"Slow crystallization" experiment
How to grow sodium thiosulfate crystal in test-tube
Winter draws marvellous icy patterns on windows! Shall we try to recreate such a beauty in a test tube?
Observe safety measures when working with open fire and hot objects. Use only special glassware meant for heating.
Reagents and equipment
- sodium thiosulfate pentahydrate (Na₂S₂O₃*5H₂O)
- a test tube
- a stand
- a gas burner.
Using a gas burner, heat sodium thiosulfate pentahydrate in a special glass test tube, until all the crystals disappear. Once the obtained solution cools down, throw in there a little crystal of sodium thiosulfate pentahydrate. Watch a slow crystallization!
Upon heating, sodium thiosulfate dissolves in its own water of crystallization — i.e. water that is already present in the structure of the salt crystals (Na₂S₂O₃*5H₂O). Once cooled down, the solution becomes oversaturated, i.e. now it contains more solid compound than it “comfotably” can at that temperature (room temperature, in our case). Such solutions are unstable, and excessive amount of dissolved sodium thiosulfate tends to “escape” the solution. Any external action may lead to its crystallization, whether it’s a dust particle, a knock on a test tube or a tiny salt crystal. Because of a high viscosity of the solution, its crystallization proceeds slowly.