

Watching crystals form quickly

Class practical

As the crystal grows, its characteristic shape is seen.

Apparatus and materials

Sodium thiosulfate crystals

Test-tube (e.g. 100 by 12 mm)

Health & Safety and Technical notes

Read our standard health & safety guidance

Some hours before the lesson, preferably the day before, put sodium thiosulfate crystals into each test-tube to a depth of about 5 cm. Melt the crystals by gentle warming. They will melt in their own water of crystallisation. However, if they do not do that easily, add a drop of water to the stock of crystals so they are damp. 

Allow the tubes of melted sodium thiosulfate to cool down to room temperature. If the tubes are left a long time after cooling, the sodium thiosulfate may recrystallise in some of them. That is one reason for some spares to be prepared. 
Recrystallisation may be started by dust, so it is advisable to cover the mouths of the tubes until they are needed. It is also advisable to avoid jarring the tubes. This may also start recrystallisation. 



Give each student a test-tube with the cool liquid sodium thiosulfate in it. They are to drop one crystal of sodium thiosulfate into the liquid and watch very carefully what happens.

Teaching notes

If students work in pairs, it is useful to get one to do the experiment whilst both watch. Then everyone gets a second look at what is a fascinating sight. 

This experiment will reinforce the idea that crystals are identified by flat faces with angles between them characteristic of that material, regardless of their size. 
As the crystals form, heat is given off which will make the test-tubes get warm (allow it to touch a cheek). It is probably best not to tell students to look out for this. Students who comment on it might be told to make a note for later. Considering where this energy comes from - particles losing their kinetic energy as they are held in the crystal structure - could better be discussed later. 
This experiment was safety-checked in May 2004


Related guidance

Crystals and atomic models for beginners