Q: Every Christmas for the past few years we have purchased a cardboard tree or mountain which has associated flowers and foliage. When this is placed in a plastic trough which contains a colourless thin fluid within about an hour it produces the most beautiful efflorescent growths resembling foliage, snow, grass and so on. Could anyone tell me the chemical basis of this phenomenon?
A: What the questioner is describing is known as a chemical garden. The colourless, thin fluid is a solution of sodium silicate; the colours on the card are water-soluble metal salts, usually present as small crystals.
When the sodium silicate meets the salt, the salt dissolves and immediately precipitates as a coloured silicate gel around the crystal. The gel acts as an osmotic membrane and water is drawn inside. The gel cannot stretch, so it ruptures and more gel forms round the rupture. The result is a growing tree of insoluble silicate, frequently splitting and branching.
Advertisement
I might not have known this, but for the fact that we used the same method to preserve eggs during the Second World War. Even in wartime in rural Scotland, it was possible to obtain sodium silicate (known as water glass). The porous calcium carbonate of the eggshell reacted with the silicate to produce calcium silicate which blocked the pores and made the shell impermeable.