Purpose
To illustrate solubility rules for solids in liquids and gases in liquids.
Procedure
Supersaturated Solution of Sodium Acetate
Preparation:
- Take some sodium acetate trihydrate in a test tube and “melt” it by heating with a Bunsen burner. This dissolves it in its own waters of hydration.
- Once fully dissolved, gently drip in some water to form a barrier against premature crystallization (keep this layer thin on top of the saturated solution).
Cooling:
- Allow the solution to cool for a while.
- Prepare multiple test tubes, ensuring that at least one contains some leftover crystals to create a saturated solution upon cooling.
Observation:
- Compare the stable saturated solution with the unstable supersaturated solutions.
- Introduce a nucleation site for crystal growth by adding another crystal of sodium acetate and observe the results.
Alternative Method:
- This demonstration can also be conducted using a commercial hand warmer with a metal disk inside.
Key Concepts
- Discuss how solubility is affected by temperature.
- Explain nucleation and seed crystals.
- Define unsaturation, saturation, and supersaturation.
- Optionally reference “Ice 9” for fans of Kurt Vonnegut.
Gas Solubility Demonstration
Soda Examination:
- Show a sealed bottle of Diet Coke and ask students whether it is saturated, unsaturated, or supersaturated.
- Open the bottle and discuss how it becomes supersaturated.
Flat Soda:
- Ask what happens to soda when it goes back to being saturated (it becomes flat).
- Explain that this is a slow process in a smooth container and that nucleation sites are needed for bubble formation.
Mentos Reaction:
- Add a Mentos candy to the soda after discussing how its rough surface acts as a nucleation site.
- Conduct this in a clear plastic tote for easy cleanup.
Prediction and Experiment:
- Have students predict whether a bottle kept in hot water or ice water will produce a bigger fountain (lose more CO₂).
- Get a volunteer to help conduct the experiment.
- Discuss how solubility differs in warm versus cold conditions. Use three Mentos per 20 oz. bottle for optimal results.
Additional Concepts
- Osmosis and Dialysis:
- Discuss concepts involving dialysis tubing, starch, sugar, and iodine (refer to Shakhashiri, Vol. 3, page 290).
- Salting Out:
- Explain the salting out process (refer to Shakhashiri, Vol. 3, page 266).
- Volume Changes on Mixing:
- Consider discussing the effects of volume changes when mixing different solutions.