Pyrophorisity of Silane

Magnesium silicide in contact with hydrochloric acid produces silane which in contact with air ignites spontaneously.

Materials
Test tube (16 mm x 160 mm), laboratory beaker of 100 mL, small spoon, metal tray with sand, burner, metal stand with clamp.

Chemicals
Magnesium powder, silicon dioxide powder (silica), 1:1 hydrochloric acid

Fig. 1. Apparatus

Fig. 2. Burning of Silane

Experimental

• Make a mixture of 2 g magnesium and 2 g of silica.
• Add the mixture in a test tube.
• Assemble apparatus as it is shown on the Fig.1 in a fume hood.
• Heat the test tube with a burner.
• When the whole amount of the substance turns black stop the heating.
• After cooling, if necessary, break the test tube in a mortar and remove the pieces of glass.
• In a fume hood, add some of the content of the test tube in small portions in a beaker with hydrochloric acid as shown on Fig. 2.

Observation and Discussion

Magnesium silicide is prepared by heating mixture consisted of magnesium and silicon dioxide.

2Mg(s) + SiO₂(s) = Si(s) + 2MgO(s)

When it is added to the hydrochloric acid a gas is evolved.

Mg₂Si(s) + 4HCl(aq) = SiH₄(g) + 2MgCl₂(aq)

This gas is silane and ignites in contact with oxygen to produce silicon dioxide.

SiH₄(g) + 2O₂(g) = SiO₂(s) + 2H₂O(g)

Safety Tips
The experiment must be performed in a fume hood. In the beginning, the test tube with the mixture should not be heated at the bottom of the test tube. Due to the possibility of reaction:

Mg(g) + H₂O(l) = H₂(g) + MgO(s)

which can take place in presence of moisture in the mixture, the content can be thrown out of the test tube. When the reaction starts one can notice burning as a glow in the test tube and the heating should be stopped.