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crystal growth and reasons why temperature and different typ

Posted: Thu Oct 23, 2014 12:06 pm
by gela1541
Hello I am the parent of a 4th grader that is doing a science project that is due in the last days of January 2015. She has decided to do her project on Crystals and she has started on experiments and I have helped her make them.
We already have at least 4 different crystals (she used Alum as the Soluble) and has done one Salt experiment.
She decided to use 2 different types of water in the experiments (she used Plain Tap Water and Distilled Water) she also used 2 different types of Temperatures when doing each crystal experiment and had 2 different types of solutions and crystals from each experiment.
The problem she is facing is trying to come up with the right way to explain her hypothesis on the different types of waters affect the growth of the crystals.
This is because when she made one of the solutions using Plain Tap Water and Alum, the crystals came out big, however when she did the next solution using Distilled Water and Alum, the crystals came out even bigger than the lest solution she had made. When we did some research on why the different types of water affects the size of growth of the crystals online to help her research the hypothesis behind it.
Online results kept coming back that Plain tap Water was the best at making large crystals, when her observations of her experiments told her different results than the computer did.
Now she's confused because she keeps researching online and in books on what affects Distilled Water has on crystal growth and she cannot find an answer that sounds like what her experiments concluded.
She would like to understand the hypothesis for the experiment she has done.
She also has Temperature experiments on the crystals growth as well but she is stuck a little there as well, although that part she is starting to understand and explain in her hypothesis.
The problem is the different water types and the crystal growth.

The solutions she made were as follows:

1 cup hot water
2 1/2 heaping tablespoons alum
food coloring (to show one experiment from another)

She made seed crystals also to show the growth of each after the solution had formed the crystals and to show the size while in the solutions.

Could an expert please help her figure the way to explain the situation she is having difficulties with and so she can understand it as well, so that she can make complete since of what her experiments showed her regarding the difference between water types and crystal growth. :?:

Re: crystal growth and reasons why temperature and different

Posted: Fri Oct 24, 2014 9:49 am
by tdaly
Hi gela1541,

Just to make sure I'm clear on the experiment: Your daughter is growing alum crystals and salt crystals using both tap water and distilled water, correct? Do the different temperatures refer to the temperature of the water the solutes were dissolved into or the temperature the solutions were kept at while the crystals were growing?

I'm assuming that all of the crystals were grown for the same amount of time.

Tap water has dissolved solutes in it that vary depending on where your tap water is coming from (e.g., reservoir or groundwater; rock types in the area). Distilling the water removes all of these solutes. Crystals grow as solute atoms stick onto the seed crystal. But, the atoms can't just stick onto the crystal in any way they want to. Crystals have very specific arrangements of atoms. Let's say you are trying to grow crystals of A (A could be salt or alum). Atoms of A are dissolved in the water and start to stick onto the seed crystal of A. The crystal grows larger. But, if there are atoms of B dissolved in the water as well (B would be the "other" solutes already present in the tap water), then B atoms might stick onto the crystal of A. This changes the surface of the crystal, and can make it very difficult for more atoms of A to stick onto the crystal. This process can actually stop crystal A from growing. In the science world, this is called "second phase pinning".

I suspect that this process is the explanation for your results. The solutes that are naturally in the tap water may be interfering with crystal growth, "pinning" the size of the crystals so that they cannot grow as large as the crystals grown in distilled water.

Re: crystal growth and reasons why temperature and different

Posted: Fri Oct 24, 2014 8:28 pm
by gela1541
Terik Daly:
Thank you for answering my question, I am responding to your question in the reply. Yes my daughter is doing the Alum and Salt crystals in both tap water and distilled water. She has done half her project using alum with tap water and then the other half of the project using alum and distilled water. She also has done the salt crystals the same way (half in tap water and other half in distilled water).
Each solution was made and left to sit undisturbed for the same amount of time. She decided to leave the solutions at room temperature to sit undisturbed to compare the experiments after they were complete.
The answer to the question you have regarding the temperatures, was that the temperatures of both solutions were kept at while the crystals were growing (so while they sat during the growing process was only at room temperature).
She has already done the experiments and has the proof to show but she does not understand the reason why the crystal made with distilled water actually grew much bigger than the other solution made using tap water.
Because when she did the experiment on both, she came up with different data then what she had researched in books and online.
Does that provide you with more detail regarding the issue my daughter has regarding the findings of her experiment and how she can give her hypothesis in a way that she understands the findings and so she can be able to explain it so that the science fair judges understand what she has concluded.

Re: crystal growth and reasons why temperature and different

Posted: Sat Oct 25, 2014 12:15 pm
by tdaly
Hi gela1541,

Thank you for this additional information. It is very helpful! It sounds like you controlled all of the right variables so that the difference must be due to using different kinds of water.

Take a look at the answer Vince Calder (of Argonne National Laboratory) gave when someone asked a similar question: ... n99889.htm. I agree with him--other ions dissolved in tap water are probably interfering with crystal growth in your daughter's experiments. Tap water may have additional nucleation sites (which other scientists at Argonne point out in that post), but the impurities in tap water can actually stop crystals from growing larger. Those impurities aren't present in distilled water, so the crystals in distilled water can grow to larger sizes. That's the explanation for the result of your daughter's experiment.

As far as presenting her hypothesis and findings goes, your daughter's hypothesis shouldn't change after her experiment. A hypothesis is an educated guess about what will happen--you make it before doing the experiment. Her conclusion should (1) state whether her hypothesis was correct or incorrect, (2) say what the results were (crystals grew larger in tap water), and (3) briefly explain why the experiment turned out the way it did (other ions--impurities--in the tap water interfered with the growth of the crystals, keeping them from growing as large as the one in distilled water).

Re: crystal growth and reasons why temperature and different

Posted: Sat Oct 25, 2014 3:19 pm
by gela1541
Terik Daly:
Thank you for providing the link to the website that will help my daughter with her answer. Thank you for explaining the results that she had come to with her experiment, thanks for providing the answer and for helping me understand the answers so I can help her understand them as well. :D