wrcmad said:JulieW said:JulieW said:Going back 50 years old high school physics, I think it's due to the direction that heat travels - cold to hot, hence at zero, heat is travelling from minus to zero and hence ice is melting.
A lot of theory has changed since my days were spent learning defunct information so they may have a newer explanation now.
Well no physicists here so I went to google and found I was wrong. Heat travels from hot to cold, so my explanation doesn't work, but the theory does. Over to the brains trust
If you think of two 'boxes' of gas. One is hot, one is cold.
In the hot one, the particles are moving faster than they are in the cold one.
If you put the two boxes together and remove the sides so they can mix then the faster particles will flow into the 'cold' box faster than the slower particles from the 'cold' box will flow into the 'hot' one. If you measure the two boxes every so often, you'll notice that the average speed of particles in the 'hot' box goes down (because it's lost some fast ones from itself and gained some slow ones from the 'cold' box) and the average speed of the particles in the 'cold' box goes up (because it's gained fast ones from the 'hot' box and lost some slow ones from itself).
Speed of the molecules is how you work out the temperature, so you can see that the 'cold' one gets faster (ie hotter), and the 'hot' one gets slower (ie cooler). Heat flows from hot to cold...
crystallization of pure liquids usually begins at a lower temperature than the melting point, due to high activation energy of homogeneous nucleation. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. The melting point of water at 1 atmosphere of pressure is very close to 0 C, and in the presence of nucleating substances the freezing point of water is close to the melting point, but in the absence of nucleators water can super cool to 40 C before freezing.
See here:
[youtube]http://www.youtube.com/watch?v=Fot3m7kyLn4[/youtube]
Skip to around 0:50 if your inpatient.