MightyBlowfish
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Joined: Sat May 12, 2007 3:14 pm

changing soil color

Postby MightyBlowfish » Sat May 12, 2007 3:26 pm

My students have been exploring different types of soil, and one of the observations they have made is that dark brown swamp muck becomes light gray when it dries. At first they explained this as the muck being "cooked" by the sunlight coming in the window, but in testing this hypothesis discovered that it dried gray even when covered by a shoebox. WHAT CAUSES THE DRAMATIC COLOR CHANGE? I have searched several sites online and have been unable to find an explanation.
Thanks!

MelissaB
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Postby MelissaB » Sat May 12, 2007 6:13 pm

I don't know the exact mechanism, but I've got an educated guess and maybe someone can chime in after me with a more technical explanation.

First, keep in mind that water changes the color of many things--bathing suits often look darker; a chocolate cake mix starts out as a grayish-brown color and becomes a dark brown when you add water and oil and eggs. Also, human hair often changes color when it gets wet (mine looks infinitely better when it's wet)!

Water scatters light differently than solids do; this light then reflects toward the human eye and whatever wavelengths get back stimulate photoreceptors, which then 'tell' the brain what color it is seeing. My guess is that the color of the soil changes because of the way the water in the swamp soil changes the behavior of the light and therefore the wavelengths reflected back to the eye.

Also keep in mind that the water is suspending the molecules of the soil in itself, so, for example, putting some soil underneath a ziplock bag filled with water (probably) won't change your perception of the soil's color (but it might be an interesting experiment to try!).

This is a really neat question, and I wish I had a better answer! You might try looking up a phenomenon which, if memory serves, is called thin-film interference, which explains why a thin film of oil reflects so many colors, but I remember it being very complex. I don't know what grade your students are in, but my guess is that it's too complicated for them.

Kudos for encouraging the scientific method in class--I know plenty of teachers who would just have let them accept the 'cooking' hypothesis because it was easier than trying to find a real answer!

MelissaB
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Postby MelissaB » Sat May 12, 2007 7:27 pm

I got thinking about this more...

It would be possible to test my hypothesis by adding different sorts of liquids to dried soil (water, oil, egg white, makeup remover, etc.). I don't know what will happen, so I suggest you try it on your own before you have the students try it. Obviously, you'll want whatever liquids you choose to be as colorless as possible and as different as possible.

Louise
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Re: changing soil color

Postby Louise » Sun May 13, 2007 7:26 am

MightyBlowfish wrote:My students have been exploring different types of soil, and one of the observations they have made is that dark brown swamp muck becomes light gray when it dries. At first they explained this as the muck being "cooked" by the sunlight coming in the window, but in testing this hypothesis discovered that it dried gray even when covered by a shoebox. WHAT CAUSES THE DRAMATIC COLOR CHANGE? I have searched several sites online and have been unable to find an explanation.
Thanks!


Clay- the mineral component of soil has very complicated structures which change based on the water (or hydration level). It has different colors depending on the structure. I would guess some of the effect is that you are destroying the clay network when you dry it.

I tried searching for "clay and structure and hydration". I found some sites, but they are somewhat complicated.

http://www.ill.fr/AR-98/page98/25liqui.htm


adding "and color" found this one, which I think will answer your questions (and isn't too complicated):
http://www.urbanext.uiuc.edu/soil/concepts/concepts.pdf

The last page is all about color and water.

You could try to test this by getting pure organic stuff (fertilizer) and pure clay and see which changes color most when dried.

Louise

MightyBlowfish
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Thanks!

Postby MightyBlowfish » Mon May 14, 2007 4:42 pm

Thanks so much for all your input-- I'm still not entirely clear on the mechanism of the color change, but I've got a much better idea than before.
Cheers!

Louise
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Re: Thanks!

Postby Louise » Mon May 14, 2007 5:13 pm

MightyBlowfish wrote:Thanks so much for all your input-- I'm still not entirely clear on the mechanism of the color change, but I've got a much better idea than before.
Cheers!


Well, the mechanisms are complicated (probably a combination of what melissa suggested and what I did).

I'm not sure what grade you teach, but a good example of this (hydration) effect is the copper sulfate chemistry. When copper sulfate is hydrated is is bright blue. When the water is removed it is white (by heating). It isn't just that the copper sulfate is "wet"- a certain number of waters are interacting with the copper sulfate in a _specific_ way- the water becomes part of the crystal. If you looked at it, you would have no idea it was hydrated. It looks dry and solid.
See the wikipedia for a lovely picture of the hydrated form:
http://en.wikipedia.org/wiki/Copper_sulfate
Many inorganic compounds (minerals) have this property. The reference I provided had information about color changes of iron containing clay, but there are many other types of minerals that may be part of your color change.


Louise

Craig_Bridge
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Postby Craig_Bridge » Tue May 15, 2007 1:59 pm

You might get some hints at some of the specific chemistry involved if you re-constitute the dried out soil using an excess of distilled water and then perform some chemical assays for disolved minerals and salts. A "total hardness" test will give you some idea about how much disolves and more specific mineral assays will give you a clue as to what disolves. You can probably also find out some clues from any published local water treatement plant data. Magnesium, manganese, and calcium salts are likely contributers to things that turn light gray when they precipitate and dry out. If you live in an area the has a lot of sedimentary rock such as limestone, you will have have lots of minerals and salts involved.
-Craig

Louise
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Postby Louise » Tue May 15, 2007 3:11 pm

Craig_Bridge wrote:You might get some hints at some of the specific chemistry involved if you re-constitute the dried out soil using an excess of distilled water and then perform some chemical assays for disolved minerals and salts. A "total hardness" test will give you some idea about how much disolves and more specific mineral assays will give you a clue as to what disolves. You can probably also find out some clues from any published local water treatement plant data. Magnesium, manganese, and calcium salts are likely contributers to things that turn light gray when they precipitate and dry out. If you live in an area the has a lot of sedimentary rock such as limestone, you will have have lots of minerals and salts involved.


Good ideas. There might be a local agricultural extension office which could probably tell you about specific soil types in your area too.

Louise


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