From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Dec 18 2002 - 09:51:43 PST
Message-Id: <l0311071aba26650f48eb@[192.174.3.125]> Date: Wed, 18 Dec 2002 09:51:43 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole salt and water's freezing point
>Quick question...
>
>Does anyone have a good explanation (for a curious
>group of 8th graders...and their teacher) for why salt
>lowers the freezing point of water?
>
>Thanks in advance!
>Meghan
>
Hi Meghan
It's a dance.
At the surface of an ice cube in liquid water, water molecules are always
leaving the ice to become liquid and also leaving the liquid to add to the
ice.
At temperatures above 0 C the water molecules leaving the ice predominate
and the ice melts. At temperatures below 0 C the molecules adding to the
ice are more numerous.
The presence of salt or any other solute (like sugar) gets in the way of
the water molecules trying to go from liquid to solid more than they get in
the way of water molecules leaving the solid. This changes the balance
between freezing and thawing so that the equilibrium happens at a lower
temperature.
Paul Doherty
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