A useful demonstration to show an energy change in chemical reaction and also a gas being made in a chemical reaction.
Magnesium and silicon dioxide are heated and silicon is produced by an exothermic reaction.
The product is then placed in acid. Small amounts of silanes are produced by the reaction of magnesium silicide (a side product) with the acid. These react spontaneously with air to give spectacular but harmless small explosions.
Silicon and Silanes from Sand
Silicon and Silanes from Sand – Risk Assessment