Crystallization from a Supersaturated Solution

Brief Description: 

Exothermic crystallization from a supersaturated solution is demonstrated by pouring a solution of sodium acetate trihydrate onto crystals in a beaker resulting in a column of solid that can be several inches in height.

Purpose/Goal: 

Reinforces the concepts of what a solution is composed of, supersaturation and crystallization.  Illustrates the concept of exothermicity during crystallization.

Explanation of Experiment: 

At temperatures below 58° C, solutions that are saturated with respect to anhydrous sodium acetate are supersaturated with respect to the trihydrate.  The composition of the solution prepared corresponds to 88 g NaC2H3O2 per 100 g water.  The 100 g of water includes both added water and water of hydration.  When this solution is cooled to 20° C, it is unsaturated with respect to NaC2H3O2, but supersaturated with respect to NaC2H3O2.3H2O.  “Seeding” the solution with the trihydrate thus causes formation of crystals of hydrated sodium acetate.  Crystallization is exothermic, and the resulting solid is warm to the touch.

Materials Preparation: 

Materials needed:

  • 175 g sodium acetate trihydrate, NaC2H3O2.3H2O
  • 50 mL distilled water
  • 2 L beaker
  • Hot plate
  • 500 mL Erlenmeyer flask
  • 100 mL beaker
  • 250 mL beaker
  • Scoopula

Prepare a boiling water bath by filling the 2 L beaker with ca 1.5 L of water and heating on the hot plate.  The supersaturated solution is prepared by placing 175 g of sodium acetate trihydrate and 50 mL of distilled water (3.5 g NaC2H3O2.3H2O per mL H2O) in the 500 mL Erlenmeyer flask, heating the mixture in the water bath and swirling occasionally until a clear homogeneous solution is obtained.  Remove the flask from the water bath, place an inverted 100 mL beaker over the neck of the flask and allow the solution to cool undisturbed until it reaches room temperature (ca. 1-3 hours).
After the demonstration, the solid can be cut up with the scoopula and returned to the Erlenmeyer flask to be used again.  The supersaturated solution is restored by heating in a boiling water bath – it can be used repeatedly unless contaminated.  After several cycles of use, small amounts of water may have to be added to compensate for evaporation losses.
The supersaturated solution is sometimes difficult to prepare.  All glassware should be clean and scratch free.  If, after several attempts, the solution cannot be cooled to room temperature without crystallization, it should be discarded.  Clean or replace the glassware and start with another sample.
Since even a small bump may cause crystallization to occur, more than one sample of supersaturated solution should be prepared.

Presentation: 

To demonstrate crystallization, place a few crystals of sodium acetate trihydrate into a 250 mL beaker and pour the solution onto the crystals.  Do not do this too slowly, since solid may start to form at the neck of the flask, eventually clogging it.  Crystallization begins immediately forming a mound of white solid sodium acetate – the beaker is used to prevent the solid from spreading too far.  All the water is trapped within the solid, leaving no visible trace of liquid.  The solid feels warm to the touch.  The shape of the mound will depend on the manner in which the solution is poured – pillars of different shapes and sizes can be produced.

Hazards: 

The solutions are hot enough to cause burns.  If the crystallized sodium acetate touches the skin, wash with water.

Disposal: 

The materials can be recycled.  When disposal is warranted, the solid should be flushed down the drain with water.

Video: 

Type of Reaction:

Primary Reference: 
Shakhashiri, B.Z. 1983, Chemical Demonstrations – A Handbook for Teachers of Chemistry, vol. 1 pp. 27-30.