The formation of molten silicates in the Earth’s mantle involves the formation of silicon dioxide and its subsequent reaction at high temperatures with metal oxides. In this experiment coloured silicates are formed in the laboratory.

Class practical

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.

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 Chemicals

Goggles for handling the sodium silicate solution 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) (Note 1)

A few crystals of some metal sulfates or nitrates (Note 2), such as:

Cobalt(II) nitrate (OXIDISING, HARMFUL)

Iron(III) nitrate (OXIDISING, IRRITANT)

Magnesium nitrate (OXIDISING)

Manganese(II) sulfate (HARMFUL, DANGEROUS FOR THE ENVIRONMENT)

Hot de-ionised water

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear goggles and disposable gloves throughout this experiment. Remember to handle the crystals only with a pair of forceps. Do not use your fingers. 

Sodium silicate (water glass) solution (CORROSIVE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Cobalt(II) nitrate, Co(NO3)2.6H2O(s) (OXIDISING, HARMFUL) - see CLEAPSS Hazcard.

Iron(III) nitrate, Fe(NO)3.9H2O(s), (OXIDISING, IRRITANT) - see CLEAPSS Hazcard.

Magnesium nitrate, Mg(NO3)2.6H2O(s), (OXIDISING) - see CLEAPSS Hazcard.

Manganese(II) sulfate, MnSO4.7H2O(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.

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

a Pour sodium silicate solution (CORROSIVE - wear goggles) 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 more able 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.

For more experiments involving recrystallisation and crystal growing see ‘linked resources’ at the bottom of the page

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015