This equilibrium occurred when the ocean's water could not dissolve any more material in solution. Similarities between fossilized sea life and organisms living today indicate that the composition of sea water stopped changing drastically about 600 million years ago. The relative abundance of the major salts in sea water are constant regardless of the ocean. Calculations of sea water salinity are made of the parts per 1000 of the chlorine ion present in one kilogram of sea water. Typically, sea water has a salinity of 35 parts per thousand.
Water is one of the few substances existing on the Earth's surface in all three forms of matter.
At zero degrees Celsius liquid water turns into ice and has a density of approximately 917 kilograms per cubic meter. Liquid water at the same temperature has a density of nearly 1,000 kilograms per cubic meter. The density of sea water generally increases with decreasing temperature, increasing salinity, and increasing depth in the ocean. The density of sea water at the surface of the ocean varies from 1,020 to 1,029 kilograms per cubic meter. Highest densities are achieved with depth because of the overlying weight of water.