Supersaturation

Purpose:

To demonstrate crystal growth from a supersaturated solution of sodium acetate.

Materials:

• 100mL beaker
• 1L beaker
• 500mL Erlenmeyer flask
• Crystallizing dish
• Hot plate

Reagents:

• Sodium actetate (NaC₂H₃O_2(s))
• Sodium acetate dihydrate (NaC₂H₃O₂·2H₂O_(s))

Hazards and PPE:

• The saturated solution is an irritant, and contact with skin should be avoided.
• Wear approved safety goggles and gloves.

Protocol:

Preparation

1. Prepare a boiling water bath with a 1L beaker.
2. Measure out 175g of NaC₂H₃O₂·2H₂O_(s) into a clean 500mL Erlenmeyer flask, taking care to limit the amount of solid that comes into contact with the side of the flask.
3. Add 50mL DI H₂O to the flask, using this water to rinse off any residual solid on the side of the flask.
4. Heat the flask in the boiling water bath, gently swirling the flask occasionally until a clear, homogenous solution forms.
5. After removing the flask from heat, place in a safe area on paper towels. Invert a 100mL beaker over the mouth of the flask and allow to cool to room temperature (1-3 hrs).
   1. Take care not to jar or bump the flask, which will cause the solute to precipitate out of solution ('crash out').
6. Firmly cover the solution with parafilm until needed.

Demonstration

1. Drop 1-2 crystals of NaC₂H₃O₂ on a crystallizing dish. Slowly pour the prepared solution onto the crystals. Crystallization will occur immediately, forming a mound of solid NaC₂H₃O₂.
   1. Alternatively, use a glass stir rod to scratch the bottom of the flask containing the solution to induce crystallization through the solution.

Additional Notes:

• If the solution is not cool enough, the solid will not effectively form. Ensure you prepare the solution early enough before the demo to allow for this.
• It is a good idea to make two batches of the solution, in case one of the batches crashes out. If they both work, they can be used to make a larger mound.
• In some cases it is best to purchase a brand-new flask to ensure it is properly clean.
• After the demonstration, the solid can be cut up with a spatula and returned to the flask. The supersaturated solution is restored by heating in a boiling water bath. The mixture can be used again if not contaminated. Small amounts of water may have to be added to compensate for evaporation losses.
• A variation on a small scale involves placing the supersaturated solution in a large test tube with an accompanying thermometer. Add 1 or 2 crystals to the test tube and observe the feathery crystals form. Illumination from below makes the phenomenon more easily visible. Note that the temperature will increase.
• Have a sample of a Heat Solution hot pack available to show a practical application of this concept.

Disposal:

• The solid can be thrown away in the trash.
• Any remaining solution can be washed down the drain with excess water.

Reactions:

• This demonstration involves the precipitation from aqueous to solid NaC₂H₃O₂·2H₂O_(s).

Citations:

1. Shakharshiri, Bassam, Chemical Demonstrations, Volume I, 1983.

Videos:

• Sodium Acetate Crystal Explosion

  The Action Lab