Ask A Scientist Chemistry Archive

name         Perry
status       student
age          9

Question -
We are growing alum crystals in water of varying temperatures. (This is a 4th grade science project.) The solution is alum in hot water. The nucleation point is a string dipped into and suspended in the solution. The solutions were then "stored" in various temperature environments: a refrigerator, room temperature, and a hot box. Basically a rock candy type of approach. We then recorded the various rates of crystal growth by photographing them each day. We observed that the solution in the refrigerator formed two large crystals on the suspended string. The solution at room temperature formed many smaller crystals on the string.
And the solution in the hot box formed crystals only on the rim of the container, not on the suspended string.
We (my daughter actually) has pulled this together, but is stuck in finding a good explanation for the differences in the various crystal formations between the different temperature environments.
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Perry,

The crystals grow when water evaporates from the saturated solution. The crystals from the 
solution in the refrigerator evaporated slowest of all three and the growth appeared on the 
string because that is probably where a nucleation site initiated first growth. The slow 
evaporation enabled those crystals to grow under rather optimized conditions.

The room-temperature set-up allowed evaporation to proceed somewhat faster. That is likely the 
reason why so many crystals got started and then grew with none really getting ahead of the 
others as they grew.

The solution in the "hot-box" yielded crystals only at the solution-container interface because 
evaporation was occurring too fast for them to form in the body of the warm solution. However, 
as the solution evaporated, they had to form somewhere and the edges provided nucleation sites 
that enabled first growth.

Once crystal growth starts, it tends to proceed at that site. Best growth (the most perfectly 
shaped crystals) occur when the solution evaporates very slowly.

Regards,
ProfHoff 750
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Well conceived experiment. What makes it even more exciting is that the results you got are not 
what you will find written up in any book!! And that is what science is all about. Here is what 
I think is going on, although it is just an educated guess (hypothesis, if you want the 50 cent 
word). The number of crystals that you get will depend upon the number of  "seeding" sites that 
occur in the solution. If there are a lot of such sites you get a lot of small crystals. If 
there is only one "seeding" site you get a single crystal. Any piece of dusts, string fiber, 
scratch on the glass or plastic container, impurities in the water can be such a site. Now you 
have two things to balance. IF and it is a big IF, you can cool the solution slowly enough and 
there is only one site available you will get a single crystal. That is the conventional wisdom. 
However, in the real world with the resources you have available, that probably will not happen. 
The other approach is to carry out the crystallization as rapidly as possible to prevent these 
pesky "seeding" sites from developing so that one or only a few sites are able to support 
crystal growth. That apparently is what you have succeeded
in doing. Of course, at the other extreme if you cool the solution too quickly you can super-
cool the solution too much and numerous growth sites will for spontaneously. You appear to have 
found a happy medium to the two competing factors. You might be able to improve the results if 
you can find some way to minimize temperature variations, and avoid any "sloshing around" of the 
solutions. Vibrations can be a killer in this very delicate process of growing crystals, which 
is more an art than science. The conditions you have found could be modified by using a small 
crystal glued to a piece of nylon monofilament fishing cord, since string will introduce fibers, 
and unless you have the warmer solutions carefully thermostatted in a couple of Polystyrene foam ice 
chests you will probably have drafts that cause the formation of seeds. Finally, what works for 
alum may not work for other salts since
each has its own solubility temperature properties that will favor or disfavor the growth of 
large crystals.

Vince Calder
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Hi Perry and daughter!
Growing alum crystals is a very pretty and exciting experience, and as you pointed it allows the 
student to observe and draw conclusions from what he/she did.
To grow crystals it is easy and at the same time can be very difficult, the sizes and beauty of 
the crystals formed depend upon many many conditions, in fact to grow well formed and big ones 
is considered an art. (There are even a famous book called " The art of growing crystals" ).
But there are some simple conditions that usually determine roughly the kind and quality of 
crystals obtained. Let us see:
1) Time: longer time gives bigger and better formed crystals
2)Concentration and  Solubility: if the experiment solution is over saturation, is becomes  
precipitating fast and with tiny crystals at the bottom of the flask
3)Temperature:The crystallization occurs when the substance is no longer dissolved.  There are 
substances that are more soluble at high temperatures, others are more soluble at low 
temperatures.
4)Particular conditions of the experiment: flask used, impurities present, conditions of 
equilibrium of the recipient (that is it cannot be moved, must stay still all the time),etc.


Ideally the crystals grown under room temperature will be larger than the crystals grown while 
cooling with ice water or at the fridge. However, if there are many nucleation points in the 
room temperature flask there may be many crystals that are rather small; this
may result from dirt in the flask, or the flask being disturbed. Larger crystals result from 
greater time for crystallization to occur, as should be the case if the solution cools slowly. 
On the other hand, if the solution cools rapidly, less time is available for the ions to 
organize into large crystals, and the crystals tend to be smaller.


At the particular experiment your daughter performed the variables were the different 
temperatures. At the hot box crystals were formed only at the flask rim: that means the alum
 was still dissolved, because its solubility is higher at high temperature; but
some of the solution when touches a cold region of the flask (the rim), cools and precipitates 
right there.(probably the seed dissolved also) At the flask at room temperature (what 
temperature?) tiny crystals were formed only at the bottom; there many factors can influence for 
the non-formation at the string, could be some small seeds formed at solution, as long as few 
ones precipitate, there is no way to prevent the others to follow. The big crystals formed at 
the refrigerator show that the crystallization was slower, big crystals form at slower rates.
I hope that help you with the experiment justification.
And thanks for asking NEWTON!

Mabel
(Dr.Mabel Rodrigues)
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