# Why do solutions become saturated?

Why can a solvent dissolve only a particular amount of solute? If we add more solute to the solution, the number of solute particles in contact with water increases. So rate of dissolution should increase, and subsequently, the solubility product should also increase. But why do we have a constant value of solubility product?

P.s. An explanation at the particle level would be appreciated

• Because there's a limit to every solvent to dissolve a solute in itself, which is called Solubility. – Syed Sahl Feb 8 '14 at 18:35
• I know that, and that's the reason I am asking this question. But can you please explain why there is a limit to solubility ? I want an explanation at the particle level – nilanjana Feb 8 '14 at 18:37
• If the solute is liquid, there could be no limitation, like ethanol. If the solute is solid and there is no limitation of solubility, I have a guess, imagine there is a big crystal like salt, and we add one droplet of water, it should become a liquid. A crystal being a crystal means there is a barrier to keep the lattice structure unless reaching the melting point etc to exceed the barrier. The attraction between a droplet of water and the big crystal of salt is much less than the total barrier energy of lattice, therefore it can not form a total liquid. – user26143 Feb 8 '14 at 19:34

Take a slightly more general problem of miscibility, all that in thermodynamic equilibrium. In your question you mix up kinetics and equilibrium, and additionally in a wrong way. Equilibrium is the most important part.

If you put two liquids together and shake them well, than in general, they do not mix. So you will observe a boundary between, say, diethyl ether (upper layer) and water (bottom layer). If you separate them and analyze their composition, you will find, that the ether fraction contains little bit of water (1.5g / 100 mL) and that the water fraction contains little bit of ether (6g / 100mL). Knowing this, if you now take 100 mL of water and mix it with 5g of diethyl ether, you will observe, that no boundary is formed, and all ether has dissolved in water.

As another example, if you shake ethanol with water, they mix in any ratio.

That's observation, now the rationale. The particles of pure liquid interact with each other, in attractive way (otherwise you will have no liquid, but gas). Now you mix two pure liquids together (A + B) and look, if some of the particles of A can easily sneak into the B. If their behavior is reasonably similar, they can do it and interact with particles of B also in attractive way. The attraction should be comparable or greater, than within pure A. That is the enthalpy part. But as we are dealing with free energy, there is also the entropy, which you have to take into account. On one hand, entropy favors mixing, as you add lot of accessible states. On the other hand, if the attraction of B to A is very strong, it starts to organize the molecules of A around itself, so they are not as free to move as they were in pure A, and limits the miscibility.

For the solubility of solids, it is technically similar, the A is solid and the interaction which hold sit together is the lattice energy.

As a side note, if you ask about solubility, miscibility and such, you should refrain from ideal approximations, as they do not predict such phenomena. Their calculation from pure substance properties is therefore terribly difficult, and physical chemistry tables are the only reasonable answer.

Why does anything dissolve at all? The binding energy (lattice energy) of the solid must be compensated by solvation energy and entropy of the solution. Like dissolves like (then cheats, like clathrates and micelles). As the solution concentrates, incremental driving energies diminish.

• U didnt answer my question – nilanjana Feb 11 '14 at 14:12

I agree that his answer is misleading ^^. Water for instance, only has so much space to hold anything inside of it; ie absorbing or becoming saturated with something, say salt. So a few things affect Saturation such as density and tempurature(which changes density of course). If the water is hot, it is able to take in more salt. Because the energy from the heat is expanding the space between the water; more space to take in material(salt). I'm sure you have experienced this with hot water.

So Saturation; The solution becomes saturated when the Solvant no longer has enough binding energy (lattice energy) for water to keep taking in Salt for instance.

If you add enough salt to water, the water will become saturated with salt> The salt will no longer dissolve and disappear into the water but will just float to the bottom of the solution.

• "heat is expanding the space between the water; more space to take in material(salt)" This is certainly an original thought worth to consider. I would have opted for the fact that the change in entropy from crystal state to soluted one is larger in higher temperatures. – Ich bin ein gäst Feb 12 '14 at 19:46
• But surely it has very little to do with space between molecules since you get examples where the smaller magnesium hydroxide is less soluble than the much larger potassium sulphate. I don't think anyone has really answered the question the op asked. He's not asking why things dissolve as everyone seems to be trying to answer, the OP is asking why there is a LIMIT to the amount of compound that dissolves which is a much more interesting question – MY2K May 13 '14 at 13:01

In dilute solutions, small particles are formed, which can suspend in the solution. As solute amount increases the particles grow bigger. At saturation, large particles are formed and precipitate from the solution.

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