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I would like to create an illustration to demonstrate something during a talk. The idea goes that there is sediment in a bottle of water that sits at the bottom until shaken up then adding another substance to the water makes the sediment dissolve or appear to vanish.

My question is; What would make this possible? What materials or chemicals would work?

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    $\begingroup$ Dissolving sand would require hydrofluoric acid or molten alkali. Unfortunately, neither is a good fit for an auditorium demo, so you are probably looking for "something similar". $\endgroup$ – andselisk Jul 15 at 10:38
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    $\begingroup$ From the description, I think you can be quite flexible as to what there is at the bottom of your bottle. Maybe it would be a good idea to more specifically define what you would like it to look like: What is the most desired color of the undissolved sediment? What is the most desired color of the liquid once dissolved? How fast do you want it to dissolve (some things take hours or days to dissolve)? $\endgroup$ – Veritas Jul 15 at 12:30
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    $\begingroup$ It would be best for the illustration if the liquid ended up clear and the reaction was fast, seconds if possible! $\endgroup$ – Rich Standbrook Jul 15 at 19:51
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To clarify: it sounds like you want a layer of insoluble sediment, so that if you shake the bottle, the water looks cloudy. Maybe it settles back out quickly, or maybe the water stays cloudy.

Then, you add something and shake the bottle again. This time, the sediment disappears.

There are lots of possibilities here. One that comes immediately to mind is lime (calcium hydroxide), which is only slightly soluble in water. Add acid (vinegar if you're timid and don't mind adding a large quantity, hydrochloric acid if you want fast results), and it will be converted to a highly soluble salt.

Chalk (calcium carbonate) would work, too, but it would fizz (produce bubbles of gaseous carbon dioxide) when you added the acid.

If you can say a little more about the context of your demonstration, we might be able to provide better ideas. Is this actually about solubility, or is it a metaphor for something else?

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  • $\begingroup$ Thank you—this sounds helpful. The demonstration is a metaphor for hidden problems in life that resurface when we are shaken and cloud our judgment! I will give the chalk a go. Perhaps with a hole in the lid for the carbon dioxide to escape! $\endgroup$ – Rich Standbrook Jul 15 at 19:48
  • $\begingroup$ You can likely use big grains table salts, with some timing and preparation. No need for an extra ingredient except water and salt. Just experiments before to find out the amounts that work well. $\endgroup$ – Alchimista Jul 16 at 10:35
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Start with sugar in ethanol or isopropanol (rubbing alcohol), then add water to dissolve. For fast dissolution, use confectioners sugar.

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Another trick is to use refractive index matching of the solid and the solvent. There are several organic solvents which match the refractive index of silica. Chlorinated solvents match silica RI very closely. However the best/simplest trick is by Karsten.

Search refractive index matching.

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You need (1) Any solid in powdered form (2) Recommended: Any other product that would change the nature of the solid to make it soluble in the solvent (let's inaccurately call it "solubility enhancer"), (3) Any solvent in which this solid would dissolve without producing gases or unsoluble co-products.

You would pick the solid and solvent based on:

A) How much solid you would like to dissolve in the liquid. The Ksp (Solubility Product Constant) would be a good indicator of this, telling you how many molecules you can dissolve in a given volume of solvent at the given temperature you'd like to do your demo. If you want to have a lot of sediment that would disappear completely, you would need a compound that is highly soluble in that solvent and at that temperature (in the form resulting from the reaction with the solubility enhancer if you chose to use one).

B) How quickly you want it to dissolve (i.e. "dissapear"). You can influence on this independently of the Ksp in at least to ways: The first is how finely ground your solid is (as it influences its contact surface), you could go from pellets down to "nano-powders", the second is how the reaction itself will influence the conditions determining the speed of the reaction: the solubility of many compounds increases with temperature for instance, and some reactions give out energy (called "exothermic" reactions), which means that you could chose a reaction that increases it's own rate of reaction as it proceeds (carefully within the limits necessary not to increase the container's temperature or internal pressure beyond what would be safe for your demo).

I would recommend one setup whereby:

(1) The solid to be dissolved is first set at the bottom of your container
(2) The solid to be dissolved is covered by a very thin layer of another solid densely packed in such a way as to insulate the first layer as much as possible from the solvent. This solid won't dissolve but the relative quantity is so small that no one will notice.
(3) The solvent is then very carefully added as not to mess up the two layers of powder
(4) You keep the recipient untouched, preferably right were you need it for the demo
(5) Right before your demo you carefully add to the solvent the chosen "solubility enhancer"

As you start shaking the bottle, the insulating solid layer will disperse, allowing for the underlying solid powder/pellet? mass to contact (1) the "solubility enhancer" it will react with to create a more soluble product (2) the solvent it will dissolve in. Now based on the solvent, the solid, and the product you make it react with, you could be shaking for a dozen seconds, or for a couple hours. :)

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    $\begingroup$ Thank you for your answer—it's sounds a little too involved for my needs. I was also hoping for the solid to cloud the liquid before adding a substance to dissolve it. $\endgroup$ – Rich Standbrook Jul 15 at 19:50
  • $\begingroup$ When you say "cloud the solution" do you mean, clouding it completely, or just enough that people see that there is tiny particles suspended in the entire volume of solvent in the bottle? (the latter case would happen for most reactions which will take at least a few dozen seconds to proceed) $\endgroup$ – Veritas Jul 15 at 20:06

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