Can mercury evaporate if it's covered by water? I was recently watching a video about elemental mercury and how it's cleaned up in water (fish tanks), and it was mentioned how mercury can be toxic in vapor form.  My question is, if I were to drop a bead of mercury in a glass of water, could it still evaporate, since it's 13x denser than water, and therefore sinks to the bottom?
 A: Mercury is slightly soluble in water. At room temperature the solubility is about 56$\mu$g/litre. So while the mercury would not evaporate if you added enough water, or kept replacing the water, the bead of mercury would eventually dissolve away.
In principle the mercury solution could evaporate the mercury into the air, but in practice the rate of evaporation would be negligably low.
It's not precisely true that mercury is toxic in vapor form - it's toxic in any form. However the vapour is easily absorbed by the body because it can be breathed in. The mercury solution in water would also be toxic, but the concentration is low enough that you'd have to drink an awful lot of it to do you any harm.
A: The process of evaporation and condensation at the interfacial boundary between a liquid and gas is governed by models developed in statistical mechanics. The probablity that an atom or molecule of liquid will leave the interface into the gas or otherwise enter the interface is determined by a number of factors including temperatures of the gas and liquid, the liquid's saturated vapor pressure at temperature, and the pressure of the gas among other factors such as surface tension. 
Evaporation and condensation are equivalent in the sense that they are a flux - a movement of atoms or molecules and different only in the sense of the flux direction. At the interface, both occur at the same time. But depending on temperature and pressure, the flux may be greater in one direction vs the other and so we say that either evaporation or condensation is occurring.
While it's somewhat easy to calculate the flux between Hg and air, introducing a third component, water just complicates the calculation, but its still possible.
Without going into details of the calculations I can confidently say that vaporization of the mercury into the air is still possible with water covering it, but the probability will be reduced. The water acts as a barrier but does not completely stop it.
And it may be, depending on the depth of the water over the mercury and its temperature, that the rate of dissolution is greater than the rate of vaporization.
A: Water cover does not prevent elemental mercury from evaporation. This can be explained by the fact that the solubility of mercury in water (and many other fluids), although insignificantly small, is still higher than in air. The solubility of mercury in water at 25 °C is $3*10^{-7} mol / l$ or $60 mg / m^3$, which is three times the concentration of Hg saturated vapor in air at the same temperature ($20 mg / m^3$).
Experimental proof could be found, for example, here (page 13) CODE OF PRACTICE - MERCURY HOUSEKEEPING
