The ideal gas model makes two strong assumptions: point-like molecules with no interactions. It works very well for rarefied gases, but fails close to liquefaction. In 1873 Van der Waals proposed a simple correction, with just two constants, able to qualitatively describe the liquid-gas transition and the critical point. This section presents the equation and its P-V diagram.
Particles with a Lennard-Jones potential (attraction at range, repulsion up close): at low temperature they condense into a droplet, at high temperature they behave as a gas. This is the microscopic origin of the Van der Waals corrections. Adjust the temperature with the slider: colours show speed (blue slow, orange fast) and the readout at the top shows measured temperature, pressure and the prevailing phase.