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Rainwater is often
presented as the purest natural sample of water, and
students may be influenced by this. Other students will
also be aware of the idea of ‘acid rain’,
and may know that rainwater can contain other dissolved
substances as well as dissolved air. Students living
in areas where there is ‘hard’ water may
also be aware that water can contain other dissolved
substances as a result of running over or through particular
kinds of rock and soil.
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The particle image
shows a far higher proportion of oxygen and nitrogen
particles than would be the case. |
More on dissolved ‘air’
Many students think that the large bubbles they see when water
boils are air bubbles. It is important to make sure that students
understand that the small bubbles which form below
100°C are ‘dissolved air’, but the
large bubbles which from when water boils are water in the
gas state.
The solubilities of nitrogen and oxygen are both relatively
low: only about 2 ml of ‘air’ dissolves in 100ml
of water at room temperature, compared to 78ml of carbon
dioxide.
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