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!
Required
chemicals:
-
silver nitrate
-
magnesium powder
Required
equipment:
-
piece of glass or smooth tile (this is spoiled during this experiment)
- glass rod
Safety:
- Silver nitrate is fairly toxic and fairly corrosive. It gives black stains on the skin. If you get black stains, you simply have to wait, until they disappear by wear. This may take a week or something like that.
- Magnesium metal is flammable.
- Fine magnesium powder is 'light' and is easily blown into the air. Although magnesium is non-toxic, it is not fun at all to inhale very fine particles of an insoluble metal. This can cause severe irritation!
-
Once the magnesium
and silver nitrate are mixed, the mixture is extremely moisture sensitive!
Mixing must be done on an absolutely dry piece of glass or a smooth
tile!
-
The reaction
produces an intensely hot mixture. Molten silver droplets can spray around!
The glass, on which the experiment is performed has small pits after the
reaction, due to the intense heat produced.
- The smoke, produced in this experiment, may contain small amounts of silver compounds and hence must be regarded as toxic. It is best to perform the experiment outside or in a well-ventilated room.
Disposal:
- After the reaction, rinse the piece of glass with a small amount of water. The rinse-water should not be flushed down the drain. It contains silver waste. It can be brought to a waste processing facility as photo-fixer waste. The piece of glass or tile can be thrown away as common household waste or stored for similar experiments.
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.