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Name: Jennifer
Status: student
Grade: 6-8
Location: PA
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?

Dear Jennifer:

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 direct responses):

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.

Vince Calder

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