Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Diffusion of bromine vapour


A classic demonstration that is explained satisfactorily by a particulate model of gases. The students first observe relatively sluggish diffusion of bromine into air. This is followed by the much more rapid diffusion (or expansion) into a vacuum.

Apparatus and materials


Brush for cleaning stopcocks, small

Vacuum pump


Sodium thiosulfate solution, 500 ml 1 M (25%) or ammonia solution

Bromine diffusion tubes with matched accessories (see list below), 2

Stopcocks, 8 mm large bore, 2

Short length of rubber tubing

Borosilicate glass cap tubes to hold ampoules, 2

Rubber bung no. 25 with 11/12-mm hole, 2

Bromine ampoules, 2

Bucket, large

Retort stand

Translucent screen

Lamp for translucent screen

Health & Safety and Technical notes

Bromine gas is very toxic and must not be inhaled. The liquid is also corrosive. The teacher must have 500 ml of 1 M sodium thiosulfate solution in a wide beaker so that a hand with a splash of bromine liquid on it can be plunged in immediately. 

Apparatus for diffusion of bromine vapour Photo courtesy of Mike Vetterlein The main diffusion tube is a closed glass tube (45 cm long, 5 cm in diameter) with only one opening to a side tube. There is, therefore, no danger of an accident releasing bromine to the pump when diffusion into a vacuum is done. A rubber bung fits into the side tube and carries the glass tube of the stopcock. The glass tube from the stopcock extends through the rubber bung, thus ensuring that only bromine vapour and not liquid comes into contact with the bung. In any case, the bung can and should be replaced, after a few days' use. 
The tube in the rubber bung leads to a stopcock with large bore. This should be of good quality, such as Interkey, with bore at least 8 mm. The tap of the stopcock must be spring-held for safety (but the stopcock may be of ordinary quality, not the special high-vacuum quality). The glass tube that leads out on the other side of the stopcock is joined to a closed glass cap by a short section of rubber tube, in which the bromine capsule is to be broken. That rubber tubing must have a fairly thin wall so that it can be squeezed with pliers to crush the capsule. 
With this arrangement, the breaking of the capsule to release bromine is done separately, before the stopcock is opened to admit bromine to the main tube. This enables the experimenter to concentrate on the crushing of the capsule first and then pay full attention to the main experiment.
Note the rubber tubing should not be very short, otherwise there is a danger of pulling it off the glass tube when squeezing it with pliers. Rubber tubing must, of course, have a bore large enough to let the capsule slide into it. The tube belonging to the stopcock and the cap tube must be still larger, so that the rubber tube fits tightly on them.) 
If the apparatus meets the above criteria, the experiment may be done in the open laboratory using safety screens. If there is any doubt, the demonstration should be done in a fume cupboard.

Rubber tubing should be 125 mm in length with an internal diameter of 12 mm to fit the stopcocks tightly. 
The bromine moving into the diffusion tube is more clearly seen if a translucent screen, illuminated by a rear lamp, is set up behind it. 
Cleaning the apparatus 
After the experiment the whole apparatus should be put into a bucket, prepared in readiness, half full of ammonia solution. The apparatus should then be taken to pieces under the solution in the bucket. The lower end of the apparatus should be plunged in first, the bung should be removed from the main tube and the stopcock and other items disassembled. The apparatus can later be washed, dried and reassembled. Vaseline should be used to lubricate the stopcock, not tap grease. 
It is sensible to wear rubber gloves for this cleaning process. Rubber gloves are not necessary during the main experiment. This would only invest the experiment with an air of danger which it does not deserve if carried out as suggested. If the apparatus has to be cleaned and dried quickly for use with another class, a hair dryer, with heater, provides much the easiest way of drying the main tube.



Apparatus set-up for diffusion of bromine vapourPhoto courtesy of Mike Vetterlein

Diffusion into air 
Initially the bromine capsule is placed in the glass tube and the tap is closed. The diffusion tube is full of air. 
a Slide the capsule down from the glass tube into the rubber tubing. 
b Using square pliers, not side-cutters, break the capsule by squashing the rubber tubing. This will release liquid bromine and it will run towards the tap. 
c Watch carefully as the tap is opened so that bromine moves into the diffusion tube. 
d Measure how far the average bromine molecule (average density of 'brownness') moves in about 20 minutes. Calculate the average speed of a bromine molecule. 
Diffusion into a vacuum 
Initially, only the bung and tap are attached to the diffusion tube. 
a With the tap open, use a vacuum pump to extract air from the diffusion tube. 
b Once the air has been extracted from the tube, close the tap and remove the vacuum pump. 
c Using rubber tubing, attach to the tap the glass tube with a bromine capsule inside. 
d Slide the capsule down from the glass tube into the rubber tubing. 
e Using pliers, break the capsule by squashing the rubber tubing. This will release liquid bromine and it will run towards the tap. 
f Watch carefully as the tap is opened so that bromine moves into the diffusion tube. It happens almost instantaneously! 
The movement of brominePhoto courtesy of Mike Vetterlein

Teaching notes

1 The crucial point to these demonstrations is that the 'average' speed of bromine molecules does not change from one demonstration to the next. Careful questioning of the class will highlight many common misunderstandings. These include the idea that the bromine is being 'sucked' into the tube in the vacuum demonstration. 

2 It may be best to discuss precisely what has been observed without any theoretical inferences. This can be followed up by developing a theoretical explanation of the observations based on the particulate picture of a gas. 

This experiment comes from AS/A2 Advancing Physics

This experiment was safety-checked in June 2004


Related guidance

Mean free path

Further discussion of mean free path

Proof of R= sN