There's a few reasons why you might want to purify chemicals by recrystallization, but the best one could be luminescence glow-in-the-dark dust.
Watch this science video tutorial from Nurd Rage on how to purify aluminum and strontium nitrate by recrystallization with Dr. Lithium.
This is important because those two substances must be extremely pure for making glow-in-the-dark powder.
First make aluminum nitrate with the "How to make aluminum nitrate nonahydrate" video.
Dissolve the dry aluminum nitrate in water and filter off any insoluble materials. It could not be filtered off before it dried in the previous video because the particles at that stage are much too small. It needs crystallize once to aggregate into particles large that can be filtered.
After filtering, dry off the aluminum nitrate. A desiccator bag might be useful (How to make a desiccator bag for drying chemicals).
After drying, carefully weigh out the crystals. Then take that mass and add in 20% mass of water. So if you have 50g like me, you add 10g of water (or 10mL since density is 1g/mL).
It won't all dissolve, so carefully heat the mixture until it dissolves. Then cover it and let it cool down. Eventually it'll crystallize out purer crystals.
After the mixture cools to room temperature, and left for a few hours. The liquid is poured off and discarded, it has the impurities and is not needed.
The crystals are again weighed and once again they are recrystallized with 20% water.
Once this is done. Dry the final product. For more details, check out the description on YouTube.
Brought to you by one of WonderHowTo's favorite scientists, NurdRage.