Demonstration

Crushing an evacuated can.

Apparatus and Materials

• Tin can with bung and tubing
• Length of pressure tubing
• Empty PET drink bottle, 2 litre
• Sharp knife
• Vacuum pump
• Balloon
• Safety screens
• Bell jar (OPTIONAL)

Health & Safety and Technical Notes

During this demonstration eye protection should be worn.

Take care when clearing away broken glass.

Read our standard health & safety guidance

The tin can should be rectangular, not cylindrical and preferably 5-litre size or larger. It should have a well-fitting bung in the top with a short length of glass or brass tubing through it.

Care should be taken in choosing the can so that its previous contents do not harm the pump.

Cut off the curved bottom of the plastic bottle with a sharp knife, ensuring that no sharp points remain. A belljar can be used as an alternative. The outlet of the bottle should have a well-fitting rubber bung with a glass tube through it.

Procedure

1. Connect the can by pressure tubing to the vacuum pump.
2. Pump the air out slowly and the can will collapse. Instead, or as well, you can evacuate a PET bottle or a hollow rubber toy.
3. Cut the neck from a balloon and stretch the remainder across the open end of the bottle. It may be held on by its own tension, but you may need to use string or thread.
4. Connect the bottle by pressure tubing to the vacuum pump. Pump out the air slowly to show the effect on the rubber sheet – its shape is roughly hemispherical. Eventually the entire bottle may collapse.
You will need eye protection and a safety screen for this part of the experiment.

5. Close the bell jar with a thick glass sheet (using vacuum grease to seal it) with a partially inflated balloon inside it. Or this can be as effectively done by putting the partially inflated balloon inside a bottle connected to the pump.
6. Place a very thin sheet of glass over the bell jar. Pump air out of the bell jar very slowly indeed so that the breaking of the glass can be seen. The glass, which must be very thin, should be sealed to the rim of the bell jar with vacuum grease. A decent trap and filter placed in the bottom of the bell jar should prevent glass chips reaching the pump.

Teaching Notes

• This demonstrates that a container needs pressure inside it to balance the pressure of the atmosphere outside. If the air inside the container is evacuated, the container collapses.
• An alternative approach to crushing the can is as follows. Fill a rectangular can with water to a depth of about 0.5 cm. Put it over a Bunsen burner or gas ring and boil vigorously so that the steam drives out all the air. Turn out the gas flame or remove the can from it, holding it with an oven cloth. Add a tight stopper. Allow the can to cool and it will collapse under atmospheric pressure as the steam inside condenses. The condensation can be speeded up by pouring cold water over the can.
• This can be a spectacular demonstration done by a teacher with a large oil drum.

This experiment was safety-tested in July 2007

Demonstration

A can collapses when evacuated.

Apparatus and Materials

• Polythene or PET drink bottle or large can (5-litre size) (see Technical notes)
• Vacuum pump
• Rubber tube and bung to fit bottle or can
• Length of pressure tubing, 1 m

Health & Safety and Technical Notes

Wear eye protection and use a safety screen to protect observers.

Take care when fitting the glass tube through the bung.

Read our standard health & safety guidance

Care is needed in choosing a can because the original contents of the can may damage the pump. A 5-litre olive oil or vegetable oil can is suitable, but try to ensure that it is clean and dry before use.

Procedure

1. Fix a rubber bung with a glass tube through it in the neck of the bottle or can.
2. Using pressure tubing, connect the bottle to the pump.
3. Switch on the pump. The air should be removed from the bottle (or the can) so that it collapses.

Teaching Notes

• Using a soft plastic bottle shows the effects of removing the air, as it collapses. When the bottle is opened air rushes in and the bottle is restored to its normal shape. This demonstration is normally done to show the effects of air pressure when the pressure on one side is reduced.
• Evacuating an oil can shows what happens when the metal is unable to return to its original shape.

This experiment was safety-tested in July 2007

Class practical

Observing water emerging from holes in a tin can.

Apparatus and Materials

For each student group

• Beaker or jug, 600 ml (approx)
• Small beakers or disposable cups, 3
• Tin cans, 2 (see Technical note 1)
• Hammer
• Bradawl, 5 cm long (approx)
• Bell jar (OPTIONAL)

Health & Safety and Technical Notes

Wear eye protection when using the hammer to batter the cans.

Read our standard health & safety guidance

The tin cans should preferably be about 0.5-litre size and can be obtained from home.

It is helpful to have a supply of sponges available for this experiment.

Procedure

1. Take a tin can and make holes in it, near the bottom, with a bradawl. A block of wood, placed as an anvil inside the can, is a considerable help. The holes should be made at different places round the can all at the same level. With care, the bradawl will make the holes all the same size.
2. Fill the cans with water over a sink and watch how the water spouts out.
3. Take a second can and make three holes – one near the top, one part way down, one near the bottom. It is most convenient if the holes are slightly staggered around the can.

