Most children with access to a chemistry set will have created a chemical crystal garden¹.

Chemical gardens are the plant-like structures formed by dropping a seed crystal of a soluble metal salt into an aqueous solution of one of several anions² such as aluminates, borates, carbonates, chromates, cyanoferrates, phosphates, or silicates. The most commonly-used (and studied) anion, however, is silicate, present in sodium silicate (water glass³) and hence the alternative names of silica or silicate gardens.

As the name indicates, the chemical 'horticulturalist' will observe a growth of fronds, similar in form to plants, taking place over a period of minutes to hours. This was of interest to 19th Century scientists who were interested in the origin of life.

Materials

These materials should be readily available from a good chemist, although they might have to be ordered.

• Glass jar or large (600ml) beaker
• Tweezers
• 100ml of water
• 150ml of sodium silicate solution
• chromium (III) chloride hexahydrate⁴ crystals⁵ (green)
• Iron (III) chloride crystals (orange)
• Iron (II) sulphate crystals (green)
• Copper (II) sulphate crystals (blue)
• Nickel (II) sulphate crystals (green)
• Aluminium potassium sulphate crystals (white)
• Cobalt(II) chloride crystals (purple)
• Rubber gloves
• Eye protection

Method

1. For aesthetic reasons an optional thin layer of sand may be placed on the bottom of the beaker or equivalent glass container.

2. If you live in a hard water area, boil the water first and allow it to cool before adding to the beaker or glass jar. Add the sodium silicate to the water, and stir it to dissolve.

3. The solution is now ready to receive the other chemicals. Using the tweezers, drop these carefully onto the surface, vertically above the place you would like them to grow, in a pattern as though you were sowing a real garden.

4. Do not shake or mix the solution. The 'plants' will begin to grow within a few minutes.

5. When the garden has ceased to grow (or if you wish to stop it growing), hold the beaker under a cold water tap and slowly allow fresh water to pour down one side of the beaker, thus flushing out the sodium silicate solution. This must be done very carefully to avoid damage to the crystal fronds.

6. When you are satisfied that all the solution has been washed away, and the beaker is filled with cold water, the top may be sealed with a lid or with a piece of plastic taped down. Under these conditions, with care, the chemical garden will survive for a long time.

Safety Precautions

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

Certain metal salts, especially those of the transition metals, form precipitates when placed in the sodium silicate solution. As the metal salt dissolves, the resulting solution is less dense than the surrounding silicate solution and so rises up through the solution. 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. The surfaces of these insoluble silicates behave as a semipermeable colloidal membrane⁶, across which osmosis can occur.

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. 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. This process repeats itself until the metal salt is fully dissolved, thus allowing the crystals to keep growing upward and sideways. As the metal salt solution is less dense than the sodium silicate solution, the precipitate tends to grow upwards.

Academic Interest in Crystal Gardens

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. Traube showed that membranes could be produced artificially, which were permeable to water but not for certain dissolved substances. 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. These results showed that the osmotic pressure is proportional to the concentration of the solution, and also that it increases with rise of temperature. 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. 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

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

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. 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. 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.

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

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. 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.