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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: INeedSerotonin on December 01, 2019, 02:20:27 PM

Title: How come it is still possible to dissolve?
Post by: INeedSerotonin on December 01, 2019, 02:20:27 PM

Hello, my friends

Suppose we dissolve NaCl in water until it is not possible anymore (that is, it begins to saturate).

Then we add KMnO4. How come the water becomes purple? Doesn't it have already been saturated?

Can you help me understand this?

Thank you

Title: Re: How come it is still possible to dissolve?
Post by: Yggdrasil on December 01, 2019, 02:42:22 PM

KMnO4 produces different ions than NaCl.  The solubility of potassium and permanganate ions are independent of the solubility of sodium and chloride ions.  One would have difficulty dissolving additional salts containing sodium or chloride (e.g. KCl or NaBr) in saturated a NaCl solution, however, because of the common ion effect (https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Solubilty/Common_Ion_Effect).

Title: Re: How come it is still possible to dissolve?
Post by: Enthalpy on December 02, 2019, 08:02:10 AM

And "water is saturated" is an unfortunate expression as it depends on the solute. If you drop AgCl in water, after 2mg/L the water is saturated, but it can still dissolve much NaCl.

Title: Re: How come it is still possible to dissolve?
Post by: Corribus on December 04, 2019, 01:04:13 PM

Quote from: Yggdrasil on December 01, 2019, 02:42:22 PM

KMnO4 produces different ions than NaCl.  The solubility of potassium and permanganate ions are independent of the solubility of sodium and chloride ions. 

Only for dilute solutions, though. If the solution is saturated with NaCl, the ionic strength of the solution will affect solubility of other salts, even with no common ion.

https://pubs.acs.org/doi/pdfplus/10.1021/ed081p1644

In fact the solubility usually increases, which is very counterintuitive.