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.  

maybe the spark is gone, love is a weird emotion and there is a possibility he may have found somebody else.

- calcium has greater affinity to bond with electron receptors than hydrogen does and hence, it has the ability to replace hydrogen in most of its compounds.

- lithium has 2 valence electrons in the product because it can make only 1 bond with hydrogen. beryllium has 4 valence electrons in the product because it can only make 2 bonds. it has fewer electrons than neon and can react with electron donors. each hydrogen has 2 valence electrons, the same as helium.

- the covalent bond formed between carbon and hydrogen is polar because carbon is more electronegative than hydrogen and it attracts the bonded electron pair closer to itself, thus giving hydrogen a partial positive charge.

- a hydrogen bond is a relatively weak bond that hydrogen atoms make with the electronegative atoms nitrogen, oxygen or fluorine.

so the best choice is a.) 2 calcium (ca) atoms