Density of natural waters, Chemistry

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Certain basic properties of lakes and oceans are important to understanding how they "work". One of these is the density of the water, and its changes. Dense (or heavier) water wants to ?ow underneath lighter water, and this tendency (plus a lot of math involving Newton's laws on a rotating earth) can be used to explain pretty much everything.

But what determines the density of so-called NATURAL waters? Temperature is important, but so is the amount of material dissolved in the water. One measure of this is the ABSOLUTE SALINITY, which is the total amount of dissolved stuff measured in units of grams of solute per kilogram of solution. Seawater has absolute salinities of 2-35 g/kg, but even fresh water in lakes (and our drinking water) has small amounts of dissolved material (typically .1-1 g/kg in most lakes, but sometimes up to 70 or more g/kg in so-called saline lakes, which usually are in hot dry places with a lot of evaporation).

To really know the density of a natural water, you must actually measure the density of a water sample. However, precision density measurements are dif?cult. In many cases it would be easier if you could estimate the density from some kind of a formula. This formula will depend on the chemical composition of the dissolved material. Waters with the same SALINITY can easily have different densities, if the dissolved materials are chemically different. Imagine having a container containing exactly 1000 g of pure water, which would have a volume of 1000/ρW = 1002.9608 cm3 at 25?C. If we dissolve a mass δm of material in that water, two things will happen:

  • the mass of the ?uid will increase (by δm, the amount of material you add)
  • the volume of the ?uid will change (by an amount δV that depends on what you add).

 


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