4. Fill the cans with water over a sink and watch how the water spouts out. By pouring replenishing water into the can from the 600 ml beaker, the level of water in it can be kept constant. Collect the water spouting from the three holes in three separate beakers or cups and observe what happens.

5. Take the first can and batter it into an irregular shape with a hammer. (Wear eye protection.) The aim is that the surface of the can at the three holes should be orientated in three different directions. Fill the can with water and observe the direction in which the water comes out and collect each stream in a separate beaker.

Teaching Notes

• These experiments are intended to be rough simple ones that can be repeated at home, to show students that simple experiments with common equipment can lead to useful knowledge. Therefore, it is essential to use ordinary things for the apparatus, such as tin cans and plastic or paper cups, and to make the holes with a bradawl. You may feel tempted to prepare special apparatus for this, which can have identical holes drilled so that it works well and can be stored away. That would miss the point of the present experiment, so rough apparatus should be used and students encouraged to make the holes for themselves using a bradawl.
• It is difficult to twist a mercury barometer around to show that the atmosphere pushes in all directions, always perpendicular to any surface that it is offered; but we can show it with water.
• With the holes all at the same level, and of the same size, then the water will spout out equally well from all the holes. The water from each of the holes could be caught in beakers and measured, so that a judgement on the equality of the hole size can be made.
• With holes of the same size at three different levels in the can, the result is not what most people expect. Asked to sketch the jets of water from those three holes, you may predict a progression of ranges. The jet from the top hole can reach the table nearest the can, the jet from the middle will arrive further out and the jet from the lowest hole, with the biggest pressure will arrive farthest out.
• A thought experiment warns that this must be wrong. If the can rests on the ground, water from the hole at the very top (the free surface) will dribble down the side and arrive at the very edge of the can. Water from the bottom, however fast it travels, will also hit the ground immediately, just at the bottom of the can. Yet water from some intermediate level will spout out and arrive some distance away.
• To calculate the velocity v with which water emerges from a hole at depth h below the surface of the water, use: mv ² /2 = mgh v ² = 2gh where g is the gravitational field strength. Then the water can be treated as a projectile moving under gravity.
• The third can has holes near to the bottom and is battered into an irregular shape so that the surfaces of the metal point in different directions. It will be found that the water leaves the can at right angles to the surface showing that pressure acts in all directions.
• As an optional home experiment, students could fill a balloon with water and pierce it with a pin. First put a hole near the top, next halfway down, finally in the lower half, to see the direction of the pressure. This experiment does not always give consistent results and it is probably best to do it in the bath!

This experiment was safety-tested in July 2007

Demonstration

This demonstration allows students to see air being removed by a pump.

Apparatus and Materials

• Vacuum pump
• Round-bottomed flask, 1 litre
• Bung with glass tube to fit (take care when inserting the glass tube into the bung)
• Smoke filter (see Technical notes)
• Length of pressure tubing, 1 m
• Light source and appropriate power supply, OPTIONAL
• Safety screen

Health & Safety and Technical Notes

Wear eye protection. Use a safety screen to protect observers. A round-bottomed flask is likely to have fewer stresses caused in its manufacture and is therefore more likely to withstand the evacuation process. Care should be taken in looking for hairline cracks before connecting the apparatus together. It is a good idea to keep a particular flask for evacuating and not just one ‘off the shelf’. When the flask is evacuated, take care not to tap the flask against a hard surface as it may shatter.

Glass wool is used inside the smoke filter (as described above). This has been a standard practice for years. Glass wool is an irritant and it should be left inside the glass tube containing it. If you are making your own filter, use ordinary cotton wool instead, though not too much of it.

Read our standard health & safety guidance

The vacuum pump should be a motor-driven rotary pump and not a hand pump. For connection to the vacuum pump, it is essential to use pressure tubing. The smoke filter is made with a 20 cm length of 3.5 cm diameter glass tubing containing glass wool. Bungs are provided at each end through which glass tubing connects on the one hand to the vacuum pump, on the other to the flask to be evacuated.

Procedure

1. Allow smoke from a smouldering spill into the one-litre round-bottomed flask so that it is clearly visible.

2. Close the flask with the rubber bung, which is attached to the other apparatus as shown above.
3. Switch on the pump so that the smoke can be seen being pumped out. To achieve this, adjust the needle valve on the pump so that the rate of pumping is as slow as possible. Otherwise, the operation is over too quickly. If the pump is fitted with a gas-ballast valve, this should be left open during this process.
4. It is helpful to illuminate the flask brightly and to have a dark background.

Teaching Notes

Most young students are unfamiliar with the action of a pump and even the idea of a vacuum is strange to many. To give them an idea of the pump’s effect and to let them see ‘air being taken out of the bottle’, some visible gas should be pumped out of a flask. Unfortunately, visible gases such as bromine are highly corrosive and should not be used with school pumps. However, smoky air can be pumped out of a clear flask without harming the pump if a smoke filter is inserted between flask and pump.

