How do I know whether a solution of potassium nitrate in water is saturated?

1 Answer
Feb 5, 2017

Can you see crystals (of potassium nitrate) at the bottom of the flask?


If you can, then the solution is saturated.

Saturation describes an equilibrium condition as shown:

#"Solution of stuff "rightleftharpoons" Crystals of stuff"#

For highly soluble solutes such as potassium nitrate, equilibrium, i.e. saturation may be hard to reach in that the salt is mighty soluble in water, and a lot of solute must be added to reach equilibrium.

We could take an excess of salt (probably around half a kilo per litre of water) heat it up and bring all the salt into solution. Upon cooling, the salt may not precipitate, and remains in solution as a # "supersaturated solution"#; that is a solution which contains an amount of solute GREATER than that which would be in equilibrium with undissolved solute. Such solutions are metastable, and can be restored to equilibrium, to saturation, by scratching the side of the flask, which can result in the precipitate of a mass of crystals.

And if the solution contains LESS solute than would be in equilibrium with undissolved solute, then the solution is termed #"unsaturated."#

You may think I am being unnecessarily wordy, but the definition of #"saturation"# is strict, and describes this equilibrium condition. For more of the same, look for the links here.