Why does ice freeze clear on the outside?

Question

Why does ice freeze see-through on the outside and white on the inside? I really want to know if anybody has any suggestions.

Answer 1

Freezing of ice is an anomalous thing>Ice floats on water.when water is heated from 0 degree celcius to 4 degree celcius it contracts.if water is  heated from 0 degree celcius to 4 degree celcius its volume decreases hence density increases.This is why in a lake water at 4 degree celcius sinks to the bottom which is very important for marine life.

Answer 2

Doesn't always.  The clarity of ice depends on how much air is trapped inside it.  Air tends to make the ice white, while really air free ice is clear and glass like when there is a thin layer.  It might also have a clear, bluish tint when there is a lots.

There is nowhere on Earth better than Antarctica for seeing the effects of air in ice.  Each iceburg can have a mix of color ranging from pure white and opaque, to deep blue and clear.  The oldest ice in the burg makes up the clear, blue portion.  That's where ten of thousands of years of compression has driven out all the air once contained in the ice.  The younger ice is the white ice where air is still trapped inside. [See source.]

Outside, back in the States, there is an ice called rice ice.  It is granular in consistency and very white, almost like snow (which, by the way is ice crystals and, together, they are white).  The rice ice is called that because each grain looks something like a white rice grain.  This often occurs when there has been a series of melts and refreezes, during which time air gets trapped inside.

Related, there is also rime ice.  Like rice ice, it's white.  But unlike rice ice, it stays pretty much intact as a sheet of ice.  Because it is filled with air, its surface is bumpy and pock marked.  As a consequence, when rime ice forms on aircraft wings, it disrupts the otherwise laminar air flow over the wings and can cause an aircraft to stall and crash.

On the flip side, we have black ice.  This isn't really black, it's clear as glass and maybe, at most, an inch thick.  (It would have a blue tint if were much much thicker like in an iceburg.)  So, on a road, we only see the black asphalt under it.  Black ice occurs when rain hits the pavement, which is below freezing temperature, and freezes immediately upon impact.  The rapidity of the freezing disallows air to mix in; so it is absolutely clear and we cannot see it on a road.  It is the most dangerous of all the ice forms because we have no warning that its there.

So the bottom line is this.  If you see clear ice outside, that's because air did not have time to mix in.  That ice probably formed rapidly.  If you see white ice inside, that's because air did mix in...the ice probably formed slowly and it may have resulted from a series of melts and refreezing.  (If you turn the heat down at night in your house and then back up during the day, that would do it.)