals equation and explains the effective space occupied by a spherical molecule. http://www.nyu.edu/classes/tuckerman/stat.mech/lectures/lecture_10/ node3.html

The rationale seems to be that to have an influence the molecules have to be “in touch” at the Van der Waals radius. In effect the occupied space is a sphere the radius of which is the diameter of one of the molecules. V= 4/3 pi D^3. The effective volume of one molecule is then 2/3 pi D^3. Which equals 2/3 pi [2r]^3 = 16/3 pi r^3. The actual volume of the molecule is 4/3 pi r^3, hence the factor of 4. The extra volume can be thought of as the extra packing volume of the molecules. Since the VdW equation is a big improvement over the ideal gas law but still has its imperfections; it has to be thought of as a first, important, step in understanding the behavior of gases as they depart from ideal behavior, not as the total answer. Another thought here is that it can be difficult, when constructing a model to explain a molecular [or any] system, to adequately describe every aspect of the system. It has to be tested empirically. Improper models have gotten many an engineer, politician, or social scientist and others involved in their escapades into deep trouble.

The simplest answer to why molecules rotate is that they can. If they are physically restricted from rotation because of bond structure or crystal orientation etc., they don’t. A quantity of molecules [or any material] that is above absolute zero contains excess energy above the material zero point energy.. This energy is manifested according to the “Law of Equipartition of Energy” http://www.schoo ljunction.com/equipartition_law.htm and is distributed among the different degrees of freedom of the substance according to the Boltzmann distribution

Re: Volatility of water at high temperatures contains some references to the Boltzmann distribution.

A substance absorbs energy by collisions with more energetic particles or by absorption of radiation of correct frequencies. Eventually as temperature equilibrium is reached and there is distribution of molecular velocities appropriate to the temperature. Any collisions that are off- center so to speak will cause the molecule or portions of the molecule to rotate. When equilibrium is reached the molecules will be distributed among the quantized rotational states. If the collisions are energetic enough they will excite the molecule to higher vibrational, or even electronic states, both of which are also quantized, and a certain fraction of the molecules will will be in the higher, excited, states. These processes are well described in the many books on infrared, visible, ultraviolet and Raman spectroscopy and are usually outlined in most basic chemistry and organic and physical chemistry textbooks.

Energy excites matter; Knowledge excites scientists. Keep thinking and asking questions and spend as much time as you can in the library reading about these things.

JimG