on structure of matter and phase transitions

First question - Free energy does the trick.

Second question - Entropy causes Brownian motion.

Is your primary interest why some compounds are gasses and others are liquids at the same temperature and pressure? That has to do with the molecular weight of the compound and the degree of intermolecular bonding energies. Oxygen, Molecular weight 32 and Nitrogen, Molecular weight 28 (diatomic molecules) remain gasses at very high pressures and low temperatures. There is very little intermolecular bonding in these molecules. Helium, Neon, and Xenon, monoatomic moecules are very light, do not display intermolecular bonding, and are very dificult to liquefy. Methane, Molecuar weight 16 takes a fair amount of pressure and cooling, while Carbon Dioxide, Molecular weight 44, is easily compressed and cooled to form a liquid. Ammonia, Molecular weight 17 is even easier to liquefy, and water, Molecular weight 16 is a liquid under standard conditions. The order of intermolecular bonding energies is CH4<CO2<NH3<H2O, so that the very strong intermolecular "Hydrogen" bonding in water due to its physical dipole shape creates the energy to bind it into a liquid.