Solubility & Saturation

 

Solubility - The amount of a solute that will dissolve in a specific solvent.  A substance may dissolve in a solvent, and it may not.  For instance, salt dissolves easily in water:  Salt is said to have a high solubility in water.  However, sand does not dissolve appreciably in water:  Sand has low or no solubility in water.  If a substance does dissolve in water, three kinds of solutions can be made:

 

1) Saturated - A solution is saturated if it contains as much solute as it possibly can.  At room temperature, 100 grams of water can dissolve a maximum of 37 grams of salt.  When these conditions are achieved, the salt water solution is saturated.

 

2) Unsaturated - A solution that has less than the maximum amount of solute that can be dissolved.  At room temperature, any amount of salt less than 37 grams in 100 grams of water would constitute an unsaturated solution.

 

3) Supersaturated - A solution that contains more solute than it should, which results in a highly unstable solution.  If we were somehow able to get 40 grams of salt dissolved in water at room temperature, the solution would be supersaturated. 

 

How to make a supersaturated solution:

 

Rock Candy is made from a supersaturated solution of sugar water.  Sugar is dissolved in a big kettle of water until no more sugar dissolves at room temperature.  Then, the saturated solution is heated up.  The higher temperature allows for more sugar to be dissolved, so more sugar is put in.  As more and more sugar is put into the kettle, it dissolves, which makes the solution saturated at the new higher temperature.  Now, if the sugar is cooled down very slowly, and no more sugar is added, the sugar will stay dissolved.  As the temperature cools, the sugar dissolved at that higher temperature stays dissolved, and a supersaturated solution is made.  At this lower temperature, the solution is unstable, and by putting a small crystal of solid sugar in the kettle, the extra sugar that is dissolved will attach itself to the sugar, and grow crystals, or rock candy.

 

 

Effects on Solubility

 

Solubility can be affected by the temperature of the solvent.

 

The graph at right shows the solubility of several solids at different temperatures. Any point on the curve is saturated. Therefore a saturated solution of KNO3 at 25 OC would contain approximately 40 grams of dissolved potassium nitrate.  

 

Any point below the curve indicates an unsaturated solution. At 25 OC, an unsaturated solution of KNO3 would contain less than 40 grams of dissolved potassium nitrate.

Any point above the line is a supersaturated solution. At 25 OC, any amount of dissolved KNO3 greater than 40 grams would be supersaturated.

 

  Diagram Courtesy of ChemCom 4th Edition

 

Complete the following worksheet: Solubility and Concentrations

Links to more information/examples on Saturation and Solubility

Digital Textbook - Factors Affecting Solubility

Digital Textbook - Saturated Solutions and Solubility