The Collaborative International Dictionary of English v.0.48:

Liquefy \Liq"ue*fy\ (l[i^]k"w[-e]*f[imac]), v. t. [imp. & p. p.
   Liquefied (-f[imac]d); p. pr. & vb. n. Liquefying
   (-f[imac]`[i^]ng).] [F. liqu['e]fier, L. liquere to be liquid
   + facere, -ficare (in comp.), to make. See Liquid, and
   -fy.]
   1. To convert from a solid form to that of a liquid; to melt;
      to dissolve; and technically, to melt by the sole agency
      of heat. [WordNet sense 1]
      [1913 Webster]

   2. To convert from a gaseous form into a liquid; as, to
      liquefy natural gas.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

liquefied \liquefied\ adj.
   1. converted to a liquid; as, liquified natural gas;
      liquified coal..

   Syn: liquified.
        [WordNet 1.5 +PJC]

   2. rendered liquid by heating; changed from a solid to a
      liquid state; melted.

   Syn: molten, liquified.
        [WordNet 1.5 +PJC]

   3. rendered liquid by dissolution in a solvent; -- of solids.

   Syn: dissolved, liquified.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

liquefied
    adj 1: reduced to a liquid state; "liquefied petroleum gas"
           [syn: liquefied, liquified]
    2: reduced to liquid form by heating; "a mass of molten rock"
       [syn: molten, liquefied, liquified]