MadSci Network: Chemistry

Re: Are alum crystals supposed to grow bigger in cold places

Date: Fri Jan 21 11:55:15 2005
Posted By: Kenneth Beck, Senior Research Scientist, Chemistry and Physics of Complex Systems, Pacific Northwest National Laboratory
Area of science: Chemistry
ID: 1106238912.Ch

Dear Kaitlen,

Well, of course you didn’t do anything wrong! You’ll be happy to know that professional research scientists today use alum-based crystals in experiments and are writing papers describing its growth characteristics. In others words, alum crystal growth is not just for student Science Fair projects, but is part an on-going scientific investigation of crystal formation that is still not fully understood. You conducted an experiment with a straightforward, reasoned hypothesis which I will re-state as:

“The growth rate and size of crystals produced from solution should both increase with a rise in the temperature of the solution.”

And what happened? One sample, the salt, followed this hypothesis. The other sample, the alum, didn’t. Let’s start our analysis with the actual results of your experiment, and what you’ll probably report:

(1) Was the environment the same for the growth of the two type of crystals?

I’m assuming this is true. You used the same type of jars, the same solution, and the same type covering for the crystals for both those inside the refrigerator and outside in the sun. The jars in the sun were warmer and stayed warm during the day, but slightly cooled at night, then re-warmed in the morning. The jars in the refrigerator were colder and stayed cold all the time.

(2) What do your crystals look like?

The salt crystals are probably cubical while most of the alum crystals will be variants of octahedrons (8-sided solids). The salt crystals are probably considerably smaller than the alum crystals.

(3) Was the amount of solution in each jar the same before and after the growth period? Or was there a difference in the amount of solution left after the crystals grew? And between the sun-grown and the refrigerator-grown crystals was there a difference in the amount of solution left?

If you don’t have an exact number, you can report you observations, like “more”, “less, “or “no change”.

Based on what results you’ve reported in your email and what I can reasonably assume about how you conducted your experiment, this is what I see:

a) The growth patterns of the salt and alum are different. This is seen in the geometry, or symmetry (as professional scientists might say), of the two type of crystals.

b) In a warmer environment, the solution will evaporate faster. The effect of this is to “supersaturate” the solution. This fosters “nucleation” of crystals – the rapid formation of many small seed crystals – because the solution is too rich in crystal compound. The driving force is for the compound to leave the liquid solution, change “phase”, and become a solid. Once the seed crystals have formed the saturation of the solution drops. It's not easy for more compound to leave the liquid for the solid phase. Now ALL these small crystals have to compete for the remaining crystal compound that can leave the solution.

c) In a cooler or cold environment, the solution does not evaporate as fast as a warmer one. The solution does not become “supersaturated” and the drive to leave the solution is not as great. In this case, only a handful of seed crystals form. The competition for crystal compound still in solution is not as great as it was in the warmer solution. So, those seed crystals that have formed now can grow, and grow, and grow really large. Larger than any diamond any mother could possible wear on her finger!

d) This growth pattern is different for salt and alum. The probably reason for this is the different “solubilities” of salt and alum dissolved in water. One has a greater propensity to leave the solution that the other. Which one is that? You can figure this out.

Crystallization involves nucleation of a crystal and subsequent growth of that crystal. Two different steps. Nucleation involves competition between the supersaturation driving crystallization and the “surface energy” created by formation of a new phase. For this reason, high supersaturation (a large driving force) promotes nucleation. It turns out that recent research on potassium alum crystals in Germany (“Growth- Sector Boundaries and Growth-Rate Dispersion in Potassium Alum Crystals”) demonstrated that the various growth planes of alum have different growth rates and that these are extremely sensitive to temperature variation. So much so, rigorous control of growing environment is needed. To quote the article:

“Crystals were grown on seeds of 6 – 8 mm diameter, suspended in the saturated solution, by slow continuous temperature lowering by 0.5 –1°C per day in the interval between 45 and 35°C. In order to provoke jumps in the growth velocities in a controlled way, in some experiments the following changes of growth conditions were applied: (1) an abrupt temperature decrease by 1°C, with continuing the slow temperature lowering as before; (2) a temperature increase by 1 – 2 °C until re- dissolution of the crystals started, followed by the restoration of the original growth conditions.”

Of course, you don’t have that kind of control in your refrigerator Kaitlen. But you can see, professional scientist essential grow big crystals of alum by lowering the temperature, just as you did. Here is the URL for this article…


I’m giving it to you not because you’ll understand this whole article, but it has some pretty cool photos and figures of the crystals of potassium alum they grew. Also, I’ve gathered some interesting “Alum Factoids”:

Alum Facts:

What is Alum? A double sulfate of ammonium or a univalent or trivalent metal but commonly used to denote aluminum sulfate (Al2(SO4)3.

The increasing use of alum in papermaking since the 17th century and the substitution of aluminum sulfate for the milder potassium aluminum sulfate about the same time have been seen as the principal cause of deterioration of books since 1850.

Alum is used in pools to form a gelatinous floc on sand filters or to coagulate and precipitate suspended particles in the water.

Crystals of potassium aluminum sulfate, commonly used in canning before it was discovered that it can cause gastric distress in some individuals. The FDA (Food and Drug Administration) no longer recommends its use for home canning. Alum is also sometimes used as a home remedy; treating canker sores for example. In Chinese cooking, it is one of the ingredients used to make deep-fried crullers.

Good Luck on the Science Fair Project!

---* Dr. Ken Beck


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