Evacuating a bottle
Practical Activity for 14-16
An approach to the question: Does air have mass?
Apparatus and Materials
- Ordinary bottle of clear glass with a well-fitting rubber stopper and glass tube
- Motor-driven vacuum pump
- Length of pressure tubing, 1 m
- Hoffman clip
- Large transparent trough (glass or plastic)
Health & Safety and Technical Notes
Use a bottle strong enough to withstand the pressure difference. Check it has no nicks or scratches.
Wear safety spectacles and use a safety screen to protect observers.
Reject any old or perished rubber bungs or tubes for this demonstration, as they develop cracks and will not hold the vacuum.
It is advisable to use coloured water in step 3.
- Connect the rubber tubing to the vacuum pump with the clip open. The bung and glass tube must be tight fitting.
- Remove the air by pumping and then close the clip on the rubber tubing.
- To show that the air has been removed, immerse the neck of the bottle (including the rubber tubing) under water and remove the clip. Water will rush in to fill the space. If the vacuum is a good one, there should be very little air inside the bottle. If the pump was not very effective or if there was a leak, then the water will not completely fill the bottle and some air will be seen in it. There will always be a small bubble left, however well the bottle is evacuated, due to air that was dissolved in the water.
- Repeat the experiment without pumping air out of the bottle before immersing it, in order to show what happens in that case. This should be done second to avoid using the pump with a wet bottle.
- At this point the emphasis is on whether air has mass. How could the method of measuring the mass of a liquid be adapted to measure the mass of air? (By measuring the mass of the beaker plus liquid and then the mass of the beaker and subtracting to find the mass of the liquid.)
- How do you know that the pump has done its job? (By putting something else, which we can see, into the empty space.) If the pump is a good one, if there are no leaks and the pump has been pumping for long enough (the pumping noise changes) then the water will fill the bottle when it is opened under water. A very small bubble of air will appear at the top, which was the air left in the bottle at evacuation or dissolved in the water.
- It is essential to show what happens if a bottle full of air is opened under water - the water will not enter the bottle.
- Be patient discussing the idea of a vacuum. It does not occur naturally to students, and when they have been given the idea they still do not picture it easily. It is an artificial intellectual concept. Remember that they take the air itself for granted as invisible and almost absent, as did our ancestors, including the great Greek philosophers. It was only at a late stage in the development of physical science that scientists realized that we live at the bottom of an ocean of air, which has density and exerts pressure.
- If students ask what the pump does, the following discussion may help:
- "The pump acts rather like a lift that is getting people out of the top floor of a tall building. A lift doesn't pull people out. It just offers them the chance to get in the lift, and the lift carries them out."
- "The lift goes up to the top floor, the lift opens its doors and waits until a few people have wandered in. Then the door slams shut and down the lift goes. Out go the people; walking out if they are human beings, but pushed out by a moving piston in the case of air molecules in the pump. Up goes the lift again; open the doors; more people wander into the lift; out go the people. Up goes the lift... and so on. Think of that happening with a pump taking out air molecules, batch after batch, trip after trip. At that rate you will never get all the molecules out, but a pump does a very good job."
The experiment was safety-checked in July 2007