# Constant concentrations in solution

I'm a high school student considering the concentration of solutions, and whether the concentration is constant at the bottom or towards the surface. Even then, how would I know that the solution is consistent and not more concentrated in places compared to others? Is there any visual way or method for identifying non-consistent concentrations in solution i.e. Copper (II) sulphate, Iron (II) sulphate

• If the solution is well mixed (stirred or shaken, etc.), then the concentration will be uniform throughout. But a solution of $\ce{CsCl}$, in an ultracentrifuge, is a different matter. Drop a few grains of potassium permanganate into a beaker of water and you can vividly see that the dissolution is neither nearly instantaneous nor uniform. But stir or shake the contents of the beaker, and you get a uniformly optically dense solution. – Ed V Aug 19 at 1:57

## 1 Answer

A solution by definition is a homogeneous mixture, so its "concentration" is same throughout the bulk. Let us have a look as to how one would prepare a solution of a given concentration, say in 1 L. We would weigh out a known weight of a solute and dissolve it ~ 500 mL of solvent, with mechanical stirring or shaking. We would add more solvent until the total volume is 1 L. We would stir it sufficiently, again, until we believe that the stirring has made the solution uniform.

a) If it a colorless solid, like NaCl, there is no visual way to tell if the mixing is complete and the solution is uniform. You have to stir it very well.

b) A colored solid, like copper sulfate or iron sulfate is more or less visible if it is not homogenous. Different colors in the bulk.

If you are still not (mentally) satisfied, you can take portions of the solution from surface, middle and bottom. Measure their refractive index and they should be identical. If they are not not, the mixture needs more stirring.

Edits: Some people may wish to differentiate microscopic vs. macroscopic homogeneity regarding the definition of solutions. IUPAC does not offer such a distinction. For an analytical chemist or any researcher interested in measuring or making known concentrations of solutions- a solution has to have identical concentration throughout the bulk. Ultimately, it will boil down to semantics or the problem of "how many angels can land on the head of the pin" - a debate which has no end! en.wikipedia.org/wiki/…

• +1 for the answer given by @M. Farooq, but one little thing: it is very easy to think that stirring is (or has been) adequate, especially with colorless solutes, just as it is easy to think that a deck of playing cards has been adequately shuffled to randomness, whatever that really means. If in doubt, stir (or shuffle) more! – Ed V Aug 19 at 2:23
• An additional comment, though OP shall be satisfied by the actual answer above. I am not sure that homogeneity of a solution refers to its composition (concentration) or just to the absence of phase boundary. I mean, out of lab glassware size, as in atmosphere, oceans or just a tall reservoir, we should still see that as solutions, in spite of density and concentration gradients.Right? – Alchimista Aug 19 at 7:23
• Solutions refer to homogeneity in composition. In a strict sense air or oceans are not exactly solutions because their composition varies in the bulk as a function of depth or height. – M. Farooq Aug 19 at 19:13
• This is not true - definition is about microscopic homogeneity - there's one phase not some tiny droplets. There's nothing about perfect mixing, which doesn't even exist. – Mithoron Aug 19 at 23:07