If hot water rises, why does ice form on the top of water than the bottom?
In: Chemistry
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Water is different from most substances because when it freezes, it does so in a crystal structure. That is, the water molecules form a specific shape with a lot of empty space in between, meaning ice is actually lighter than water.
The reason that hot water rises is simply because its less dense or lighter than cold water, but because of that crystal shape, ice is actually even less dense or lighter than much warmer water. This means that when you have a larger body of water, like a lake or pond or even a swimming pool, the ice crystals will rise to the surface as they form instead of sinking to the bottom. The ice on top also acts as an insulator, preventing the water underneath from freezing as easily.
This is important, as without this unusual quality of water, all bodies of water would freeze solid during the winter killing off all aquatic life immediately, and leave no liquid water for terrestrial life. The Earth would end up covered in glaciers too as there would be so much ice that even summer would be enough to melt it. Maybe the tropical regions would still have liquid water. I’m not even certain of that, as all that ice would also lower the global temperature.
So be thankful for the odd way that ice freezes!
If water or gas rises and drops depend on the density. Water is densest at 4 degree C and above that it is less dense. So colder water the 4C will flow on top of water at 4C so it then cold water rises.
In most cases out in nature the reason that water form ice on the top and not the bottom is that in combination that the ground in most situation warmer the the air. So it it the top exposed to the colder air and the ice form there and where it in contact with the ground that is above freezing temperature.
If you put out a bucket in cold air it ice is first formed on the top but then on the bottom and the other sides and will freeze from the outside to the center. That is because all sides is exposed to cold air trough a thing bucket and not to warmer ground
Ice forms wherever water is in direct contact with freezing temperatures. For most instances, that contact is with the air located above the water’s open surface
Hot water rises, but freezing water rises as well! Water is densest at 4 °C, anything warmer than that will rise to the top, but if the water is cooled below this temperature then it will “swap around” and the freezing water (along with the ice) will rise, while the (slightly) warmer water will sink to the bottom.
Why does this happen? Unfortunately, that’s a pretty complicated question to answer, well beyond the scope of an ELI5, I’m afraid. It has something to do with how the individual hydrogen and oxygen atoms in water molecules interact with the atoms in the water molecules that are next to them. Water is just weird like that.
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