Ice Diving question

[bridgediver]

To put it in plain english.
pure ice is 9% lighter than water

I like fisher's explanation about why this is so

 

[dschonbrun]

Big t,

In your density of Ice and of Water, what did you assume as the Pressure and Temperature of those bodies? Without those details, the calculation can't be verified.

Thanks,
D

 

[DA Aquamaster]

When water molecules freeze, they form up into a crystalline lattice kind of structure, which increases the space between the individual molecules, as compared to the space between them when in a liquid state, thus lowering density. Compare this to all (I believe) other liquids which become denser when they freeze/solidify.

It is well understood why ice is less dense than water (a 104 degree bond angle in the crystalline structure of ice that means the same mass takes up more volume.)

The more important issue is why does ice freeze at the surface? Normally as things get colder they get more dense. It would follow then that the coldest water would sink to the bottom and that ice formation would start there. That would be bad news as the water above would still be exposed to cold temps and continue to cool all winter. In the spring, all that water would have to warm back up before meting of the ice on the bottom would begin with the result that ice forming in deeper bodies of water would still be there next winter. Eventually, over severl or even several thousand years, the ice accumulates, freezes all the way to the surface then reflects sunlight and eventually you get an ice covered planet that never thaws.

Luckily for us, water begins to expand at 35 degree F so that the coldest water is on top and ice forms on the surface at 32 degrees - insulating the water below from colder temps and melting early in the spring due to direct exposure to warm air and sunlight.

So...who can explain why water, still in a liquid state, begins expanding at 35 degrees?

 

[J.R.]

Density of ice = 57.25 lb/cu. ft.
Density of Water = 64.43 lb/cu. ft

The difference in density would be 64.43-57.25...or 7.18 lbs/cu. ft.

Which, of course, yeilds the ever popular "tip of the iceberg"... (See: Archimedes...)

 

[J.R.]

So...who can explain why water, still in a liquid state, begins expanding at 35 degrees?

There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open".

It is this open solid structure that causes ice to be less dense than liquid water. That is why ice floats on water, for which we should all be thankful because if water behaved "normally" many bodies of water would freeze solid in the winter, killing all the life within them.

Water's "density maximum" is a product of the same phenomenon. Close to the freezing point, the water molecules start to arrange locally into ice-like structures. This creates some "openness" in the liquid water, which tends to decrease its density. This is opposed by the normal tendency for cooling to increase the density; it is at approximately 4 degrees Celsius that these opposing tendencies are balanced, producing the density maximum.

... now, that being said.... I have very little concept of what this all means from a molecular level... but it *is* amazing what one can find on the internet... but it does suggest that the ice in my glass of water will not float HIGHER than the ice in the glass of scotch next to it...

 

[samaka]

water is less dense in it's solid state than it is in it's liquid state.

Isn't that good news for polar bears and pinguins...

.......a.......

 

[Tomeck]

It's better to remove ice block from water and put on the surface, or to push ice block more far under ice surface.