After a think about all the confusing language, I suppose a simple way to visualise the pressure and density thing, is as follows: In a cylinder with a piston, some liquid water is in the cylinder, trapped by the piston. If the air is pumped out, and the cylinder stood upright, then the water is frozen. After freezing, the piston is moved so the ice is stuck to it but not to the cylinder. Then the water is melted, then vaporised. When solid, liquid or gas the water has the same molecular mass, but flow and pressure are different material properties. Compress the water when it's a vapor and you have to push against the expansion the heat gives the gas. Liquid and solid flow are 'compressed' by being denser, but the molecular densities are very similar so liquid flow is translation of density-volume, equivalent to a solid with momentum. Compressing a gas is equivalent to pumping fluid.