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Q & A: Freezing Orange Juice

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Q:
What will freeze first water ot orange juice or a solution in between?
- Alex Garced (age 8)
Chasco Elementary, NPR, FL USA
A:
Alex -

In general, the more impure a liquid is, the lower its
freezing point will be. Basically, this means that the more stuff is
mixed into a liquid (like water), the colder you will have to make it
in order for it to freeze. There are lots of equations and stuff that
describe this, but the general gist of the reason is this...

When a liquid like water freezes, its molecules stick
together in neat patterns. (Think of the patterns you can see in
snowflakes or other ice crystals and you will know what I mean.) But
if you add other stuff like sugar to that water, all the molecules
aren't the same shape anymore. (Sugar molecules are a different shape
than water molecules.) Since they aren't all the same shape, it's
harder for them to form neat patterns and stick together. In fact,
when the water starts to freeze it forces almost all the sugar
molecules out of the ice into the remaining liquid. That's hard to
do, so you have to get the sugary water (or orange juice) colder
before it starts to freeze. To freeze all the liquid requires
lowering the temperature way below the freezing point of water.

(republished on 07/25/06)

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