SMJK Chung Hwa Confucian Science and Math Society

9. Once the garden grown , you can replace the sodium silicate solution carefully with pure water.



10.Initially a colloidal and semipermeable shell of silicate is formed around the crystal.



11.Inside this is a strong solution of the salt and out side is a weak solution of sodium silicate.



12.Hence water permeates into the shell and pressure rise until the shell bursts.



13.At this stage the salt solution escapes but immediately comes in the contact with the sodium silicate solution and react with it to form again semipermeable shell of the metallic silicate.



14. Thus the original condition is reproduced over and overagain and a projection of silicate continusly grow.



15.The optimal concentration of magic solutio lie between 1.56 M and 0.625 m with respect to si9lica in experiment where growth of silica tubes is vigorous.



16.More concentated solutionproduce meagre growth and thre is vigorous growth
from intermediate concentration while the more diluted solution produces mearly a gelatinous mass.



17.In more concentrated solution the semipermeable membrane of sodium silicate surrounding the seed crystal is broken only with difficulty to produce the growth of tube.



18.On the other hand in dilute solution the membrane that is formed acquire a more plassticcharacter and is not easily rupture rather distords without breaking.

The breaking "product"

Safety Precautions
1.Several of the chemicals involved, especially the chromium (III) chloride and nickel (II) sulphate, are skin irritants, and can cause contact dermatitis.

2.Furthermore, iron (III) chloride is corrosive and stains the skin and many transition metal salts are toxic. Hence, the crystals should not be directly handled; use the tweezers!

So, What is Going On?
1.Certain metal salts, especially those of the transition metals, form precipitates when placed in the sodium silicate solution.

2.As the metal salt dissolves, the resulting solution is less dense than the surrounding silicate solution and so rises up through the solution.

3.As it reacts with the silicate anion, 'stalagmites' (like those found in caves) form from the bottom of the jar upwards - these are insoluble metal ion silicates.

4.The surfaces of these insoluble silicates behave as a semipermeable colloidal membrane6, across which osmosis can occur.

5.Water from the sodium silicate solution travels across the semi-permeable membrane of the metal ion/sodium silicate precipitate, to the higher concentration of metal ions that are present on the inside.

6.The water pressure inside the gel-like structures increases until the membrane bursts, thus allowing more of the metal ions to react with the silicate solution to create new membranes.

7.This process repeats itself until the metal salt is fully dissolved, thus allowing the crystals to keep growing upward and sideways.

8.As the metal salt solution is less dense than the sodium silicate solution, the precipitate tends to grow upwards.

Academic Interest in Crystal Gardens
1.So-called metallic trees were first observed by people such as the German chemist, Johann Rudolf Glauber (1604 – 1668) and first studied by another German chemist, Isidor Traube in the mid-19th Century.

2.Traube showed that membranes could be produced artificially, which were permeable to water but not for certain dissolved substances.

3. In this respect they were similar to those membranes surrounding plant and animal cells.
Among the semipermeable membranes prepared by Traube was one of copper (II) hexacyanoferrate (II), and such a membrane, formed in the walls of a porous pot, was used by the German botanist, Wilhelm Pfeffer, in 1877, for the quantitative measurement of osmotic pressure.

4.These results showed that the osmotic pressure is proportional to the concentration of the solution, and also that it increases with rise of temperature.

5.This research culminated in van't Hoff's law of osmotic pressure, formulated in 1887, in which he was able to apply the second law of thermodynamics.

6. This pointed the way to a method for determining the relative molecular masses of substances in solution.

Indeed, osmotic pressure (for very dilute solutions) was found to obey the Ideal Gas Law, and
πV = (m/M)RT
where:
π = osmotic pressure
v= volume (in dm3) containing a fixed mass of solute
m = mass of solute present
M = Relative Molecular Mass of solute
R = Universal Gas Constant
T = Absolute Temperature
and hence:
M = mRT/πv

7. Despite all this knowledge, the physical chemistry of the formation of chemical gardens is still imperfectly understood.

8.In 1984 Independent Television News (UK) organised a competition for all schools in Great Britain to suggest an experiment to be performed in space, aboard the Space Shuttle.

9.This was won by a school from Kent, Ashford School, who suggested a chemical garden.
The students wished to find out what shape and direction the 'plants' in a chemical garden would grow under conditions of microgravity.

10.They wrote a computer program to simulate what might happen to the garden in space and it predicted a range of possibilities from near-spherical shapes to a spherical bundle of long arms.

11.The shuttle Endeavour launched on 12 September, 1992 and the chemical garden experiment worked very successfully.

12.The growths were in random directions and tended to be twisted. To the surprise of the students there were also a few perfect spiral forms. At the time of reporting they had no satisfactory theory of the origins of these spirals.

13.Crystal-growing under conditions of microgravity has a wider and deeper significance for the life sciences, where a major goal is to understand structure/function relationships of biological systems at the atomic level. For example, many important biological molecules such as proteins (which include enzymes) have yet to be adequately structurally analysed.

14.Microgravity may provide an environment where perfect protein crystals could be produced, that are large and pure enough for more precise analysis, such as X-Ray Crystallography. This would have important applications in, for example, cancer research.