Which chemicals can produce a color change from clear to opaque and back again in water?

I am a professional Magician designing a new stage illusion, and I have specific requirements on a particular step of this illusion. This will require that I can take what appears to be normal water, and change the color of it so that what is submerged in the water cannot be easily seen through a clear container. Later I need to be able to reverse this color change so that the "water" appears to be transparent again. 

Specifically, in my research done on my own I have come across a method in which potassium Iodide is created through a clock reaction. My concern here is whether or not the Iodide will stain as this effect requires that I submerge my hands in the water/solution. 

Next involves Phenolphthalein and Sodium Hrdroxide but I have several concerns with this method in that the deep red color achieved at Ph 10 may not be opaque (hard to tell from pictures) enough for my needs, and whether or not the ph 10 solution would be caustic enough to cause chemical burns.

If there are any other chemical methods that can be used that fit the bill of being reversable, safe for skin contact, and non staining I would greatly appreciate any information you have.

Thank you

Expert Answers

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You are looking at two basic types of reactions for something that would suit your needs.  The first is a clock reaction, or a reaction that goes from colorless to colorful and back again according to the equilibrium dynamics of the particular reaction being used.  Unfortunately, most of these clock type reactions involved iodine, since the iodide anion (I-) is colorless and the iodine molecule (I2) is very darkly colored.  I would not recommend dipping my hands into a solution of I2 since it is very staining, even to skin.  Your hands would definitely darken in color which would take several days to wash/wear off.  Other variations to this reaction add things such as mercury salts, so no help there health wise.

The other type of reaction you are looking at is acid/base with a color indicator.  When the pH of the solution hits a certain threshold, the color of the reaction abruptly changes.  I can tell you that I would definitely not dip my hands into a pH 10 solution of sodium hydroxide.  Acids and bases like this are very caustic substances that at the very least would irritate your skin and at worst would give some very nasty chemical burns. 

Having said all of this, I cannot think of a variation here that would be safe for skin.  These reactions are very old, before safety was really thought about for lab work.  If you could use tongs or some other type of instrument to place an object in and out of the water without touching it, then some of these reactions might work for you.

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