One litre of air holds about 210 mL of oxygen.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
One litre of air holds about mL of oxygen. One litre of river water holds about mL. (a) How many litres of river water hold as much oxygen as one litre of air? (b) A fish uses mL of oxygen in an hour, and its gills take out half of the oxygen in the water. How many litres of water must pass over its gills in an hour?
Need a reminder? The key ideaShow
Gases are exchanged by diffusion across a large, thin, moist surface. Water holds far less dissolved oxygen than air, so fish move a lot of water over their gills. Insects use tracheae, land vertebrates use lungs, and plants use stomata and lenticels. By day photosynthesis uses more carbon dioxide than respiration makes; at night only respiration goes on.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Water for one litre of air. L of river water.
Step 2. Oxygen the fish gets from each litre. Half of mL is mL.
Step 3. Water in an hour. L.
Answer: (a) L of river water. (b) L of water in an hour. That is why fish breathe so fast.
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