TeacherExpt:Sodium ethanoate ‘stalagmite’

From Learn Chemistry Wiki

In this demonstration experiment a supersaturated solution of sodium ethanoate crystallises rapidly forming a ‘stalagmite’.

Lesson organisation

This is a demonstration used to show the rapid crystallisation of a supersaturated solution in a spectacular way and explore the energy change involved. It can also be used to stimulate interest in public presentations.

Apparatus and chemicals

The teacher requires:

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

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

Technical notes

Sodium ethanoate-3-water (sodium acetate-3-water) (Low hazard) Refer to CLEAPSS Hazcard 95

1. A black background is probably better than a white one for this demonstration.
2. The ‘stalagmite’ can be re-heated and used again. Keep the solution clean and free from dust – this could cause it to crystallise prematurely.

Procedure

Health & safety

Wear eye protection throughout.

Procedure

Before the demonstration:

A. Weigh 125 g of the hydrated sodium ethanoate into the beaker and add 12.5 cm³ of water.

B. Heat the beaker over a low flame and stir until a clear solution is obtained.

C. Cover the beaker with a watch glass and allow to cool to room temperature to give a supersaturated solution.

The demonstration:

D. Remove the watch glass and place a few crystals of sodium ethanoate on it.

E. 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 re-heating 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 re-used by heating in boiling water to re-dissolve the crystals.

Health and Safety checked, November 2006

Reference

This experiment has been reproduced from Practical Chemistry: http://practicalchemistry.org/experiments/introductory/solutions-and-water/sodium-ethanoate-stalagmite,64,EX.html

Web Links

This website has a movie showing the reaction.

http://www.jce.divched.org/JCESoft/CCA/CCA3/MAIN/ACETATE/PAGE1.HTM