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I am trying to find a way that you could cause metal (Aluminum) or glass to float in a mixture of water and some water-soluble material. I am aware that salt can be used to heighten the density of water, but I doubt it could go to that extent because both aluminum and glass are more dense than salt (possibly flawed logic, I have limited knowledge of the subject). So basically: if there is any way to cause Aluminum or glass to float in water, possibly through adding a water soluble material to the water, please let me know.

Edit: Basically what I'm doing is trying to think of creative ways to sort materials by density, for example a set of marbles. With the example of solutions with salt, you would vary the amount of salt in the mixture to alter what materials would float.

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  • $\begingroup$ You might be able to find something clear and nontoxic that appears to be water that is dense enough to float glass or aluminum, but an easier option might be trickery, via a hollow metal ball. Not sure whether this is an option for your application. $\endgroup$ – Jason Patterson Oct 11 '15 at 1:54
  • $\begingroup$ Try to make the appropriate surface area of metal or glass . So that they flow rather than this because it is not economic feasible $\endgroup$ – Raj kumar Singh Oct 11 '15 at 8:42
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The best current non-toxic alternative to Clerici Solution appears to be Sodium Polytungstate (SPT). It also has a neutral (~7) pH, which is important when working with aluminum. By dilution/evaporation, density covers the range $1.1\!\!-\!\!3.1$g/cm$^2$, but above $2.5$g/cm$^2$ it can develop a harmless reversible blue tint when exposed to reduced metals. Aluminum has an oxide patina, so I am not sure if it qualifies as a reduced metal in this setting. SPT is rather expensive though.

Aluminum is $2.7$ g/cm$^2$ and Common glass is $2.52$ g/cm$^2$. So both of those materials, many gemstones and some minerals, will float in SPT.

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  • $\begingroup$ @JohnDowns cesium tungstate is NOT sold at www.formatebrine.com. They sell Cesium Formate, a completely different animal. Instead of editing my answer, please create your own independent answer. $\endgroup$ – Eubie Drew Jan 28 '16 at 22:14
  • $\begingroup$ Yeah, that was clearly a spam edit and should have been rejected. $\endgroup$ – Jan Jan 30 '16 at 17:37
  • $\begingroup$ @Jan, I would have still tried to incorporate the edit if it had been about cesium [poly-]tungstate, but it wasn't even that. $\endgroup$ – Eubie Drew Feb 4 '16 at 1:38
  • $\begingroup$ That would have been way too kind to a spammy edit suggestion ;) $\endgroup$ – Jan Feb 4 '16 at 14:42
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One of the requirements for the liquid base of oilfield drilling fluids is high density. Besides preventing the chips from falling out of suspension, the increased hydrostatic pressure helps prevent the sides of the well from collapsing before a steel casing can be applied.

Oilfield drilling fluids are based on alkali metal formate solutions. Sodium, potassium and even cesium formate solutions are used on a multi-ton scale. Reasons for their success in this application are the high solubility of formates and the low toxicity of the solutions. Sodium and potassium formate are certainly dirt cheap; I am unaware of the price of cesium formate.

According to the relevant document dowloadable at http://www.cabotcorp.com/solutions/products-plus/cesium-formate-brines/formate-technical-manual cesium formate brines of up to 2.50g/cm3 density can be prepared at 15.6C. I would assume solubility increases at higher temperature.

That may not be enough to float regular glass, but should be enough to float borosilicate glass, which according to http://www.udel.edu/chem/GlassShop/PhysicalProperties.htm has a density of 2.23 g/cm3. Similarly, aluminium will not float in this solution but boron will.

Looking at the other answers, I note that Clerici solution uses a combination of formate and malonate, presumably to improve solubility by having a mixture of ions. This strategy should in principle work with cesium. Probably the ultimate non-toxic solution would be a cesium polytungstate solution.

One issue you will find is that materials of similar density tend to have similar refractive index. Transparent glass in a high density transparent solution may be almost invisible. Also, be careful with aluminium in contact with alkaline solutions. If the pH is high enough, they will dissolve the oxide layer, allowing the aluminium underneath to corrode extremely rapidly with evolution of hydrogen.

Edit: Thanks to John Downs for the following info: Cesium Tungstate solution with density of SG 2.80 is available from various sources. This is interesting though it less than the density of the sodium polytungstate solution mentioned in Aabaakawad's answer.

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Well, if a toy boat made out of aluminum or glass is not what you had in mind, let us consider the method of increasing water density with additives. Surely, ordinary table salt will not get you this far, but there are other salts. Clerici solution comes to mind; it is probably not that easy to come by, but certainly dense enough. And yes, it was invented for this very purpose: to separate materials by density.

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    $\begingroup$ Make sure you don't drink it - thallium is highly toxic. $\endgroup$ – Gimelist Oct 11 '15 at 3:43
  • $\begingroup$ Alright, is there maybe some other way of floating metal on water? possibly applying some force through the water, like air, though would the weight be affecting this rather than density? anyway, do you have any ideas for alternatives to the other solution? (forgive the pun) $\endgroup$ – Anonymous Oct 11 '15 at 4:12
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From a practical point of view: try methylcellulose. It forms a transparent gel with water. From intuition I'd say you can get a saturated enough solution at room temperature to get glass or aluminium marbles to float (warning, I haven't tried it myself or calculated whether it's possible). Alternatively, use another gelling agent with food applications, such as xanthan gum, or even cook a starch pudding (preferably tapioca starch, as other common types such as cornstarch are not as transparent). You can change the concentration to get the density you want.

Unlike lab reagents, food gelling agents are easy to obtain (some more than others) and nontoxic to the point of being safe to ingest. The downside is that the result is visibly viscous. So if you are trying to do construct a magic trick ("look, this glass marble floats in pure water") it won't help.

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    $\begingroup$ Is the "floating" due to density effects or viscosity effects? This Dow chemical publication lists the specific gravity of a 10% solution at 1.0245 (table 6), which is well below the expected density of glass/aluminum. While marbles might float "kinetically" on a high-viscosity solution, their equilibrium state might not be - meaning such a solution would be ill suited for separating items of different densities. $\endgroup$ – R.M. Oct 11 '15 at 18:12
  • $\begingroup$ @R.M. I really can't say. I'm just speaking from experience using this kind of gels around the home, but I have never floated something as dense as aluminium in them. If somebody here can confirm or disprove my suggestion either by calculation or empirical observation, I'd be very happy to hear it. $\endgroup$ – rumtscho Oct 12 '15 at 9:55

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