Making a crystal garden
This is not only a good end-of-term experiment to let students grow crystals, which they find a fascinating process in itself, but is also a useful means of illustrating the appropriate chemistry involved when teaching introductory Earth Science. The formation of molten silicates in the Earth’s mantle involves the reaction of silicon dioxide with metal oxides at extremely high temperatures. In this experiment coloured silicates are formed in the laboratory.
Read our standard health & safety guidance
Lesson organisation
This is a very straightforward experiment and can be carried out easily by groups of two in 30 minutes. The crystals start to develop overnight, but the experiment can be left for several days or for more than a week, with perhaps a competition being held to judge which is the finest “garden”.
Apparatus and chemicals
Eye protection and disposable gloves (preferably nitrile)
Beaker (500 cm3)
Watch glass
Glass stirring rod
Forceps
A piece of card, to cover the beaker
Sodium silicate solution (water glass) (Corrosive) (see note 1)
A few crystals of some metal sulfates or nitrates (see note 2), such as:
Cobalt(II) nitrate (Oxidising, Harmful)
Nickel(II) nitrate (Oxidising, Harmful, Dangerous for the environment)
Iron(III) nitrate (Oxidising, Irritant)
Manganese(II) sulfate (Harmful, Dangerous for the environment)
Magnesium nitrate (Oxidising)
Access to:
Hot de-ionised water
Technical notes
Sodium silicate solution (Corrosive) Refer to CLEAPSS Hazcard 95B and Recipe card 66
Cobalt(II) nitrate (Oxidising, Harmful)
Nickel(II) nitrate (Oxidising, Harmful, Dangerous for the environment) Refer to CLEAPSS Hazcard 65B
Iron(III) nitrate (Oxidising, Irritant) Refer to CLEAPSS Hazcard 55C
Manganese(II) sulfate (Harmful, Dangerous for the environment) Refer to CLEAPSS Hazcard 60
Magnesium nitrate (Oxidising) Refer to CLEAPSS Hazcard 59B
1 Sodium silicate is supplied in solution as an egg preservative. This type of solution is ideal for these experiments, as it is very difficult to dissolve the solid.
2 The metal salts chosen are known to be reasonably soluble in water. If a particular metal compound is unavailable, a nitrate is usually a safe choice as an alternative, or even the chloride if a Data Book indicates that the solubility is as high as that of the nitrate or sulfate.
Procedure
HEALTH & SAFETY: Wear eye protection and disposable gloves throughout this experiment. Remember to handle the crystals only with a pair of forceps. Do not use your fingers.
a Pour sodium silicate solution (Corrosive) into the beaker to a depth of about 3 cm.
b Add hot de-ionised water to this solution, stirring well with a glass rod, until the final depth is about 12 cm.
c Continue stirring until the sodium silicate and water are thoroughly mixed, and no separate layers are visible.
d Allow the mixture to stand until the liquid is completely still.
e Use a pair of forceps to drop one or two crystals of each of the metal salts supplied into the mixture. Try to ensure that the crystals do not fall close to each other.
f Cover the beaker with a piece of card and leave overnight.
Teaching notes
There is a great temptation for students to want to handle the crystals, especially when these are not properly held by the forceps and drop before entering the beaker. Gloves can be avoided if students are careful to use forceps.
The very best effects are observed when students use a relatively small number of crystals and arrange these in a well separated manner at the bottom of the beaker.
For abler students it may be appropriate the explain that:
a the metal ions are mostly chosen from the d-block of the periodic table (these may be better known to students as the transition metals), since it is these which are coloured.
b the reaction taking place is a precipitation of the metal ions with silicate ions. A simplified equation for the reactions taking place:
eg cobalt(II) ions from the metal salt and silicate ions from the sodium silicate solution form insoluble cobalt(II) silicate
Co2+(aq) + SiO32-(aq) → CoSiO3(s)
Obviously whereas the reaction occurring in the laboratory is taking place in solution, the analogous process taking place in the Earth’s crust involves ions in a molten state at extremely high temperatures linking together.
Health & Safety checked, April 2008
Updated 29 Oct 2008
3 out of 5
The sodium silicate supplied in solution is often too concentrated,and the chemicals won't drop down into the solution easily - but using too weak a solution does not give good results. Some experimentation beforehand to get the right solution is essential. Once it is right, a backlight and/or a flexicam gives some nice effects.
Submitted by: andrew on 18 December 2008