Why Does Salt Water Dissolve Less Salt? Something Is Already in the Way
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Adding a substance that shares an ion with something already dissolved pushes that something back out of solution, and the principle explains everything from soap making to kidney stones.
What happens
A sparingly soluble substance sitting in water is in a balance, dissolving and recrystallising at equal rates, with a fixed product of the concentrations of its ions. Adding a different soluble compound that supplies one of those same ions raises that ion's concentration, which pushes the balance back towards the solid, and some of the original substance comes out of solution. The substance has not become less soluble in any fundamental sense. The water is simply already carrying more of one of its components than it otherwise would.
Where the effect is used
The principle is exploited deliberately in several processes:
- •Salting out soap, by adding common salt to the finished mixture
- •Purifying a compound by adding excess of one of its ions
- •Precipitating an unwanted metal from an industrial solution
- •Buffering a solution against changes in acidity
- •Suppressing the dissolution of a mineral in groundwater
- •Recovering a dissolved product without evaporating the water
How soap making uses it
The clearest industrial example is the oldest. Boiling fat with alkali produces soap dissolved in a great deal of water along with glycerol and leftover alkali, and separating them by evaporation would be absurdly expensive. Adding ordinary salt supplies a large quantity of sodium ions, the soap is a sodium compound, and the balance shifts so that the soap comes out of solution and floats as a solid layer that can be lifted off. The glycerol and impurities stay dissolved below and are recovered separately, which is why glycerol is a soap industry byproduct.
The opposite case
Adding a different soluble compound can also increase solubility rather than reduce it, and keeping the two straight avoids a common confusion. Where the added compound shares no ion with the dissolving substance, the extra charges in solution shield the ions from each other, so they recombine less readily and slightly more of the solid dissolves. That is a much weaker effect and works in the opposite direction. Where the added compound reacts with one of the ions, removing it from solution, the balance shifts the other way entirely and the solid dissolves far more, which is how insoluble minerals are dissolved for analysis.
Where it works against people
The same principle causes problems as often as it solves them. Kidney stones form when urine carries an excess of one component of a sparingly soluble salt, which drives the rest out as crystals, and much of the dietary advice given to stone formers is an attempt to reduce that excess. Scale in pipes and boilers forms by a related mechanism. Hard water reduces the effectiveness of traditional soap because calcium ions drive out an insoluble compound as scum. And in analytical chemistry, an unnoticed shared ion in a reagent can quietly ruin a measurement of how much of something is present.
The takeaway
Adding a compound that supplies an ion already present pushes a dissolving balance back towards the solid, so less of the original substance stays in solution. Soap is separated from its glycerol and alkali by exactly this trick, using ordinary salt. The same principle forms kidney stones and boiler scale, and explains why hard water spoils traditional soap.