"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?

Safe­ty pre­cau­tions

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.

Step-by-step in­struc­tions

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!

Process de­scrip­tion

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.