C
06.01e

Experiments for chemistry lessons

Growing crystals

S
n

Need of Time: 15 minutes + growing time
Goals: E:  formation of periodic structures in nature. K: Engl. Versuchsanelitung.
Material:
  • beaker 600mL
  • 2 beakers 100mL
  • wooden rods (pencils)
  • thread (polyester or very thin fishing line)
  • distilled water
  • tweezers
  • (sand paper)
  • very much patience!!
  • hot plate stirrer
  • stirring bar
  • spatula with spoon
  • seed crystals or petri dish
  • superglue
  • (Zapone varnish, paint brush)
Chemicals:
  • KAl(SO4)2*12 H2O
    L=110g/L
    CAS-Nr.: 7784-24-9

 

 
  • KCr(SO4)2*12 H2O
    L=250g/L
    CAS-Nr.: 7788-99-0
    H315, H319
    P302+P352, P305+P351+P338

Warning
 
  • CuSO4*5 H2O
    L=317g/L
    CAS-Nr.: 7758-99-8
    H302, H315, H318, H410

Warning
Procedure:

Producing the saturated solutions: Fill into the beaker 600mL 500mL of dist. water. While heating to 105F try to dissolve as much of the substance as possible. Let the solution cool down. It will be saturated no sooner as you can see a solid precipitate at the bottom at room temperature. If not so,  leave it for evaporation for about one week. Decant the saturated solution in a new beaker or bottle.
How to get seed crystals: Fill approx. 10mL of your solution into the petri dish. Leave it for evaporation without lid. After a few days: select one of the crystals (the most beautiful one) from the bottom and use it as a seed crystal.

 

 

  

  Growth: Attach the seed crystal to the thread (try superglue); bind the other end to the wooden rod and fix it about 2cm above the bottom of the beaker (Remember: you expect the crystal to grow large). Hook the crystal in the saturated solution. Place it at a quiet but not too warm location (bad: flue, windowsill; good: in a cupboard, or covered on a cupboard). Important: If it's necessary to re-add solution you have to make sure that the solution really is saturated (look for solid precipitate at the bottom). Otherwise the nice crystal could disappear (dissolve again). What a misery!
Protection: Some conditions (e.g. very young students, corrosion) could make it necessary to cover the crystal with varnish. Here Zapon varnish is needed. It is soluble in acetone.
Observation: After 4 to 6 (10) weeks you will get crystals with an edge lenght of 2-3cm.
Disposal: Evaporate solutions which aren't needed any more. Salts can be used again.
Source: Common.
Discussion: Phantom crystals. Chimerae. Refreshing of the solutions. Reason why big crystals grow in a solution, while small ones vanish simulaneously.
Notes: Fast crystallization:
  • both prussiates of potassium (slow growth; red columns with peaks at the edges respectively yellow square pads),
  • NH4Al(SO4)2*12H2O (colourless oktahedrons, deodorant crystal),
  • NaH2PO4*2H2O (colourless oktahedrons).

Slow, tricky crystallization in aqueous solution under normal conditions:

  • common salt,
  • sucrose.

Options

  1. Sand a crystal at one ridge or drill a hole in one face. Hook the crystal in the solution again. It's surprising what happens!
  2. First let grow a crystal of chromium alum, then hook it in a solution of aluminium alum. This way you get a chimera. Groups of students may try different concentrations (from 1:10 Cr:Al to 1:20, 1:50, 1:200); the results will be different shades of bordeaux colour.
  3. After adding 1% KOH or NaOH, alums will form a cubic habitus, common salt  with urea or glycine will form an oktahedral habitus. The reasons for this phenomenon are not well understood yet.
WWW: http://www.chemieunterricht.de/dc2/kristalle/dc2kt_32.htm
http://www.crystalgrowing.com/recipes/sugar/sugar.htm

 

Walter.Wagner ät uni-bayreuth.de, Stand: 07.02.18

Didaktik der Chemie
Universität Bayreuth