Crystals Can Grow 
                    
 How Crystals Form
   Cochise College   

   Geology Home Page               

   Roger Weller, geology instructor (wellerr@cochise.edu)
   last edited:  12/22/15   

 
                          
How Crystals Form
 
 links
     http://answers.yahoo.com/question/index?qid=20081201170315AAGx9qr
     http://www.sciencekidsathome.com/science_topics/what_are_crystals.html
 

        There are two major methods that are commonly employed in growing crystals
   in the classroom environment:
      
Evaporation Method  and  Supersaturation Method

        The evaporation method produces the best looking crystals, but it is slow. 
   The supersaturation method is fast (but for only certain chemicals) but only
   produces mediocre-looking crystals.

          Evaporation Method

        In the evaporation method a saturated solution of the chemical being grown is
   created by dissolving as much of the chemical   as possible in room temperature
   water.  The solution is then allowed to slowly evaporate.  As water evaporates, the
   chemical in solution is forced to precipitate.  If there are any tiny crystals present
   within the growing solution, they will act as seed crystals and layers of atoms of
   the chemical being precipitated will be added to the surfaces of the seed crystals. 
   Consequently, the crystals at the bottom of the container will start to grow.

        Growing crystals by the evaporation method is often a two-step process.  In
   the first evaporation process there should be no trace of the chemical left in the
   solution; there should be no undissolved crystals at the bottom of the container
   containing the saturated solution.  Every undissolved particle, no matter how tiny
   it is becomes a potential seed crystal.  Too many seeds will result in a crystalline
   mass in the container.  If the solution is clean and undisturbed, chances are that
   only a few crystals will start to form at the bottom of the container.  When they
   reach the size of about a quarter of an inch, these crystals can be used as seed
   crystals for the second step.  When you feel that the crystals are of proper size
   poor off the solution, slowly into a clean container.  Dry the crystals off on a paper
   towel.  Save the crystals that are well-shaped, single crystals with no tiny crystals
   attached to them.  These clean crystals will be seed crystals for a later evaporation
   process.

  
   seed crystals at bottom of container

  
   potential seed crystals

        Before describing the next step, I must warn you about some problems.  The
   saturated solution must be placed in an an area that is as dust free as possible,
   because each speck of dust can become a nucleus for creating a seed crystal. 
   Too much dust and you will get hundreds of tiny crystals instead of just a few nice
   large crystals.  You cannot put a cover on the growing solution because this will
   slow down or even stop the evaporation process.  Another big problem is
   disturbing the growing solution.  There is a strong temptation to check the
   growing solution each day to see how the crystals are doing.  However, even the
   tiniest ripple at the top of the growing solution can cause a disturbance that will
   trigger the formation of dozens of tiny crystals, thus overwhelming the solution
   with too many seed crystals.
 

        Now that you have at least one good seed crystal, you are ready to start the
   second evaporation process.  Again, you will need a saturated solution.  You
   might use the solution that you poured of the first growth.  However, you might
   have some tiny crystal present because the solution has been evaporating.  Add
   a tiny bit of water to the solution to make sure that all of the particles are dissolved. 
   If you add too much water you will have an under-saturated solution and your
   seed crystal will dissolve instead of growing when you put it in the solution. 
   Allow the solution to sit quietly.

       To grow a single large crystal, you need just one seed crystal.  If it were to be
   placed at the bottom of the container, it would grow in a distorted form because
   only the top face and side crystal faces would be exposed to the growing solution. 
   Therefore the crystal needs to be lifted off the bottom and suspended in the
   solution.  This is accomplished by tying a thread around the seed crystal and the
   other end of the thread tied to a pencil or popsicle stick. 

  
   method of tying seed on a thread


  
       Lower the crystal slowly into the solution, trying to disturb the surface of the
   solution as little as possible to avoid triggering off the creation of additional seed
   crystals. 


  
   seed crystal suspended in solution

        Okay, now wait a couple of weeks.
 

        Here is one additional warning.  The room temperature should be as stable as
   possible.  Commonly, temperatures drop off at night and more of the chemical
   comes out of solution and your seed crystal grows.  During the daytime
   temperature in the room often increases and the solution becomes under-saturated
   and your seed crystal will start to dissolve.  If the temperature gets hot enough,
   your seed crystal will dissolve so much that it will fall out of its threaded knot.

        Alum, mono ammonium phosphate, and copper sulfate crystals grow nicely
   using the evaporation method.  However, no one has much luck growing a
   single crystal of sugar by the evaporation technique, but you can get nice clusters
   of sugar crystals. 

        Salt almost always results in hundreds of tiny crystals.


         Supersaturation Method
 

        The supersaturation technique works best only with those chemicals that are
   more soluble in hot water than in room temperature water.  Salt has an almost flat
   solubility; raising the temperature of the solution will allow salt to dissolve faster,
   but you cannot really dissolve much more salt in hot water than you can in cold
   water.  Sugar also does not grow well by this method either.  You do use hot water
   to dissolve the sugar, but you have to wait for evaporation in order to grow sugar
   crystals.  Epsom salt is the opposite extreme; too much of this chemical dissolves
   at high temperatures and when the solution cools, crystal form too rapidly and all
   you get is a mass of needle-like crystals.
 

        Growing crystals by the supersaturation technique is fast.  You can get crystals
   forming a just a few hours.  Alum, mono ammonium phosphate, and copper
   sulfate work well with the supersaturation technique.
 

        To grow crystals by the supersaturation technique is simple.  You prepare the
   growing solution by dissolving as much of the chemical in hot water as you can. 
   Again, there must be no undissolved crystals left in the growing solution.  You can
   use boiling water to dissolve the chemical, but this creates a dangerous situation
   for possible burns due to scalding,  glass containers shattering and plastic
   containers melting.  A possible compromise would be to heat the water to a safer
   lower temperature using an electric coffee pot and then pour the hot water into a
   safe container such as a Pyrex measuring cup or bowl with the chemical to be
   dissolved.

        If you are just trying to grow seed crystals or if you want an interesting cluster
   of crystals, just leave the hot growing solution in the bowl or measuring cup.  Do
   not move or disturb the solution in any way because supersaturated solutions
   are especially susceptible to even the slightest ripple or splash which would
   create large numbers of undesired seed crystals.  Allow the solution to cool
   overnight if possible.  If you were successful at creating a supersaturated solution,
   crystals will form as the solution cools.  You can either pour off the solution to
   obtain the crystals or you can simply allow the solution to slowly evaporate over
   the next couple weeks, adding clear sharp surfaces to the crystals that have already
   formed.

   

    alum crystals formed by supersaturation technique at bottom of container

        If you wish to grow single crystals by the supersaturation technique, the method
   is similar to the evaporation technique.  You need to have a prepared seed crystal
   tied to a thread and support structure like a pencil.  Gently and slowly lower the
   seed crystal into the hot supersaturated solution soon after the solution is
   prepared.  If the hot solution is under saturated, your seed crystal will dissolve.  On
   the other hand if you wait too long to put the seed in the solution merely breaking
   the surface tension of the solution will trigger off the production of large number
   of potential seed crystals.  Growing crystals is somewhat of an art.
 

      I have been able to grow alum crystals up to one inch in diameter by the
   supersaturation technique in just 6 hours.  However, the crystals are cloudy and
   ill-formed because of their rapid growth.

 

   
   large alum crystal grown by being suspended in supersaturated solution


   
    large alum crystal grown by being suspended in supersaturated solution


    copyright 2015-R.Weller

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