Expert: Henry Boyter - 2/17/2008

Question
Hi Henry,

My friend (a Physics teacher) maintains that water (H2O) is an entirely transparent liquid, and that any appearance of blue colouring is due to external factors (colour of sky etc). I recall many years ago my Chemistry teacher taught me that it is in fact slightly blue, and that this becomes evident when shining a white light through a large body of clear water.

I believe my Chemistry teacher to have been correct, but in the absence of a white light and large body of water am unable to confirm this or prove it to my friend. I have searched Google but found nothing that he is prepared to accept.

Please could you confirm the true colour of water one way or the other!

Kind regards,
Dave

Answer
The answer is of course "None of the Above".  There are several problems discussing this.  One, color is a pure human response and sometimes emotional.  As you know, noone sees a color the same way.  For scientists, color is defined as a set of data when compared to a standards.  Now we have a problem here.  For water based solutions, the standard is for zero color is distilled or deionized water, so when we measure the color of water solutions, we define pure water as having no color. You can also use air as the standard, but that is uncommon.

Another problem is the word transparent.  The scientific definition is that light passes through it as compared to some standard.  There is that standard problem again.

When we talk about water having color there are two factors that enter into the discussion.  The location of the water, its background and the light source used will influence how we perceive the color.  For water we find in nature, it is never pure and the amount of silt, algae, and other things will influence the color.  We also have to talk about apparent and tru color.  These are defined by scientists as the color as compared to a standard (always a standard) as found in nature and after being filtered through a 0.45 micron filter (tru color).  Why 0.45 micron?  It is a standard filter used in labs for separating solids, nothing else.

The bottom line is both your teachers were correct.  The Physics prof is talking about comparing water to a water standard, your chemistry teacher was comparing it to the air which is truly transparent.  It is all based on your frame of reference like all other human responses.