The student finds two more unlabeled bottles that are either co2 or h2 based on their appearance , how can she determine which substance is which?

Carbon dioxide and  hydrogen are both gases  hence when two bottles contain these two gases in it it is really difficult to know which is which by just seeing it.

Explanation:

Although there are very slight differences between two gases which we can see physically to differentiate between them.

For example a very light amount of gas can be let out on a burning matchstick.

Hydrogen being a highly combustible will light the matchstick while letting out carbon dioxide on the matchstick take would instantly turn off the flame because carbon dioxide does not aid burning.  

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that's what i got you can count the number of atoms