MadSci Network: Earth Sciences

Re: It has been said that the glacial ice that is blue in color is due to the l

Date: Fri Aug 29 21:18:01 2003
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Earth Sciences
ID: 1062208914.Es

Hi Ray.

Surprisingly, your question, which is a good question, and easy to understand, has three questions 
hidden in it! There is a physics question, a chemistry question, and an Earth sciences question!

Let's start with the chemistry. There are two different types of oxygen in water. The one you know 
about, and were thinking of in the question, is the oxygen atom that is bound together with two 
hydrogen atoms to make water: H-O-H (H2O). That oxygen atom is tied up so tight it can almost 
never be removed from water. It takes a huge amount of energy, like for example a temperature of 
over 3000 deg C (5000 deg F). The other sort of oxygen is a small amount of ordinary oxygen 
from the air: O=O (O2). It dissolves in the water, just like salt or sugar would, but it remains as O2 
molecules. When we say that water has had some of the oxygen removed, it is this dissolved 
oxygen we are talking about. It can be used up by various animals and microbes that live in the 
water (just like we use up the oxygen from the air), and the water can become very short of 
dissolved oxygen. The oxygen that is bound in H2O is never used in this way, and hydrogen gas 
(which would be flammable and dangerous) is never formed.

Now for some Earth Sciences: Glacial ice is rarely oxygen depleted. It formed originally from 
snowfall, which picks up plenty of dissolved oxygen from the air, and which also contains plenty of 
trapped air. That oxygen remains trapped in the ice. If anything, glacial ice might be richer in 
dissolved oxygen and/or tiny oxygen bubble inclusions.

I have actually heard it said the other way: that glacial ice is sometimes blue because it contains a 
lot of trapped oxygen! Liquid oxygen (minus 190 deg C) is a duck-egg blue colour, and sometimes 
people think that oxygen gas might be a little bit blue as well! It is not, or at least not blue enough 
to be detected.

There are two different places that you see blue connected with glaciers: in the ice itself, and in the 
streams that form from the melt waters. I have seen streams in New Zealand whose waters are 
quite a distinct chalky blue colour. The blue is caused by two different things: in the ice it is mostly 
tiny bubbles of trapped air; in the melt waters it is mostly tiny particles of rock and soil that have 
been gouged by the glacier.

That brings us to the physics: large numbers of any sort of tiny particle will "scatter" light, and 
produce a blue appearance when viewed sideways to the source of light. That is why smoke is 
blue, why the sky is blue, why the ocean is blue, and why ice with a lot of tiny air bubbles or water 
with a lot of tiny rock particles looks blue. You can find out a little more about scattering, and why 
it produces a blue colour from any reference book that talks about why the sky is blue, or from a 
previous MadSci answer of mine.

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