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

Investigation of a bottle labelled ‘vacuum’

Demonstration

 

Apparatus and materials

Glass bottle with well-fitting rubber bung through which is a glass tube

Short rubber tubing with wall thin enough to close

Screw clip

Rotary vacuum pump

Pressure tubing to connect pump to the bottle

Bowl of water (coloured if possible), large

Health & Safety and Technical notes


Suitable glass bottles are not easy to obtain since thin plastic bottles are commonly used for drinks. A clear, sparkling wine bottle should be satisfactory since they are designed to withstand pressure.

Once the air has been pumped out of the bottle and the screw clip closed, it may help to keep the vacuum if a glass rod is inserted in the end of the tubing. 

Old or perished rubber bungs or tubes should not be used for this experiment, as they develop cracks and will not hold the vacuum. 
 
apparatus set-upIt is essential to use rubber pressure tubing for connection to the pump otherwise it is liable to collapse (plastic tubing is not advised). A long tail of pressure tubing would be inconvenient when the bottle is opened under water; on the other hand there should be plenty of pressure tubing, say 1 metre between the bottle and the pump to make the work easier. The best arrangement is to have two pieces of glass tubing: the first in the outlet to the bottle is connected by a short piece of rubber tubing with the clip attached to the second (which is replaced afterwards by a rod as a stopper) and this second piece of glass tube takes the long length of pressure tubing, in turn connected to the pump. The short piece of tubing can be of medium-wall rubber instead of pressure tubing if it is short and the glass tubes are pushed into it close together to hold it open. 
 
It is advisable to use coloured water when opening the bottle. 

 

Procedure


To show there is nothing in the bottle, immerse its neck in the water and release the screw clip. Water will rush in to fill the space. Coloured water makes this clearer to a whole class. 


Teaching notes


Having come across the bottle labelled ‘vacuum’, students should be asked what they think is inside. Not all will think there is nothing there! When the group does come up with that idea, it is fun to ask them how they think they could test it. Only after this discussion should the procedure above be followed. 

If the vacuum is a good one, there should be very little air inside the bottle. There will always be a small bubble left, however well the bottle was evacuated, due to the air dissolved in the water. If there is a significant amount of air at the end then the pump was not very effective or there was a leak. 
 
The more thoughtful students might wonder how the air was removed in the first place. For such people it would be an investment to help them realise that the air will move from a higher to a lower pressure. This will help students with fluid flow in general, and also flow of charge in electrical circuits. (They may even want to know how vacuum pumps work at low pressures.) 

 

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