[thedoc (OP)]

We all know that heat rises. So if I heat a kettle of water, the hotter water rises to the top, and the colder water will fall towards the element, get heated & rise.

How is it possible that when I freeze water, it solidifies from the top down? If the hotter water is near the top, why does it solidify first?

Yours
Pinchas Goldberg

Asked by Pinchas Goldberg


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[Pmb]

As water starts to cool down it sinks because the molecules start to move in a way which reduces the disance between them due to their kinetic energy. As the water gets even more colder the atoms which make up the molecules start to bind together and that happens in a less dense configuration and that means the water becomes less dense and starts to rise. The coldest configuration means that water is in its least dense state.

[evan_au]

Water is an exception to the usual rule that "heat rises".

Over a narrow temperature range around 0C, water bucks the general trend, meaning that ponds (and oceans) freeze from the top down, insulating the lower pond water from the atmosphere, and preserving life in the liquid water beneath the ice.
See: https://en.wikipedia.org/wiki/Properties_of_water#Density_of_water_and_ice

[CliffordK]

If ice was to form sub-surface, it would float to the surface.

However, it is possible if you say put a freezer coil into a bucket of water, the ice would freeze on the surface of the coil.  Likewise, an ice cube may be able to skin over all the surfaces including the sides and bottom, then freeze the middle later.

[yor_on]

Pete's explanation seem pretty straight forward to me. And a density also represent a energy, doesn't it? So something 'dense' that you want to to transform into something else should cost you more energy, than doing it to something not that dense. And you can add that on earth you also have a atmosphere, with wind, transporting away heat from the topmost layer.