What is Dissoved Oxygen (DO)?

Imagine that you are looking at a new, unopened bottle of clear soda. What does it look like? Does it look different from a bottle of water? Or juice?

Now imagine that you have opened the bottle of soda. What happens now? Would a newly-opened bottle of water do the same thing?

Where did the bubbles that appear in the soda come from? They come from gas that has been dissolved in the liquid soda and held their by pressure and temperature (you can tell that temperature affects the amount of gas in a liquid in two ways - see the note at the bottom). When oxygen, or any gas, is dissolved in a liquid, it's invisible. It only becomes visible when it forms bubbles of pure gas (no longer dissolved).

Although there is not as much of it, there is gas dissolved in plain water, from the tap or in the natural world. One of the gases in water is dissolved oxygen. All animals need oxygen to live; dissolved oxygen in water is as important to animals living in water as the oxygen in air is important to animals living on land. Dissolved oxygen gets into water in the same way that water gets into air; oxygen is a byproduct of photosynthesis, whether photosynthesis occurs underwater or in air.

Oxygen also moves directly into water when that water comes into contact with air. Water that is moving, as in a river, stream, or over a waterfall, comes into contact with more air than still water does, and so moving water has more dissolved oxygen in it.

Warm water, just like warm soda, can hold less dissolved oxygen than cold water can. This means that dissolved oxygen varies through the course of the day, and along a stream where the stream moves in and out of sunlight areas.

Experiments

How does temperature affect the amount of gas dissolved in a liquid?

In the first method, gently heat a bottle of soda (put it in direct sunlight in a windowsill) and cool a second (in the fridge). When you can feel a dramatic difference in temperature, open both bottles. The warm one will bubble more. The other way to do it is to open two bottles of soda of the same temperature, then put one back in the fridge, and the other in the direct sunlight. When you see the bubbles stop appearing in the soda on the windowsill, take the one from the fridge and put it in the sunlight. It should start bubbling again.

Why is Water Temperature Important? | What is Dissolved Oxygen (DO)? | What is pH?

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