system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there an upper limit to solubility? Why does a substance become saturated?

Take water and nitrogen or sugar and water as examples. Why can't I dissolve any amount of water I want into the air or any amount of sugar I want into a cup of water? What physically stops me? Is it simply a matter of the solvent keeping the solute suspended like dust in a stream?

My perspective is from Physics so I've been trying to wrap my head around the thermodynamics of the situation but the enthalpy angle doesn't seem to help me get a instinctual understanding of what's occurring...
gpt
Remove the word "saturation" completely from your mind. "Saturation" being used in this context is very misleading. It can make it seem like one of the phases alone can't "hold" any more of the substance. And this can lead to confusion as you've stated: "*Is it simply a matter of the solvent keeping the solute suspended like dust in a stream?*". Another example would be, the water "saturation" vapour pressure is the pressure of water that the air can "hold" at a given temperature. This is not how you should think of it at all.

To understand these limits you have to take into account the *equilibrium between the two phases*.

Let's think about water. If you placed a volume of liquid water into a closed volume of vacuum, some of the water will evaporate into the gas phase. But the reverse will also be happening: some water molecules in the gas phase will return to the liquid phase. The rate of this reverse process will increase as more and more water evaporates. Eventually, the rates of evaporation and condensation will be equal and an equilibrium between the two processes and between the phases is reached. The pressure of the water vapour above the liquid at this equilibrium is this "saturation" vapour pressure. But it has nothing to do with the vacuum or vapour phase being "saturated" and not being able to "hold" more water.

In more thermodynamically strict language, the chemical potential of the water in both phase (and the gas a liquid phases) has to be equal and the water will evaporate until this is achieved (equilibrium is reached).

You can apply this exact same reasoning to salts or sugars being dissolved. An equilibrium is established between the solid and dissolved phases.