Water-initiated flash reaction
Silver nitrate and magnesium, when powdered, can react with each other extremely violently, with the production of a lot of heat and light.
The most remarkable is that this reaction is initiated by a adding a small drop of water to the mix of chemicals.
This is a spectacular, but also quite dangerous experiment. Be very careful and never scale up the experiment for added effect!
Procedure for performing the experiment
Take approximately 50 mg of magnesium powder. Do not scale up. Put the magnesium powder on an absolutely dry piece of glass or smooth tile.
Take approximately 125 mg of silver nitrate and carefully mix this intimately with the magnesium powder, without applying much pressure. The resulting mix is a greyish/white powder. The mix must be made into a small pile, with a size of approximately 6x6 mm2 and a height of a few mm.
Take a glass rod and assure that a small drop of water is on the rod. Carefully, with stretched arm, touch the mix of silver nitrate and magnesium with the drop of water on the glass rod.
As soon as the mixture is touched with the drop of water, an intense white flash is observed and quite some smoke is produced (mainly magnesium oxide, with some silver contamination). The picture below shows what can be produced with this experiment. A smoke cloud was produced of almost 20 cm high, even from such a small amount of chemicals. The sound of the reaction is just a soft and short hiss.
Sometimes, the mixture does not flash at once, but a slower reaction can be observed, with the production of small amounts of nitrogen dioxide, which can be observed as brown vapor. If this occurs, wait for some time, because the mixture suddenly can set off unexpectedly! You don't want to have your hands near the mixture, when it sets off, because of the small droplets of molten silver, which are sprayed around! If the mixture still does not set off after some time, then add another drop of water. If that does not work, then quench the mixture at once with several ml of water, before getting your hands close to the mixture.
Discussion of results
Magnesium is easily oxidized by silver ions in the cold and by nitrate as well at elevated temperatures.
Initially, the magnesium powder and silver nitrate crystals do not allow sufficient contact. The reaction cannot proceed at a macroscopic level.
When some water is added, then the silver nitrate dissolves and a concentrated solution is formed. The silver ions in this mixture react with the magnesium metal, producing a lot of heat. This heat makes adjacent mixture more reactive.
Once sufficient heat is available, no further water is needed anymore. The hot reaction frontier moves through the powdered mixture rapidly. At the high temperatures, the nitrate also acts as oxidizer.
A similar reaction is the magnesium/iodine reaction, which also is initiated by water. In the latter reaction, however, the heat produced is not sufficient to continue the reaction. Only the wetted parts react in that reaction.