Supersaturation 11/12/22

Science with Richard Bleil

Let’s take a moment to discuss a concept that is often misunderstood and a term frequently misused, specifically supersaturation.  But before we can discuss this, we must discuss saturation, and before we can talk about that we must discuss mixtures and solutions.  But before we discuss mixtures and solutions, let’s talk about solutes and solvents.

Often chemistry professors will say that in a solution, the solute is what is present in the lesser amount.  This is usually true, but there are exceptions.  We use solutions for ease.  It’s easier to measure out a teaspoon of a medication than five milligrams, even if we had the ability to do so at home.  And a little bit of error in a teaspoon of solution will be far less critical than an error in measuring five milligram of the pure medicine.  In this solution, the solute is the medication.  On the box, it’s called the “active ingredient”.  That’s really what a solute is.  The solute is the “active ingredient”, it’s the reason you have chosen the solution, it’s the reason you are using it.  The solvent, then, is the delivery mechanism, the liquid the solute is in.  To suggest that the solute is what is present in the smaller amount suggests that my ex-wife drank the alcohol that I kept in the house as a cleaning solvent for the 5% water that was present in it.  Does that really make sense?

There is difference between a mixture and a solution.  In a solution, the solute dissolves, completely, in the solvent.  Solutions are always clear (but not necessarily colorless), and mixtures are always cloudy.  Eventually, a mixture will separate, but a solution never will.  For example, milk is a mixture, not a solution.  The white color is caused by a protein called Casein (not calcium as many people assume).  If you leave milk in the refrigerator too long, the white casein will begin settle in the bottom of the milk, leaving a yellowish liquid on top which is water.  The milk isn’t necessarily going bad at this point (lumpiness is an indication that it’s going bad), but it’s settling.  My pop, on the other hand, will never separate.  It’ll always be mixed no matter how long it stays in the refrigerator.  I drink diet, so it can go bad, but it won’t settle because it’s a true solution.

The terms unsaturated, saturated, and supersaturated all are relative to the solubility.  The solubility refers to the maximum amount of solute that can dissolve in a given solvent.  As it turns out, everything is soluble in everything else, but not necessarily to a useful degree.  Oil does dissolve in water, but so little dissolves that we say it is “insoluble” (or, since it is liquid in liquid, we say it’s “immiscible” which means it’s not allowed in a court of law).  When something is soluble, that means we can dissolve enough of the solute in the given solvent so as to be practical (or, for liquids in liquids, we say “miscible”). 

If we have dissolved some solute in a given solvent, but could dissolve more, we say the solution is “unsaturated”.  Pop has a LOT of sugar dissolved in it, but if you take another teaspoon of sugar, it, too, will dissolve in the pop.  Why you would want something so sweet is, well, I couldn’t understand it, but because you could dissolve more sugar in the pop, we say the pop is “unsaturated”.  Then a saturated solution has exactly as much solute dissolved in the solution as possible. 

Now, if you start with a saturated solution, and add more solute, the solute will not dissolve because the solution already has the maximum amount of solute in it.  This is actually the easiest way to be sure the solution is, indeed, saturated.  If it’s not saturated, or if solubility increases (say because the temperature changes), then some of that undissolved solute will dissolve until, once again, the solution is saturated.  If solubility decreases, excess solute will form on the undissolved solute until, again, the solution is saturated.  When you decant off the liquid, leaving the solid behind, that liquid is saturated.  But this mixture is not supersaturated, as many people believe it to be.

Supersaturated means that the solution has more solute dissolved than is possible.  Sound impossible?  Well, that’s because it is.  In theory.  Here’s how it works.  Start with a saturated solution, add excess solute, and change the temperature (usually by heating) until that excess solute dissolves.  At this new temperature, the solution is still saturated (or unsaturated if you pass that point) and not supersaturated, but once it dissolves completely, cool the solution down again.  To reform the solid, crystals require something to form on, a “seed point”, so if you have a perfectly smooth container, and no excess solid, then the crystals cannot form.  Then, you end up with a supersaturated solution because there is more solute dissolved than should be. 

Supersaturated solutions are unstable by nature, because they are in principle impossible.  Sometimes, just shaking or disturbing the solution will cause the excess solute to precipitate out as a solid.  Or, if you add just one crystal of the solute, the excess solute will drop out of solution as a solid.  Or, if you just scratch the inside of the container (as chemists do), the excess solute will precipitate out.  Once the excess does precipitate out, the solution again becomes a saturated solution (or, more correctly, a saturated mixture). 

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