WordNet sense 1 (a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures):

mercury, hydrargyrum, Hg

WordNet sense 2 (liable to sudden unpredictable change):

erratic, fickle, mercurial

WordNet sense 3 (such that alteration is possible; having a marked tendency to change):

changeable, changeful