Supersaturated+Solution

=Supersaturated Solution=

Video
If you don't have the materials, then watching a video is a fun way to show supersaturation (and you don't need to worry about the demo going awry). YouTube Video of Supersaturated Sodium Acetate Crystalization

Materials
sodium acetate trihydrate water funnel filter paper pyrex bottle hot plate small crystallization dish 100-200 mL beaker

=Preparation=
 * Boil about 50mL of water in the beaker. Add sodium acetate trihydrate until it won't dissolve anymore.
 * Filter the solution while it is still hot, then pour the hot solution into the bottle to cool.
 * Put a small amount of the solid in a vial to use later.
 * If you plan to do this demo more than once, you may want to bring a few small bottles of the supersaturated solution. I've had the solution completely precipitate out in the bottle while doing this demo for one class, and then I couldn't do it for the second class.

=Demo=
 * Put a few grains of the seed crystal in the crystallization dish.
 * Slowly pour the supersaturated solution onto the seed crystals. The solute will almost immediately precipitate out when it comes into contact with the seed crystals. If you pour slowly enough, you can make a drip-tower out of the precipitate.

=Explanation= A supersaturated solution contains more dissolved material than could be dissolved by the solvent under standard conditions. The supersaturated solutions can be prepared by changing a condition under which a saturated solution is prepared. For example, increasing or decreasing the temperature, decreasing the volume of solvent via evaporation, or increasing the pressure of a gas can all result in supersaturated solutions.

Supersaturated solutions lie far from equilibrium. If they are allowed to sit, over time, the solute will precipitate or evolve out of the solution so that the solution reaches its equilibrium state. Adding small particles, or seeds, to the supersaturated solution can induce the separation of the dissolved material from the solvent. These seeds can induce the formation of crystallites, which are tiny crystalline particles. If the solvent is evaporated slowly, large single crystals can be formed, such as with rock candy.

=Clean up=
 * The precipitated solution can be returned to the bottles, and the supersaturated solution can be remade. Rinse the crystallization dish out with water.