Dark Bottles

When you enter a chemistry lab, on the bench, there are some bottles containing chemicals, and some of these chemical bottles are tinted so that they have a dark brownish colour.

My chemistry teacher had told the class that the dark colour is used to prevent light from getting to the chemicals stored in the bottle, so that the chemicals will last longer. But I don't understand why.

Let's say the bottle is not tinted and clear, the light will get to the chemicals, which is common sense, and provide energy for the chemicals to decompose or react.

Now let's say the bottle is tinted, the most of the light that falls on the bottle will be absorbed by the bottle, 'cause dark colours absorb more light. And the energy absorbed from the light will then be transfered by conduction to the chemicals, cause there will be a energy gradient between the chemical and the bottle. So in the end, it's all back to square one. So how does making the bottle dark in colour actually help in anyway?

My physics is not the greatest, please point out any flaws in my physics.:smile:
 

chroot

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In such cases, visible light (read: relatively high frequency light) is capable of making the chemical decompose. When the bottle is darkened, the light is converted to heat when it strikes the bottle. Heat, unlike light, does not cause the chemical to decompose.

It is true, however, that the bottle's walls reradiate the energy into the chemical, but mostly in the infrared spectrum (since all objects at room temperature radiate mostly in the infrared). The infrared photons do not have enough energy to activate the reaction, so the chemical stays fresh.

- Warren
 
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chroot, your explanation is IMO correct. Still I suspect this might be an urban legend... If they are so concerned about light, why not paint the bottles black? And why are there no dark plastic bottles, just white ones (AFAIK)?
 
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You may be familiar with hydrogen peroxide from the apothecary, stored in dark plastic bottles to prevent photolysis.
 

selfAdjoint

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Originally posted by arcnets
chroot, your explanation is IMO correct. Still I suspect this might be an urban legend... If they are so concerned about light, why not paint the bottles black? And why are there no dark plastic bottles, just white ones (AFAIK)?
Opaque bottles would certainly keep the light out, and sometimes that is done, with especially sensitive liquids. But then you can't tell just by looking what is the level of fluid in the bottle.

In the old days before it was pasteurized, beer came in brown bottles.

I believe specifically UV radiation is the culprit.
 
dont forget about beer bottles!
 
In such cases, visible light (read: relatively high frequency light) is capable of making the chemical decompose. When the bottle is darkened, the light is converted to heat when it strikes the bottle. Heat, unlike light, does not cause the chemical to decompose.
What is that special property which light has that can decompose chemicals?

It is true, however, that the bottle's walls reradiate the energy into the chemical, but mostly in the infrared spectrum (since all objects at room temperature radiate mostly in the infrared). The infrared photons do not have enough energy to activate the reaction, so the chemical stays fresh.
What about through conduction too?
 
What is that special property which light has that can decompose chemicals?
the ability to carry energy?

in paticular, it carries the proper activation energy
 
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Originally posted by selfAdjoint
In the old days before it was pasteurized, beer came in brown bottles.
So they didn't want to let light in to prevent the beer from .... fermenting? what?

is fermenting what makes beer bad so that they would want to limit fermentation?

(note: im not old enough for beer so i wouldn't really know too much about it)

can you briefly explain how pasteurizing does the same thing to beer as preventing light from entering the beer? i mean, what's the point of keeping the light out of the beer?
 
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Bubonic Plague
What is that special property which light has that can decompose chemicals?
As I mentioned above, photolysis[b(].
 
the ability to carry energy?
As I mentioned above, photolysis.
Heat, unlike light, does not cause the chemical to decompose.
I know that light carries with it energy, but what is so different about the energy from light and the energy from heat which is conducted or radiated to the chemical that only energy from light can cause the chemicals to decompose?

Pardon me, i just can't seem to "see" the answer.
 

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