Water and Oxygen

Does water give off oxygen?
- Amy Wengert
Lebanon Catholic, USA

Hi Amy,

We’re not sure if you’re asking about separate oxygen molecules dissolved in water or the oxygen which is part of each water molecule.

Most water you find around, in the ocean, lakes, streams, and even coming out of the tap, has air dissolved in it. Air is about 20% oxygen, and so yes, there is dissolved oxygen in the water. This dissolved oxygen is what fish need to respirate, otherwise they would suffocate. So you can say that the water gives off oxygen.

Tap water sometimes gets more air dissolved in it than would be in equilibrium if you just let it sit still in a jar. If you put tap water in a glass or a jar and let it stand for a while, some of the dissolved air will bubble out. When I was small, we always did this before putting the water in the fishtank, to keep bubbles from forming in the fish’s gills.

Water is composed of molecules containing hydrogen and oxygen atoms. These don’t come apart on their own, however. You can break water molecules up by the process of electrolysis. Passing an electrical current through the water will cause oxygen to bubble up on one electrode and hydrogen on the other. The process can go more easily if some acid is added to the water. Electrolysis of water is dangerous! The hydrogen is extremely flammable and can explode.

Tom (w mike)

(published on 10/22/2007)

I had the same question as one you've answered: Does water give off oxygen? A New York Times Science article leads me to ask you for more information. The article concerns Mars as a factor in the origins of life on Earth. Here's an excerpt from the article: A number of studies suggest that the early Earth was covered in water and had few if any continents. As for molybdate, it only forms in the presence of oxygen. The atmosphere of the early Earth appears to have been nearly oxygen-free. So, would the water that covered Earth not give up oxygen to the atmosphere? (sub-query: Does lightning induce electrolysis?)
- David Fasulo (age 63)
Brunswick, Maine, USA

Although there's oxygen in water, in equilibrium almost none of it is present as O₂. That's very familiar from the behavior of water nowadays. Even when heated to boiling, water doesn't decompose and give off O₂.

I don't know how much O₂ is produced when lightning hits water. Evidently, it's not much. You can directly read the oxygen status of the early atmosphere off rocks by looking at how much the iron deposits were oxydized. O₂ concentrations were very low before photosynthetic bacteria started producing it.

Lightning does do interesting things to atmospheric chemistry, however. It's said to make NO₂. It also makes ozone (O₃). My new ozone meter, purchased to test whether the filter on an possible air-conditioner purchase would be safe, just went nuts a couple of days ago, right before a storm.

Mike W.

p.s.   I just read the same NYT article.

(published on 09/17/2013)