Leo Pel
Transport In Permeable Media
Eindhoven University of Technology
5600 MB Eindhoven, TheNetherlands
E-mail: l.pel@tue.nl

Step 1: Heat up water to 35oC

 

Step 2: Add Na2SO4 until you have a saturated solution
(i.e., no more salt can be added and it can be see on the bottom)

Step 3: Wait untill all salt has fallen to the bottom and there is a clear solution on top

Step 4: Take some of the solution and put it for example in a glass tube (preferably borax free)

 

Step 5: Cool down the solution in ice.

Now in about 15-30 minutes you should see the first heptahydrate crystals grow.

                 
To show it is really heptahydrate you can just add a mirabilite crystal to the solution and it will immediately be transformed: see movie 

Launch in external player

The final situation