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The density varies with temperature, but not linearly: as the temperature increases, the density rises to a peak at 3.98 °C (39.16 °F) and then decreases; [33] the initial increase is unusual because most liquids undergo thermal expansion so that the density only decreases as a function of temperature. The increase observed for water from 0 ...
An increase in the temperature of the water above 4 °C causes expansion and the density will decrease. Water expands when it freezes, and a decrease in temperature below 4 °C also causes expansion and a decrease in density. An increase in salinity, the mass of dissolved solids, will increase the density.
Increasing the pressure always increases the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalization. For example, the density of water increases between its melting point at 0 °C and 4 °C; similar behavior is observed in silicon at low temperatures.
A number of materials contract on heating within certain temperature ranges; this is usually called negative thermal expansion, rather than "thermal contraction".For example, the coefficient of thermal expansion of water drops to zero as it is cooled to 3.983 °C (39.169 °F) and then becomes negative below this temperature; this means that water has a maximum density at this temperature, and ...
A change in the temperature of the water impacts on the distance between water parcels directly. [clarification needed] When the temperature of the water increases, the distance between water parcels will increase and hence the density will decrease. Salinity is a measure of the mass of dissolved solids, which consist mainly of salt.
It freezes at a lower temperature (about −1.9 °C (28.6 °F)) and its density increases with decreasing temperature to the freezing point, instead of reaching maximum density at a temperature above freezing. The salinity of water in major seas varies from about 0.7% in the Baltic Sea to 4.0% in the Red Sea.
As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer. A point is reached where the density of the cooling surface water becomes greater than the density of the deep water and overturning begins as the dense surface water moves down under the influence of gravity.
Near Antarctica, temperature is the main contributor for the high density there. Water near the tropics already has high salinity. Evaporation leaves salt behind in the water, increasing salinity and therefore density. As water temperatures are a lot higher, density in the tropics is lower than around the poles.