How can aldehydes and ketones be distinguished from one another?

The Tollens test utilizes the redox chemisty of #Ag^+#. An aldehyde can be oxidized to a carboxylic acid, and such oxidation must be accompanied by a corresponding reduction of #Ag^+# to #Ag^0#, which usually manifests as a beautiful silver mirror lining your test tube (I save the good ones, they are quite pretty - in fact, high quality mirrors are made by this reaction). A ketone cannot normally be oxidized by these means, and thus would fail the test.

The reaction chemistry is:

#2Ag(NH_3)_2^(+) + RC(=O)H + H_2O rarr 2Ag(s)darr + 4NH_3 + RC(=O)OH + 2H^+# (I think this is balanced!)

Silver nitrate is thus reduced to atomic silver, and the aldehyde is oxidized to carboxylic acid.

So the Tollens reagent is silver nitrate in ammoniacal solution (which forms the #Ag(NH_3)_2^+# ion).

A word of (unnecessary!) advice. It is a fact that cyclohexanone, a ketone, gives a lovely silver mirror with the Tollens reagent, even though cyclohexanone is a ketone. So the Tollens reagent is not discriminating in this particular case.