Saturation of multiple solutes

Question

Can a liquid which is saturated with a single solute, dissolve a different solute, or is saturation a universal thing? I ask because I’ve seen that different solutes have different points (amounts) at which they become saturated in water, for example, which caused me to think that maybe each solution was perhaps independent, even though that would produce some interesting extreme implications. Something like, take a water & sucrose mix that is saturated; can this mixture then dissolve salt?

The specific implication I am asking about is this: If I dissolve xylitol (a sugar alcohol) in water until it is saturated (200 g xylitol to 100 g water at 25 °C / 77 °F) can I then dissolve erythritol (a different sugar alcohol) into the solution?

Obviously, we could heat the liquid and enter super-saturation, but that’s not what I’m asking. I’m asking if this can be done without super-saturating (i.e. risking precipitation/crystallization) once cooled.

Answer 1

Different solutes in the same solution indeed can affect each others' solubility. One example where this effect can be used to advantage is the "salting out" of a protein from water solution. Adding a salt (ammonium sulfate is especially good for this purpose) reduces the solubility of the protein in water, causing it to precipitate or crystallize out. Salting out can also be used to more fully separate an organic solvent from water. I imagine that in your example with the sugar alcohols, adding one component to water will reduce the solubility of the second component by a similar degree. But there may be cases where one solute increases the solubility of another.

Answer 2

The conundrum arises because dissolution is treated as a physical process when it is a chemical process. Solubility is dependent on the activities of the solvent, solute, and the precipitates [! no good word for the yet to dissolve or will not dissolve material]. The properties of the pure solids and [pure] solvents are assigned activities of one and shoved into the equilibrium constant and activity coefficients. Who really believes that the activities of water, solid NaCl, fused Al2O3, H2 gas etc. are all the same? It does make life easier. Adding something to a solution will change the activities of everything there; an application of LeChalelier's principle is a place to start. Someone working in formulation or even cooking will know that strange things happen when working near solubility limits. The timing, amounts, rate of addition order of addition can have profound effects on a product [Throw in some "normal" reactions and havoc can ensue].

The two sugar alcohols will probably compete for water lowering the total solubility. Look at the molecular structure and work on a molecular level, each alcohol molecule is probably equivalent to 6 waters. Also check on the formation of hydrates.