The Collaborative International Dictionary of English v.0.48:

Whiten \Whit"en\, v. i. [imp. & p. p. Whitened; p. pr. & vb.
   n. Whitening.] [OE. whitenen; cf. Icel. hv[imac]tna.]
   To grow white; to turn or become white or whiter; as, the
   hair whitens with age; the sea whitens with foam; the trees
   in spring whiten with blossoms.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whiten \Whit"en\, v. t.
   To make white; to bleach; to blanch; to whitewash; as, to
   whiten a wall; to whiten cloth.
   [1913 Webster]

         The broad stream of the Foyle then whitened by vast
         flocks of wild swans.                    --Macaulay.
   [1913 Webster]

   Syn: See Blanch.
        [1913 Webster]

WordNet (r) 3.0 (2006):

whiten
    v 1: turn white; "This detergent will whiten your laundry" [syn:
         whiten, white] [ant: black, blacken, melanise,
         melanize, nigrify]