Chemical Properties of Ascorbic acid
History Basic Information Abundance 3-D Models Compound Structure Uses Chemical Properties References
Tautomeric equilibrium – Vitamin C is sensitive to heat and light, and will oxidize readily by reacting with the oxygen in the atmosphere.
Since Vitamin C is a molecule, and is "stuck" together with intermolecular bonds, it has a much lower melting point than, for example, a salt would.
(from http://www.netsci-journal.com/97v4/97014/vitc2.html)