One question at a time... First, which freezes
faster - cold or hot water? We've actually answered this question
already, so check out Freezing Hot Water and the Mpemba Effect
. And now to your second question...
sounds to me like you've 'heard' correctly. Hot water will (at least in
theory) make more solid ice cubes with less bubbles. The reason for
this has to do with how bubbles come about in ice in the first place.
All water has at least some air disolved in it. When you cool the water
down to freeze it, the disolved air forms bubbles in the ice. So the
amount of bubbles you get in the ice really depends on how much air
there was disolved in the water in to start off with.
warmer a gas is, the less likely it is to stay disolved. This actually
makes sense if you think about it. Gas molecules normally have a lot of
energy. This is what makes them move around so much. But if they get
disolved in water, they have 'less room to move'. The warmer they are,
the more energy they have, and the more they want to move around. By
coming out of solution and going into the air, they aren't as confined
as when they're disolved, so it's easier for them to move around.
when you heat up water, you lose a lot of the air that would otherwise
be disolved in it. Hence, when you start off with hot water, you get
less bubbles in the ice.
(republished on 07/25/06)