MadSci Network: Chemistry

Re: When fish breath do they actually remove an oxygen molecule from H2O?

Date: Tue Feb 13 22:17:15 2001
Posted By: Scott Miller, Post-doc/Fellow, Chemistry, Air Force Research Laboratory
Area of science: Chemistry
ID: 981932050.Ch

Dear Tom,

The short answer to you question is, no. Fish do not remove an oxygen 
molecule directly from H2O. Now, here's the longer version. In actuality, 
the oxygen that fish (and all other animals, humans included) breathe 
looks very different than it does in water (H2O). The oxygen used for 
respiration is represented by the formula O2, which means there are two 
oxygen atoms bonded (stuck) together to form one oxygen molecule. As for 
the oxygen atoms in H2O, they are pretty much stuck there in nature, 
except when a hydrogen (not an oxygen) atom leaves to make H + OH. 
However, H and OH are the important parts of acids and bases, so we don't 
want to rely on these for breathing.

To move on to your other questions, water does contain air to some degree. 
More specific to this question, however, is that air contains oxygen (the 
O2 kind I talked about above), and some of this oxygen is dissolved in 
water. So, what the fish does is uses its gills to filter out this oxygen 
for breathing. So, while fish don't specifically require air to to live, 
neither do any other animals. We all require the oxygen that is in the air 
(about 21% of air is actually oxygen, while 78% is nitrogen, and the other 
1% or so are trace gases).

Your final question is something very important to anyone who owns an 
aquarium. Air is constantly dissolving and "undissolving" from the surface 
of any body of water, be it a glass, a swimming pool, or a lake. Also, 
water is evaporating from the surface of the water, so by the time all the 
air could escape from a cup of water, the water would probably be gone. 
However, in fish tanks where the fish are actively removing oxygen from 
the water, it is important to intentionally re-supply the waters oxygen 
content. This is done using a device called an aerator, which bubbles air 
into the water so it can be dissolved and therefore replinish the oxygen 
content of the water. This is not necessary for large bodies of water, 
because so much of the surface is in contact with air, and also plants 
growing in the water will release oxygen and keep the O2 levels up.

Well, I hope this helps! Good Luck! Scott

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