Give two importance of dissolved salt in water. Give the advantage of anomalous expansion of water.
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Answer:
At 4 °C water expands on cooling. This density is maximum together with the low ice density results in (i) the necessity that all of a body of fresh water (not just its surface) is close to 4 °C before any freezing can occur, (ii) the freezing of rivers, lakes and oceans is from the top to down, so permitting survival of the bottom ecology, insulating the water from further freezing, reflecting back sunlight into space and allowing rapid thawing, and (iii) density driven thermal convection causing seasonal mixing in deeper temperate waters carrying life-providing oxygen into the depths. The large heat capacity of the oceans and seas allows them to act as heat reservoirs such that sea temperatures vary only one third as much as land temperatures and so moderate our planet's climate (for example, the Gulf stream carries tropical warmth to northwestern Europe). The compressibility of water reduces the sea level by about 40 m giving us 5% more land . Water's high surface tension plus its expansion on freezing encourages the erosion of rocks to give soil for our agriculture.