Fluid phase: Gasses and Liquids

In order to give a general idea about the characteristics of the different phases of matter we will concentrate on one of the more common materials on Earth, water.
Let's first of all consider water in its gas-like phase.
In order to have a complete understanding of this phase we should keep in mind that the temperature of a gas is porportional to the average of the square of the velocity of the particles which compose it.

T = (constant)* < v²>

In the gas phase, molecules have a high velocity so that the interaction among them is irrelevant. The force of attraction which try to stick them one each other is not strong enough.
Due to its high speed-molecules, the gas try to occupy all the volume it is given to it. The net result is that the density of a gas (number of particles per unit of volume) is a very small number and it is constant all over the volume it occupies.
Gasses are isotropic and homogeneous, knowledge of the distribution of the particles in a small region of space gives us information about all the gas.
Let's now imagine to lower the temperature of the gas (i.e. the average square velocity of the molecules). If the velocity decreases enough, the intermolecular forces become important. In the case of water these forces are called dipole-dipole interactions. Molecules under the effect of this force will start to stick together. The density of the gas will slowly change becoming larger wherever the molecules start grouping together. Molecules floating in the gas will prefer to spend most of their time close to the other molecules.
Keeping on lowering the temperature the attractive forces become more and more relevant and the grouping of the particles increases until eventually we get to a different status of matter. The gas turns into liquid. The main difference between a liquid and a gas is their density. The liquid doesn't look homogeneous anymore. there are regions with higher density then others. To be continued