Silicon forms hydrogen compounds called silanes (SinH(2n+2)), which are similar to alkanes (CnH(2n+2)). Silanes are quite reactive compounds.
In this experiment, the preparation of monosilane (SiH4) in the laboratory (some higher silanes may form as well) was demonstrated, and the spontaneous combustion of silane, when exposed to oxygen in air.
For the preparation of silane, magnesium silicide (Mg2Si) was prepared first. Powders of silicon dioxide (SiO2) and magnesium were mixed and intensely heated in a porcelain crucible. The reaction products were magnesium silicide and magnesium oxide (MgO):
4Mg(s) + SiO2(s) → Mg2Si(s) + 2MgO(s)
The products in the crucible were left to cool off. Thereafter, the mixture was poured into diluted hydrochloric acid (HCl solution).
Magnesium oxide slowly reacts with the acid without any visible reaction characteristics. Magnesium silicide reacts vigorously with acid, emitting gaseous monosilane:
Mg2Si(s) + 4HCl(aq) → 2MgCl2(aq) + SiH4(g)
or in ionic form:
Mg2Si(s) + 4H+(aq) → 2Mg2+(aq) + SiH4(g)
Upon contact with oxygen in air, silane ignites instantly. The combustion is accompanied by a cracking sound:
SiH4(g) + 2O2(g) → SiO2(s) + 2H2O(g)
In the literature, it is recommended that this experiment should be carried out under fume hood, because the formed tiny SiO2 particles may irritate the respiratory system.