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1074 Journal of Chemical Education • Vol. 74 No. 9 September 1997
In the Classroom
tested demonstrations
Burning Phosphorus under Water Safely
Submitted by: Larry C. Taylor
5241 Tarbell Rd., Edinboro, PA 16412
Checked by: Cole McWherter and George Gilbert
Department of Chemistry, Denison University, Granville, OH 43023
I have developed a new procedure for the lecture/labo-
ratory demonstration for burning phosphorus under water.
For years, among high school and college chemistry instruc-
tors, there has been a general dissatisfaction with methods
for performing this demonstration. Thus, there is a need for
change—and unlike many changes, this one is nonthreat-
ening. Before discussing my new procedure, I would like to
briefly illustrate some of the hazards and problems that ex-
ist with two of the procedures commonly employed.
One undesirable method is simply to pass oxygen gas
through glass tubing into a beaker of hot water, which con-
tains white phosphorus. The oxygen generator utilizes po-
tassium chlorate, which in itself poses a potential explosion
hazard. Because the oxygen generator and delivery tube are
usually secured by a ring stand, the beaker has to be con-
tinually moved around by hand in order that the oxygen
continues to come into direct contact with the moving phos-
phorus. As the oxygen bubbles from the glass tubing, some-
times violently, the hot “water” (which has become phospho-
ric acid), as well as the phosphorus itself, sometimes
“bumps” out of the open beaker, presenting other hazards.
Because the beaker is a completely open system, the smell
of phosphorus is present in the air. Attempts to “chase” the
phosphorus around by means of the oxygen generator tube
or by moving the beaker around are not very practical or
successful. At the same time, oxygen production is dimin-
ishing and the water containing phosphorus is cooling, both
being deterrents for a spectacular display. This results in
disappointment and lack of interest for chemistry instruc-
tors and students alike.
Although the second method I will illustrate is a func-
tional improvement over the previous one, its hazards may
be equal or greater. Thus it, too, is unsatisfactory. Oxygen
gas is passed through glass tubing into a test tube contain-
ing very hot water and white phosphorus. The test tube is
supported and stands vertically in a beaker waterbath,
which is heated to boiling and sustained at boiling tempera-
ture by a burner. Oxygen is generated in the same poten-
tially hazardous fashion as in method one (using potassium
chlorate), and is delivered via a glass tube running to the
bottom of the open test tube, which is half full of water con-
taining phosphorus. The open test tube presents the same
hazards as the open beaker in method one. The test tube is
suspended by a piece of cardboard or thin sheet metal, con-
taining a central hole, which is supported by the rim of the
beaker waterbath.
In the procedure that I have developed (Fig. 1), a much
safer 3% hydrogen peroxide solution is used to generate oxy-
gen, rather than the potentially explosive potassium chlor-
ate. Oxygen is released more gently from the hydrogen per-
oxide solution and oxygen generation can be manually con-
trolled by means of the stopcock on the separatory funnel.
The test tube in the Erlenmeyer flask is sealed with a two-
hole stopper, which does not permit phosphoric acid or phos-
phorus to escape if bumping occurs. In the two previous
methods, contents could escape. With the new procedure the
smell of phosphorus is barely noticed, if at all. It is kept to
an absolute minimum because phosphorus pentoxide does
not escape into the air directly. With the other methods, the
smell is a problem. Not only does this procedure afford a
most spectacular display, but it can last an hour or so, if
desired. The other methods last only minutes.
Figure 1. Setup for lecture demonstration. Each flask should be
supported by a ring stand and clamp. Pass oxygen directly into
the melted phosphorus. A: 3% aqueous hydrogen peroxide. B:
Oxygen is passed directly into the melted white phosphorus in
the bottom of the test tube. C: Manganese dioxide. D: Water in
flask is heated to boiling and the boiling is sustained. E: Test tube
should be loosely set into the flask to prevent buildup of vapor
pressure. Depending on the duration of the demonstration, you
may or may not need to add additional water to the Erlenmeyer
flask. A round-bottomed flask could be used; this might retard the
rate of water loss. F: Water. G: Do not apply suction. At the con-
clusion of the demonstration, when all or most of the phosphorus
has reacted, test the water in the suction flask with litmus paper.
An acid reaction will be found, due to phosphorus pentoxide dis-
solving in the water to form phosphoric acid.
Editor’s Note
Caution: White phosphorus is extremely hazardous. Inges-
tion of even small amounts may produce severe gastrointes-
tinal irritation, bloody diarrhea, liver damage, skin eruptions,
circulatory collapse, coma, convulsions. Approximately fatal
dose: 50 to 100 mg. External contact may cause severe burns.
Chronic poisoning may result from ingestion or inhalation and
causes deterioration of bones (especially the jawbone), spon-
taneous fractures, anemia, and weight loss. Keep white phos-
phorus under water and handle only with forceps.