Does all salts disssolve in water?

Answer 1

Technically, all salts are soluble (i.e. will dissolve) in water - the problem is what context you are thinking of. Salts such as sodium chloride are so highly soluble in water that it is obvious that they dissolve, but other salts which are so poorly soluble that they are termed insoluble still have some solubility.

An example of this is Fluorite (calcium fluoride), which most people would regard as insoluble in water. In fact, fluorite is soluble in water at a level of approximately 10-4 moles/kg (8mg/litre), so with enough water and enough time to dissolve it, fluorite will dissolve. Another example from geology is quartz, which makes up most beach sand. This is soluble in water at a level of 10kg per million kg of water, and it is estimated that this is process alone is responsible, on average, for the lowering of the land surface by 1cm per 1000 years.

SiO2 + 2H2O --> Si(OH)4 (aq)

So, in a similar way, all salts - even supposedly insoluble salts - will dissolve in water; it really depends upon how much water you have available and what kinetic barriers there are to the salt dissolving.

In some cases, though, the solution process is incongruent. For example, orthoclase may dissolve in water to give kaolinite:

2KAlSi3O8 +11H2O --> Al2Si2O5 + 4Si(OH)4 + 2K+ +2OH-

Other factors to take into account are the pressure and temperature. Salts that are insoluble at room temperature and pressure may be very soluble at elevated temperatures and pressures. In the case of fluorite, its solubility at 1000 °C under 2 GPa of pressure increases two thousand times to 0.247 mol/kg H2O.