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Q:
I read your question about ice melting faster in water than milk. I tried freezing milk cubes and water cubes, and putting them each in a glass and the milk cubes melted faster than water cubes. Is this becuase milk has fat and is more dense? The milk cubes were sticky and looked very dense and seemed to melt in my hand. The ice cubes were clear. Did I do something wrong?
- Andrew (age 8)
Chesterbrook Elementary, West Chester, Pa
A:
Andrew -
No, you didn't do anything wrong. That's just what frozen milk
looks like. You're right that milk is more dense than water, but this
is not why it melts more quickly, although it is indirectly related.
The reason that milk is dense is because it contains a lot of
impurities - raw milk is about 3.7% fat and 3.2% protein (for Holstein
cows; other breeds actually have more). I think that this is also the
reason that frozen milk melts faster. Here is our theory:
In solid ice, the water molecules are bound together tightly, in a
regular pattern. This regular pattern is part of what makes the
chemical bonds holding the molecules together so strong. But if you add
a lot of fat or proteins, the ice that forms when the water freezes
will contain pockets of these impurities. When you take the milky
ice-cube out of the freezer, you noticed that it was sticky. That was
probably due to the fact that these pockets of impurities were not
really frozen. They probably also help the ice melt since they provide
lots of nucleation centers for the melting to start.
-Tamara & Mike
(published on 10/22/2007)
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