Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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bleach : NaClO
hydrochloric acid : HCl
phosphorus red : P
sodium hydroxide : NaOH
Class:
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elem=P
redox
Summary:
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White phosphorus reacts with hot solutions of sodium hydroxide and gaseous
phosphine and diphosphine are produced. The diphosphine spontaneously combusts
in air, the phosphine does not. The diphosphine is very unstable and within a
few minutes it reacts and only phosphine is left.
REMARK: THIS IS A VERY DANGEROUS EXPERIMENT. WHEN NOT CARRIED OUT WITH
SUFFICIENT PRECAUTIONS. KNOW WHAT YOU ARE DOING!!
Description:
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Add a small piece of white phosphorus (size of a pea) to a solution of sodium
hydroxide in a test tube, which can be stoppered with a rubber stopper with a
single hole in it. For the NaOH, use a 40% concentration or something close to
that: Nothing happens, the piece of phosphorus sinks to the bottom, there is no
visible reaction.
Next, pour in some liquid butane, or some of the liquid, contained in canisters
for dust-off (bottled "air" for blowing away dust from cameras, laptops etc.)
and IMMEDIATELY tightly stopper the test tube with the rubber stopper, with a
thin flexible PVC tube attached to it, the other end of the tube being under
water. Allow the butane of "dust-off" liquid to evaporate. When this is done,
all air is driven away from the test tube and the flexible PVC tube. Any other
means of assuring that air is driven away
After this preparation, heat the test tube carefully, until the liquid just
starts boiling: First, the butane gas or "dust-off" gas is bubbling out of the
thin flexible PVC tube, some time later, bubbles of phosphine are produced. If
heating is sufficiently strong and the PVC tube is not too long, then the
bubbles of phosphine ignite as soon as they reach the water-surface and pop. A
white cloud of smoke is produced when the bubbles ignite, sometimes
ring-shaped. Unfortunately, the bubbles do not always ig
Let the reaction go on for a while (while still heating), just to be sure that
all butane gas or "dust-off" gas is removed and the bubbles are pure phosphine.
After that, keep a small bottle full of water inverted over the PVC tube and
collect the gas: A colorless gas can be collected, which does not dissolve in
water at any appreciable rate. The phosphine does not react with water, it can
be stored as such. When a bubble of the gas is allowed to go to the surface of
the water, it does not ignite.
When all phosphorus has dissolved, then the liquid is pale yellow and clear.
Let the test tube cool down with the PVC tube still under water! Water is
sucked into the test tube when it cools down. Finally approximately half the
volume of the test tube remains filled with water. Bubble some of this gas to
the surface of the water: The bubble ignites as soon as it reaches the surface.
Bubble the remaining gas into a clean test tube, inverted under water, and then
stopper this test tube, also under water: A colorless gas is collected, but
some white smoke forms inside the gas. After a few minutes, the inner side of
the test tube, which is not under water, is covered by a pale yellow solid.
When the colorless gas now is bubbled into air, it does not ignite.
In this reaction, the phosphorus disproportionates to phosphine and
hypophosphite. There, however, also is a side-reaction in which P2H4 is formed.
The gas-mix, produced in this way contains PH3 and a small amount of P2H4. This
mix self-ignites in contact with air. When this gas-mix is stored above water
in an inverted test tube, then the P2H4 quickly disappears (a yellow solid is
formed) and the PH3 remains. It is not clear whether the P2H4 reacts with
water, reacts with oxygen, dissolved in the water, or
REMARK: This experiment is interesting and rather spectacular, but one has to
be sure that no air can enter the test tube with the phosphine mix. The
stoppers and tubes must be 100% air-tight, otherwise nasty accidents can occur!
Another big risk is the intense toxicity of phosphine!
REMARK2: After the experiment, it is important to cleanup everything, such that
no remains of white phosphorus are left in the test tube, the thin PVC tube,
etc. For this purpose, prepare a mix of 10% HCl and bleach and use this to
rinse the test tube and also press this mix through the PVC tube. This destroys
all remains of phosphorus, which may stick to the glass or the PVC.
REMARK3: The white phosphorus, used in this experiment, was not purchased
commercially, but it was made from red phosphorus. For more info see:
http://woelen.scheikunde.net/science/chem/exps/RedP2WhiteP/index.html