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Supersaturated Solution

Solubility is defined to be the amount of solute that will dissolve in a given amount of solvent at a specified temperature. Typically, solubilities are recorded as grams of solute per 100 grams of solvent. For most solutes, increasing the temperature increases the solubility, that is, more of the solute dissolves in the same amount of solvent.

Solutions can be defined as unsaturated, saturated, or supersaturated.

  • Unsaturated solutions contain less than the maximum amount of solute possible. As more solid is added to this solution, it will dissolve.
     
  • Saturated solutions contain all the solute that is possible to dissolve under a given set of conditions. As more solid is added to this solution, it will not dissolve.
     
  • Supersaturated solutions are unstable solutions that have been forced to dissolve more solute than should be possible under a given set of conditions. A solute crystal placed into a supersaturated solution will begin to grow.
    A common way to prepare a supersaturated solution is to prepare a saturated solution at a high temperature and then slowly cool the solution. It is possible that all the solute may remain dissolved, even though the solubility decreases as the temperature is reduced.
 

The graph below shows the solutility curve for potassium chlorate. solubility curve

For this lab, you are to prepare a supersaturated solution of potassium chlorate. A check of the Material Safety Data Sheet for potassium chlorate indicates its solubility is "moderate", at 7.4 grams per 100 cm3 of H2O at 20 oC.

  • The following will be automatically provided:
     
    • A maximum of 25 grams of potassium chlorate
    • Distilled water
    • 1 electronic balance
    • 1 reagent spatula
    • 1 250 ml beaker
    • 1 thermometer
    • 1 glass stirring rod
    • 1 lab burner
     
  • Be specific with your procedures. You must know if additional materials will be needed for your lab. If they are, you must make arrangements for them BEFORE lab day.
HINT: When preparing a supersaturated solution, care must be taken to insure that all glassware is perfectly clean. If it is not, crystals will form on the irregular surfaces as the solution becomes supersaturated - keeping the supersaturated solution from forming.
 

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Jim Askew  

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