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Consider a glass of water and a glass of coffee. Their contents differ by no more than a few grams of particles coming from the roasted and ground coffee, yet the former lets almost all visible light pass through, and the latter blocks most of it.

I am wondering what are the most efficient ways to turn water opaque by pouring matter in it, under normal temperature and pressure. By efficient I mean minimising the mass of added matter. By opaque, let's say 99% of daylight is blocked by two inches of liquid.

There is no other reason for my asking than sheer curiosity from the many hours I spend gazing fixedly through the coffee pot.

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A solution of potassium permanganate blocks light pretty well, though I don't know if it is the most mass-effective soluble light absorbent. –  Piotr Migdal Nov 2 '11 at 21:35
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Permanganate of course is not among those dyes. But first: do You want to know for dyes or for the definition "what are the most efficient ways to turn water opaque by pouring matter in it, under normal temperature and pressure""? This definitions are different. Second: Give a reason for Your curiosity. –  Georg Nov 2 '11 at 21:43
    
I am not a native speaker and I didn't think “dye” was conveying a specific meaning. Would “light absorbent” be a better term? –  Sam Hocevar Nov 2 '11 at 21:45
    
Not really, that ""most efficient ways to turn water opaque by pouring matter in it"" is better, it covers all possibilities, just leave out the water.. Dyes are chemicals like indigo, fluorescein, eriochrom T, etc. –  Georg Nov 2 '11 at 22:04
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Pour in enough antimatter to turn it to steam... –  user2963 Nov 3 '11 at 0:51
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