All matter in the universe exists in one or more of three states: solid, liquid or gaseous and each state is determined by its body temperature and pressure. The best known example of a substance with the three states is water, which in human-survivable temperatures and pressures can exist as ice, water and vapour.
Liquids and gases are called fluids because they can flow and can take the shape of any container into which they are poured. There are however also considerable differences between liquids and gases. A liquid can have separating upper and lower surfaces with other liquids – if they do not mix – or with gases but a gas cannot have a separating surface with another gas. Some liquids mix with other liquids (water and wine) some do not (water and oil). All gases mix with each other and some gases can dissolve in some liquids. Liquids are virtually incompressible whereas gases are compressible.
Many physical, chemical or other properties identify all substances. In pumping we are mainly concerned with properties associated with mass. A force imparts acceleration to a mass. Weight, as a particular form of force, imparts gravitational acceleration to a mass. For easy comparison of various materials, we usually express their masses relative to a unit volume. This physical property is called Density ρ [kg/m3 ]. Occasionally density is expressed in tonnes per cubic metre [t/m3 ] or kg per litre [kg/L], which are numerically equal.
The most common liquid handled by centrifugal pumps is water. At normal ambient pressure and at freezing point (0ºC), water and ice have densities of 999 and 895 kg/m3 respectively, which explains why ice floats on water. At the same pressure but at boiling point (100ºC), water and saturated vapour have densities of 957 and 0.590 kg/m3 respectively, which shows that water expands approximately 1600 times after boiling. The most common solid material handled by centrifugal pumps is silica in the form of sand or rock, whose density is around 2650 kg/m3 . Very often we express unit mass of a material by its Specific gravity or SG, a dimensionless number, which we obtain by dividing the density of the material by the density of water. For this particular purpose, we usually take the density of water as 1000 kg/m3 and so the SG of any material is simply equal to its density divided by 1000. By this reasoning, the SG of water is 1 and that of silica is 2.65.