system
Given the following post, respond as if you're commenting on Reddit:
human
How is dissolved oxygen related to the oxygen in the chemical formula H2O?

Im kinda confused about what dissolved oxygen is a measurement of-- if H2O is ~89% oxygen by mass, is "water" with 0% dissolved oxygen in it just hydrogen? If not, what does it mean for oxygen to be dissolved "in" a substance that is primarily made of oxygen?
gpt
You're mixing up two different things, but that's okay, I'll try to explain it: There is water—the molecule—which has two hydrogen *atoms* and one oxygen *atom* bound together and oxygen—the molecule—which is two oxygen *atoms* bound together. When oxygen is dissolved in water, it means that among all the water molecules there are some oxygen molecules swimming about with them.

Consider O's being water and X's oxygen molecules:

> OOOOOOOOOOO

> OOOOOOOOOOO

> OOOOOOOOOOO

This is pure water, but remember each molecule still has a single oxygen atom and two hydrogen atoms. Now let's add some dissolved oxygen.

> OOOOOOO**X**OOO

> OOOO**X**OOOOOO

> OOOOOOOO**X**OO

Now mixed among our water molecules are some oxygen molecules. Sure, the majority of the mass is still just oxygen, but oxygen molecules and water molecules are very different. You just have to remember that some atoms are usually referred to by their molecular name. Hydrogen has the same naming convention where the hydrogen molecule is two bound hydrogen atoms. The reason this confusing convention exists is that chemists didn't always know that things like hydrogen and oxygen were naturally two atom systems on Earth.