Ask A Scientist General Science Archive

name         Grayson C.
status       student
age          6

Question -   When I take out an ice cube from the freezer, why is part of
it white and part of it clear?
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The ice cube freezes from top to bottom (or the outside to the inside
depending upon how cold your freezer is). In either case, air that is
dissolved in the water is trapped in the liquid. Finally, as the ice cube
continues to freeze the air no longer stay dissolved
but cannot get out through the ice, and air bubbles form. That is what you see
as white -- air bubbles. Dissolved salts also do a similar thing, but it is
much less noticeable than the air.

You can do an experiment to test this. Take two ice trays and fill one with
tap water, and the other with boiled water. Put them both in the freezer and
let them freeze. See which one forms the clearer ice cubes. Boiling the
water, removes most of the air, so it will form clearer ice cubes.

Vince Calder
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The less air in the ice the clearer it is

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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The water you put in the freezer had some air dissolved in it.  As the water
froze, the air could not stay dissolved in the solid ice.  So the ice that
froze first (on the outside of the ice cube) was clear, but the ice that
froze later (in the center of the ice cube) was mixed with air bubbles.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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