On Growing Rochelle Salt Crystals
General References on Crystals
In celebration of National Chemistry Week, October 15-21, 2000, the Hamilton Section of the Chemical Institute of Canada is sponsoring a Crystal Growing Competition among the local high schools.
Highschools currently within our local section lie within the regions of Wellington,
Halton, Hamilton, Wentworth, Lincoln, Niagara South, Haldimand, Norfolk, Brant and Waterloo.
The competition is a fun, hands-on learning experience for high school students. The objective is to grow the largest, best quality, single crystal within the given time period (September 18 - October 23). In attempting to do so, students will solve a practical scientific problem from which the resulting crystal is a reward in itself.
Materials:Schools will be provided with free starting material (Rochelle salt). Kindly donated by Anachemia Science.
Judging:Crystals will be judged on both quality and size.
Prizes:The winning local team will receive special Crystal Champions T-shirts. The winning crystal will be entered in competition for the National Trophy. National winners will receive souvenir trophies, and hold the National Trophy for one year, also the school will recieve a cash prize.
If your school would like to participate, please contact:
Kimberly Banfield
Phone: (905) 525-3018
FAX (905) 525-1041
e-mail: ciccrystals@mail.dbc.net
Thank you again for participating. If you have any suggestions on a crystal for next year, or any improvements that could be made to the competition, let us know and we will forward them on to Ottawa.
Rules, Instructions and Guidelines
The Crystal Competition is meant to be a fun, learning, hands-on experience for high school students. To grow the largest best quality single crystal is the objective, but by participating and being involved in a practical scientific problem in which the result is a "gem" crystal is a reward in itself.
STEP 1 - Register
Contact your local section coordinator by September 15 to let them know that your school is participating.
STEP 2 - Obtain your Starting Material
This year we will be using potassium sodium tartrate (also known as Rochelle Salt) kindly donated by Anachemia Science.
To order your material, call Anachemia directly at 1-800-361-0209 and ask for Part Number M10373 - 500g Rochelle Salt. They will ship 500g of Rochelle salt directly to you for free.
This is sufficient for the preparation of 3 crystals.
Note: Only teachers can order materials.
STEP 3 - Prepare your Crystal
Rule 1.
The MAXIMUM AMOUNT of starting material that may be used for each given crystal is LIMITED to 150 g. The 500 g supplied is sufficient for preparation of three crystals.
However there is no limit to the number of students asssigned to a team, nor is there a limit to the number of teams permitted at a given school.
Rule 2.
So that all students across the country have an equal preparation time, crystal production must conclude within five weeks after receipt of starting material.
STEP 4 - Have your Crystal(s) Judged
The crystals will be judged on October 25.
If this will make your production time less than 4 1/2 weeks, contact your local coordinator.
Solubility of Rochelle Salt
| Temp deg C | g Salt/100 g H2O |
| 0 | 26 |
| 26 | 66 |
Crystals are fascinating. To understand their nature and better appreciate their beauty, one can learn to grow and manipulate them. It is easy to modify them and to control the growth of a little crystal into a macroscopic replica of itself. For such an experiment, you will need the following:
Preparing the growth solution
Look up the solubility (in grams/litre at a given temperature) of the material to be crystallized. Dissolve about twice this weight of material in two litres of water at the specified temperature in clean glass container. Distilled or demineralized water is preferred. Dissolution is liable to be time-consuming so heat the water a little more and stir gently with a clean glass rod. Let the solution cool slowly to the reference temperature. (Notice that the solubility varies a lot with temperature so temperature is an important factor in crystal growth.) To allow growth of the seedling crystal, it is absolutely necessary that this solution be saturated at the temperature at which the crystal will grow (usually room temperature).
The seedling crystal
With a magnifying glass (or a microscope under low magnification) select a beautiful, transparent small crystal. Tie it with the fishing line using a simple overhand knot. Suspend it in a shallow (1-2 mm deep), small volume (1-2 ml) of saturated solution in a Petri dish for a day or two. Check with the magnifying glass to be certain that the seedling crystal is well fixed to the line at the beginning of growth. This is very important because one can lose several days of growth if the beginning growth is not regular or not along the faces and edges of the seedling crystal.
Growth of the single crystal
Suspend the seedling crystal by the fishing line in 1-2 litres of saturated solution. It is crucial that the solution be saturated for as much time as possible during the growth stage. A stable room temperature is then very important, unless the growth solution can be immersed in a thermostated bath at a temperature a few degrees above room temperature. Many experiments have been perturbed by the beginning of the heating period in the fall or the beginning of the air conditioning period in spring. One must regularly re-saturate the growth solution, daily if possible. This is especially important as the crystal gets bigger. To re-saturate the solution, one must either lower the volume by heating and then gently cool the solution again to the growth temperature or dissolve a little more material, which can take some time.
Slow rotation of the growing crystal provides a small advantage by aiding the equilibration of the solution and growth with good symmetry. An electric motor with 1-4 turns per day is best, but may be hard to find. (Sometimes one can be scavenged from an old humidifier or such apparatus.) This is most useful when the crystal gets rather large.
Each time that the solution is re-saturated, it is a good idea to "clean" the crystal of any undesirable growth and to clean the suspending line. One should manipulate only the dry crystal. If it is humid or wet, it should be held only by the suspending line. It is a good habit to clean one's hands after each manipulation.
Remember that the observations you make along the way are probably as valuable as the end result. Enjoy the experience and good luck!
The idea is to grow a SINGLE CRYSTAL, not a bunch of crystals. It is therefore essential to avoid excessive rapid growth, which encourages the formation of multiple crystals, not a single crystal.
One single crystal will be judged only on the basis of quality as outlined below.
The other single crystal will be judged on the basis of combining mass and quality factors as outlined below.
The quality is judged by experts who will rank the crystals on a scale of 0-10. A score of 10 will be given to a perfect gem quality crystal which fits the ideal crystal structure known for the chemical.
The logarithm of the mass is chosen so that large poor quality crystals don't swamp out smaller good quality crystals. The value 1 is added to the mass so that crystals weighing less than 1 g get a positive score.
The crystal with the highest score is the winning crystal.
Example:
| Mass[M] | Quality[Q] | Mrel | Qrel | Total Score | |
| Entry A | 125 | 5.0 | 1.0 | 0.51 | 102 |
| Entry B | 125 | 7.0 | 1.0 | 0.71 | 142 |
| Entry C | 50 | 9.5 | 0.40 | 0.99 | 155 |
| Entry D | 30 | 9.8 | 0.24 | 1.00 | 138 |