Grow Crystals in a Few Days

Crystals have always had a special attraction for people. In this experiment, we grow a white crystal from alum. To make the crystal grow faster, we will do it with hot water. You could also do it with cold water, but then it will take a long time. However, the reward for the extra patience is a much more beautiful crystal because it has had enough time to grow along the crystal directions.
If you dissolve alum in hot water to the solubility limit and then let it cool, the amount of soluble alum in the water reduces faster than the crystal can actually grow. The alum that is too much in the liquid is then deposited on the crystal and on the walls of the vessel. As a result, the crystal grows very quickly, but it consists of many small partial crystals. If you do it with cold water and wait until this allows the crystal growth by evaporation, the crystals grow very slowly, but it is real crystal growth, which leads to few but larger individual crystals.

	This experiment requires boiling water and chemicals that must not be eaten or brought in contact with your eyes! Young scientist should do this experiment together with their parents and leave the boiling water part to them.
	The chemicals used to grow crystals are poisonous. Don't eat them or bring them near food. Make also sure not to bring chemicals near your eyes (this also means not to touch the chemicals and then put your fingers near your eyes before washing your hands thoroughly!
	The chemical used in this experiment to grow the crystals should not stain. However, if you add additional colors to get other crystal colors, the additional colors can of course stain your clothes.

Materials you need for this experiment:

water	As much water as you can put in your glass jar
big glass jar	The glass jar should be as big as possible because it determines the maximum size of your crystal. It has also to be temperature resistant so that you can use it with boiling water.
Alum (KAl(SO₄)₂·12H₂O)	The easiest way to get it is at your nearest pharmacy. It is not very toxic but still be sure not to eat it and not to bring it in contact with food or your eyes.
yarn	The stronger the yarn the better. It has to handle the weight of your crystal and as the crystal grows it will get quite some weight.
wood chopstick	This you put on top of the jar to fix the yarn to.
stirring rod	This is needed to stirr the water while you add alum. It is best if you use a wood stick because metal transfers the heat quite fast. In case you use a cooking spoon mark it in a special way and tell everybody in your family that this one is just for chemistry and may never be used again for cooking.

How to grow the crystal:

	First, put the glass bowl in a place where it can stand undisturbed for a few days, where water vapor will not disturb and where the high temperature will not cause any damage (the boiling water will also make the glass very hot). Please note that the jar must be placed somewhere where in no case small children or pets can reach it. The next step is to fill the glass with boiling water (young scientists: please let your parents do this) and then dissolve as much alum as possible while stirring continuously. If there is some alum left at the bottom that you can't dissolve despite stirring (see picture on the left), don't add more alum.
	Now tie the yarn to the wooden stick and make a few knots at the other end where the crystal then starts to grow. The length of the thread should be chosen so that the end of the yarn is about halfway up the glass when it hangs inside (then the crystal can grow upwards and downwards the same amount until it hits something).
	Now you have to wait until the crystal starts to grow. After one day you should see the first small crystals on the filament (and probably also on the glass walls). Now carefully take out the thread and remove all small crystals except for the ones at the very bottom of the thread (you want the crystals to form one big crystal only at the very bottom and not to grow wildly everywhere). Now carefully put the thread with your first small crystal aside and bring the water to boil again in the microwave (young scientists: this is a part for your parents). Now repeat the process from above and again dissolve as much alum as possible in the hot water. Then wait an hour until the water has cooled down a bit and put the thread with your little crystal into the water. By the next day the crystal will have grown quite a bit and slowly earn the name crystal. If you put your crystal into the hot water too early, it will dissolve, which is especially annoying if you have been successfully growing it for a few days.
	Now repeat the process of letting it grow, take the crystal out of the water, heat the water, dissolve the alum, let the water cool down a bit, let the crystal grow again. The more times you repeat this process, the larger your crystal will become.
	If you illuminate the crystal with a LED you can make a beautiful night light. You can also dissolve different colors in the water as the crystal grows, affecting the color of the crystal. Test which colors are picked up particularly well by the crystal.

Have fun experimenting!