Ketones have a general formula of #RC(=O)R'#; while maintaining the carbon chain, the carbonyl group cannot be oxidized any further. On the other hand, an aldehyde #R(C=O)H# can be oxidized up to a carboxylic acid, #R(C=O)OH#. Since silver ion, #Ag^+#, is the oxidant, macroscopic evidence of such oxidation is given by the appearance of a beautiful silver mirror coating the interior of the test tube. In fact, the best mirrors (i.e. sheet glass that you paint your face in) are precisely made by this reaction:
#RC(=O)H +H_2O rarr R(C=O)OH +2H^+ +2e^-# #"Oxidation (i)"#
#Ag^+ + e^(-) rarr Ag(s)darr# #"Reduction (ii)"#
#(i) + 2xx(ii):#
#RC(=O)H +H_2O + 2Ag^(+) rarr R(C=O)OH +2H^+ + 2Ag^0darr#
Sometimes, you don't get the silver mirror, but the reduction to silver metal is clear. Cyclohexanone WILL undergo this reaction, whereas acetone will not.
