Crystal Growth, Temperature and Mass
Date: January 2008
I am doing my Science Fair project on growing crystals in
different temperatures and seeing what the results are. After researching
this topic for a while I have come to the conclusion that growing the
crystals in warmer temperature will make them grow faster. (Notice I say
faster not heavier, larger, etc.) My thesis is "Do warmer or cooler
temperatures affect the size, mass, and rate of growth of ammonium
phosphate crystals?" My hypothesis is, "The researcher predicts that
if ammonium phosphate crystals are placed in a warmer temperature then
it will grow larger, have more mass and have a faster growth rate than
the crystals that were placed in a cooler temperature." I conducted my
experiment and did find that the crystals that were placed in warmer
temperature grew faster, but to my surprise after a few days they
seemed to be finished growing and have a glowing shine to them. Now
the crystals that were placed in a cooler temperature have caught up
and have even grown taller than the crystals that were placed in a
warmer temperature!? They seem and look not even done growing yet
but I am not sure. Could my hypothesis only be partially correct?
I said that the crystals grown in warmer temperature would grow faster,
which was correct, but then said it would also grow larger and weigh
more. I have massed them both in a pan on a scale, and it turns out that
the crystals grown in colder temperature have more mass. Is it possible
that I have conducted some error in the experiment? I do not think so but
I plan to do it again just to see if there is a different outcome. Just
because crystals grown in warmer temperature grow faster, does that
necessarily mean they have to grow larger and have more mass? Is it
possible they will?
I am not a chemist, but I do have a few comments about your experiment.
1. Your hypothesis is complex. Perhaps you need three separate hypotheses,
one each for size, mass, and rate of growth. By breaking the hypotheses apart,
you should avoid a partial confirmation and partial failure situation. In
general, the simpler the hypothesis, the better. It is not at all unusual
to have an hypothesis statement change from some initial statement to a
refined statement, some time later. Keep a record of these changes in your
experiment journal (notebook).
2. I am not clear on what you mean by size. Clarifying this may help solve
part of your problem.
3. It is meritorious that you wish to repeat the experiment. Typically, an
experiment is repeated many times to confirm the results. I recommend that
you do this.
4. I have made many hypotheses in my professional life. Few work out. There
is a saying, "Nature always sides with the hidden flaw." I have learned a
tremendous amount from the failures. It is worthwhile to document failures
and report them.
Although this does not address the specifics of what you have asked, I hope
it helps your process. I will let the chemistry experts answer the details
about the specific crystal.
Keep up the good work and the critical thinking. It will serve you well.
Thanks for using NEWTON.
Nathan A. Unterman
I do not grow crystals but, based on some tangential familiarity with the
subject, I offer the following observations (in case you do not receive more
As you know, crystals have orientations. They grow faster in some directions.
I am not familiar with the compound you are using. If you are using a seed
crystal, make sure it is oriented in the same direction in all your
experiments. Try growing more than one crystal at each (for both the crystal
and the solution) temperature to make sure your results are repeatable. You
may want to repeat the experiment at small temperature intervals to see if
there is a trend. Variation of the temperature DURING the experiment also
would affect the results, so you may want to make sure the temperature is
kept constant during the entire process.
Finally, to affirm a hypothesis, one would have to grow crystals of different
compounds to ascertain the generality of the hypothesis. Maybe you could try
one more compound.
Ali Khounsary, Ph.D.
Argonne National Laboratory
In order to address your question it is important to know WHICH "ammonium
phosphate" you are using. There are three possible "ammonium phosphates":
(NH4)H2(PO4); (NH4)2H(PO4); (NH4)3(PO4) with CAS numbers 7722-76-1;
7783-28-0; unknown, respectively. I cannot find the tri-ammonium phosphate
in handbooks, so I assume you must be using either the mono-ammonium or
di-ammonium phosphate. The more rapid growth at higher temperature is
probably due to the more rapid evaporation of water, although you did not
specify what temperatures "high" and "low" are. Hydrates of crystals often
form at different temperatures, so mass due to water of hydration of the
crystals can cause a difference in mass, as well as rate of crystalization.
Different temperatures also affect the speed at which the various crystal
faces grow, so that the "shape" of the crystal can be different depending
upon the temperature the crystal is grown. Different temperatures can also
affect how many "seeds" are present in the solution, that is, you need to
distinguish whether you are only considering a single crystal, or whether
you are considering a poly-crystalline cluster of multi-seeded crystals.
Putting it all together, I think that the most critical factor here is the
higher vapor pressure of water at higher temperature. Other factors being
equal, the faster rate of evaporation of water may be the rate determining
factor in the growth rate of the crystals.
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Update: June 2012