Ask A Scientist Chemistry Archive

 
name         Susanne
status       other
grade        4-5
location     IN

Question -   I am a Webelos den leader and am presently working on the
Scientist badge with my den.  As a den, we are doing various experiments
required for the badge.  While conducting the experiment for gowning
crystals, the directions provided by one of the parents stated that a
glass container, not a plastic one, should be used.  This became a topic
of discussion with no resolve.  Hence, my question:  Do crystals grow
better in glass than in plastic?  And if so, do you know why?

I encouraged the Webelos to investigate the answer and present their
'findings' at the next meeting.  In the meantime, I have tried to find an
answer via the Internet, but have not been successful.  I hope you can
shed some light on this so I can pass along the facts.
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Hi Susanne-
      For crystal-growing, the difference between glass and plastic is not 
 real big.  Both will allow the growth to occur.
Neither dissolves in the solutions.  Both can be cleaned to the point that 
they will not provide nucleation sites : starting-points for tiny new crystals.

      Glass has the advantage that you can raise its temperature to that 
 of boiling water without melting or deforming it.
Of course this is a relatively dangerous way to do things, unless you are 
fairly careful.
Plastic has the distinction that the crystals will be unable to stick to 
it, or stick very weakly.
Glass is an oxide, and most of your crystal-growing salts are oxide salts, 
so they may be able to stick strongly.
Related to that, plastic is easier than glass, to clean to the point that 
it does not provide nucleation sites.

Plastic is flexible; a bottle will deform when handled, forcing crystals 
to detach from any weak attachment points.
Glass is so stiff that crystals will be much more likely to retain their 
attachment points, and the inside arrangement can survive.
Perhaps with glass you can allow more handling and looking by the scouts.
Similarly, if your procedure calls for pouring off the solution while the 
grown crystals remain stuck to the container, glass will probably be better.
Some people use plastic then catch the loose crystals with strainer or filter.

Crystal growth is often practiced in two stages.
First stage: growing some crystals, small crystals, any crystals, from new 
solutions of raw powder.
Second stage: re-position the best few small crystals, and re-dissolve 
rest, to then grow a larger crystal with a clean shape.

The point being, I can imagine doing first stage in glass, second stage in 
plastic.

This staging allows you a first encounter with difficulties such as too 
much nucleation (a sandy pile of small crystals)
or too little nucleation (i.e., nothing starts growing for a long time).
Re-using the jumbled small crystals as raw stock for the second phase may 
purify the material too,
so your second phase has a low nucleation rate (less dust in the solution) 
needed for growing big crystals,
and has less foreign salts and soaps which might change the shape-habit of 
the crystals
or force them to stop-and-restart and not become one big clear crystal.

Personally, I would think the program can be done in either material.
There could be subtle differences that lead to better results,
but I tend to think it is luck and not science if the 
instruction-followers actually reap those benefits as the 
instruction-writers did.
There is usually a body of unwritten details that is communicated only by 
example.
No wonder you cannot resolve it by discussion.
Repeated practice, and experience, is what really works these details out.
Somebody gets lucky and it turns out well, then suddenly it is easy for 
everybody near them.
If you can have parents try it first, then the scouts, that will probably 
show what I am talking about.
If you can stretch out your program and run it long-term in parallel with 
others,
your scouts might get the most such experience.
If you can split them into successive teams, later teams benefiting from 
seeing the experience of earlier teams, that might help too.

In essence, you are always re-inventing your own recipe for this kind of 
thing.
Invention can be done, but it works best with time.

Jim Swenson
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Susanne,

Some of the older crystal growing kits used solvents that can dissolve 
plastic - if you are using one of those, then that would be the reason. I 
am going to assume that you are using water-based crystal growing recipes.

I am not certain about this, but I think that glass surfaces are smoother 
(at the molecular level) then plastic surfaces. Since crystals have to 
grow on a nucleation site - a rough surface that will allow the molecules 
of the crystal to fit and start the crystallization process, then using a 
plastic surface will provide to many nucleation sites and the crystals 
grown will be small and all over the inside of the plastic cup.

Of course, the reason could be something as straightforward as glass being 
clear and most plastic cups are not. In which case, the reason is that you 
want the kids to see the crystal grow over time.

I would say go ahead and try one with a plastic cup and see.

Greg (Roberto Gregorius)
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