Page 8 - Curriculum Visions Dynamic Book

Oxygen in water
In the past, oxygen atoms combined with hydrogen atoms to form water. Oxygen is a much more massive element than hydrogen (the atomic weight of hydrogen is 1, that of oxygen is 16). As a result, oxygen makes up about nine-tenths of the mass of the oceans.
The oxygen in the compound water is very strongly bonded to the hydrogen atoms and it will not break up even on heating. This is why when water boils the molecules of water change from liquid water to water vapour, rather than breaking up into hydrogen and oxygen atoms.
The modern atmosphere and oceans contain almost no free hydrogen, so oxygen and hydrogen can no longer combine. Because there is a surplus of oxygen atoms, the atmosphere contains free oxygen. Oxygen also occurs as free oxygen molecules dissolved in water. These
molecules do not combine
with the water molecules.
In most surface water there
may be 45 grams of oxygen
dissolved in every cubic metre
of water. These are the oxygen
molecules that are vital for life
in the water.
 One of the key processes in a water treatment plant is aeration. Artificial fountains aerate the water, providing the oxygen needed by the bacteria that consume waste organic material.
 Stagnant water is still water with little oxygen in it. This situation occurs when microbes living in the water use the oxygen faster than the rate at which oxygen can dissolve into the water. Because oxygen dissolves only slowly in water, still water has a low oxygen content. In warm conditions, microbes grow quickly and can soon use up the dissolved water making it stagnant. This is why stagnant conditions are more common in summer than winter.
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