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Saturday, May 4, 2013

Growing crystals at home

What is a crystal?
Crystals are regular structures formed by a regular repeating pattern of atoms or molecules.

These structures grow by a process called nucleation. During nucleation, the atoms or molecules of what we want to transform in a crystal (solute) are dissolved in a solvent. The particles of the solute will tend to cluster together, forming subunits of atoms or molecules. These larger particles will also group with each other and eventually become large enough to "pop out" the solution (crystallize).

Other solute molecules will continue to adhere to the surface of the crystal, causing it to grow until equilibrium is achieved between the solute molecules in the crystal and the solution.

Growing crystals
Three factors that can influence the growth of "home" crystals:
  • A good/poor solution saturation- The first stage of home crystal growth is a saturated solute. In a saturated solution the probability of molecules colliding with each other in order to form a core for initiating nucleation is greatly increased.
  • Surface type-A rough surface tends to be more attractive for nucleation. It is more likely that a crystal is formed on a piece of rough rope than the in the smooth walls of a glass.
  • The presence of deposits in the bottom-This occurs when the solution is not scrambled or means that too much solute was added to saturate the solution. The presence of these deposits create areas for optimum crystal growth however prevents crystal formation in the "target."
Lets see how to grow sugar crystals, these crystals can be sucked and eaten like a lollipop. This demonstration may take up to 3 weeks.

We will need:
  • 3 cups of sugar, we have to adjust this quantity, we want to saturated at 100% but no precipitate,
  • cup of water, to boil,
  • food coloring,
  • small jar,
  • small bowl,
  • wood stick, or rope,
  • kitchen paper or paper filter.
How to:
  1. Boil the water, careful with burns!;
  2. In the bowl, mix the boiling water with the sugar;
  3. Stir the water until all the sugar is dissolved;
  4. If you want to give sugar some color, now it's the time, add the food coloring;
  5. Place this solution in the jar, attention! wash the jar really well to avoid nucleation in it's walls;
  6. Avoid any amount of sugar precipitate in the jar- any not dissolved sugar-, this sugar will be a good nucleation "start point";
  7. Suspend the stick or rope in the solution, do not wash those, we want this to be a suitable "start up" spot for nucleation;
  8. Chose a nice and quiet spot to place your jar for at least 3 weeks;
  9. Wait until the solution cool and cover it with a paper filter. 
What happens?  
After cooling the solution will use nucleation spots to form crystals.

NOTE: You must check the jar on daily basis, if you see any nucleation in jar walls, change the jar;

Wait about 3 weeks for excellent results.

Go further:
  • Follow the growing crystals with a graph;
  • Use salt and compare the growing velocity with the sugar;
  • Use 3 jars, in the first one use boiling water, in the second tepid water, and in the last one repeat the essay with cold water;
  • Try to dissolve the sugar/salt/other while the water is boiling;
  • I am sure you can remember other ways to change this demonstration into a experiment...
Et Voil√°!
Science you can eat!  


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