Sodium ethanoate ‘stalagmite’

In this spectacular experiment, students observe the rapid crystallisation of a supersaturated solution of sodium ethanoate and explore the energy change involved.

The demonstration can also be used to stimulate interest in public presentations.

Equipment

Apparatus

• Eye protection
• Beaker, 250 cm³
• Measuring cylinder, 25 cm³
• Watch glass, large, about 10 cm diameter
• Stirring rod
• Bunsen burner, tripod and gauze
• Access to a top-pan balance (1 d.p. is sufficient)

Chemicals

• Sodium ethanoate-3-water (sodium acetate-3-water), 125 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Sodium ethanoate-3-water (sodium acetate-3-water), CH₃COONa.3H₂O – see CLEAPSS Hazcard HC038a.
• A black background is probably better than a white one for this demonstration.
• The ‘stalagmite’ can be reheated and used again. Keep the solution clean and free from dust – this could cause it to crystallise prematurely.

Procedure

Before the demonstration

1. Weigh 125 g of the hydrated sodium ethanoate into the beaker and add 12.5 cm³ of water.
2. Heat the beaker over a low flame and stir until a clear solution is obtained.
3. Cover the beaker with a watch glass and allow to cool to room temperature to give a supersaturated solution.

The demonstration

1. Remove the watch glass and place a few ‘seed’ crystals of sodium ethanoate on it.
2. Pour the supersaturated solution slowly onto the sodium ethanoate cystals. The solution should crystallise immediately on contact with the crystals. It will form a growing ‘stalagmite’ of solid sodium ethanoate as more and more of the solution is poured onto it.  The watch glass becomes warm as heat is released during the crystallisation process.

Teaching notes

If reheating is shown to the class, emphasise that the solid is dissolving (in its own water of crystallisation) and not melting. A supersaturated solution of sodium thiosulfate, obtained in a similar way, is also stable until a seed crystal is added.

Commercial ‘heat packs’ are available which use the principle of supersaturation. Here a mechanical disturbance, usually a spring loaded button inside the pack, induces crystallisation. The packs can be reused by heating in boiling water to redissolve the crystals.