On earth, there are three dominate phrases of matter: solids, liquids and gases. * The predominate distinction between these types lies in the forces between and the energies within their molecules. In gases, the molecules are extremely energetic and are essentially independent of each other; while in liquids, lessenergetic molecules are loosely attracted to each other and easily wander past each other. Both of these phases take on the shapes of their containers, with gases also taking on the container's volume. They are often grouped together as fluids  that is, they are materials that flow readily under the action of an applied force. In solids, the molecules vibrate about fixed positions and are so strongly attracted that the material appears to be rigid.
Density is defined as the ratio of mass per unit volume. It is generally represented by the Greek letter rho, ρ, and measured in terms of kilograms/cubic meter, or kg/m^{3}.
Since the volume of a fluid expands and contracts, the density of fluids vary with temperature. The most common fluid, water, has maximum density of 1000 kg/m^{3} at 4ºC. Air, a mixture composed principally of the gases nitrogen (78%) and oxygen (21%), has a density of 1.29 kg/m^{3} at 0ºC and 1.20 kg/m^{3} at 20ºC.
How a liquid's density compares to that of water at 4ºC is called its specific gravity. If a liquid has a specific gravity of 0.9, then its density is 0.9 times that of water, or 0.9 x 1000 = 900 kg/m^{3}.
The second property is a fluid's compressibility. This quantity represents how easily the material can be compressed or reduced in volume by a change in pressure. It is generally represented by the letter and measured in terms of pascals ^{1}.
Air has a compressibility value of 1.0 x 10^{5} Pa^{1} while water has a compressibility of 4.5 x 10^{10} Pa^{1}. Because of this extremely small value, liquids are often referred to as being incompressible and we do not need to worry about volume changes in our calculations.
Hydrostatic pressure
Pressure is defined as the ratio of force per unit area where the force is perpendicular to the crosssectional area.
Pressure is a scalar quantity measured in pascals, Pa, where 1 Pa = 1 N/m^{2}.