The experiment was safety-checked in July 2007

Demonstration

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.

Read our standard health & safety guidance

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.

Procedure

1. Connect the rubber tubing to the vacuum pump with the clip open. The bung and glass tube must be tight fitting.
2. Remove the air by pumping and then close the clip on the rubber tubing.

3. 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.
4. 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.

Teaching Notes

• 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

Demonstration

Pumping air from one side of a manometer.

Apparatus and Materials

• Mounted U-tube manometer, 1.5 m tall
• Bottle of mercury
• Length of pressure tubing, 1 m
• Vacuum pump
• Mercury spill tray
• Trap (to prevent the water being 'sucked' into the vacuum pump)

Health & Safety and Technical Notes

When mercury is being used, the laboratory should be well ventilated, and equipment for dealing with spills ready at hand. The apparatus itself should be placed in a tray so that any beads of mercury can be collected easily.

Wash hands thoroughly after using mercury.

Read our standard health & safety guidance

The pressure must be reduced gradually by careful operation of the tap on the pump. This is made easier if there is a needle valve on the pump, alternatively a side tube can be used with rubber and clip to provide a leak.

It is essential to insert a trap between the pump and the tube to ensure that mercury does not enter the pump. This has the additional advantage of making it easier to evacuate slowly.

The spill tray should be plastic and smooth surfaced. It should be deep enough to hold all the mercury.

When the manometer is not in use, use a stopper to keep the ends open.

Procedure

1. Connect the vacuum pump to the manometer as illustrated.
2. Reduce the pressure gradually until the difference in heights stops increasing.
3. Measure the difference in heights.

Teaching Notes

• For many students, the idea of the atmosphere is half taken for granted, half a mystery. Asked if the air is here in the room a student will say, "Of course it is, I breathe it in and out, I can feel it." Yet when asked if air is real stuff that you can weigh and put in a box, like water or sand, many students will show uncertainty. Air is not as real to them as water or sand, nor was it to early scientists.
• The idea that we live at the bottom of an ocean of air that exerts as good a pressure as an ocean of water some 10 m deep is new and strange; essentially unthought of rather than difficult. (James Conant, in his excellent discussion of the tactics and strategy of science, quotes the idea of an ocean of air as an example of a conceptual scheme that enabled science to advance.)
• Students may have heard that the air exerts a pressure, pushes on things. Ask, "Well, if the air does press on everything, could we use the U-tube and mercury to measure the pressure of the air in this room, the pressure of the atmosphere as we call it?" (If someone asks, "Why mercury?", reply, "Let us try it with mercury first, in case the pressure is so big that the water pressure scale is not tall enough.)" This is not quite the same as the discouraging reply, "because mercury is the right liquid to use" - it is nearer to the sensible admonition "Try the 10-amp ammeter before you try the 1-amp one!" That is good scientific procedure, and so worth pointing out.
• If students do not know what to suggest, point out that if you blow into a U-tube pressure gauge, there are two pressures. The lung pressure plus atmospheric pressure on one side and atmospheric pressure only on the other side.
• "Now look at the U-tube. Both ends are open, not connected to anything. There is mercury at the same level on both sides. Suppose we wanted to measure the pressure of the air in this room. There it is pushing on the mercury in the left side, and there it is pushing on the mercury in the other side. What must we do?"
• Elicit the suggestion of pumping air away from one side. Pump a little and ask what is happening. Then pump some more. Then stop and raise the question of the pump being damaged by pumping the mercury into the pump. By this time a group of students are frantic to see that happen. Go on pumping and show there is a definite limit. Ask whether one can be sure that there is a good vacuum, just nothing, on that side.
• Measure the levels of mercury. What would it be if we had used water?

This experiment was safety-tested in July 2007

Demonstration

A mercury column supported by atmospheric pressure.

Apparatus and Materials

• Glass tube (1.5 m long, 4 mm bore, 8 mm external diameter)
• Small trough of mercury
• Length of pressure tubing, 1.5 m
• Retort stand, boss, and clamp
• Vacuum pump
• Translucent screen and lamp (see Technical note 2)
• Mercury spill tray
• Trap (to prevent the water being sucked into the vacuum pump)

Health & Safety and Technical Notes

When mercury is being used, the laboratory should be well ventilated, and equipment for dealing with spills ready to hand. The apparatus itself should be placed in a tray so that any beads of mercury can be collected easily.

Use a trap to prevent mercury being sucked into the vacuum pump.

Wash hands thoroughly after using mercury.

Read our standard health & safety guidance

It is essential to insert a trap between the pump and the tube, so that the pump is fully protected.

It is much easier for the students to see the mercury if the tube is placed in front of an illuminated sheet of translucent material. A suitable screen can be made of tracing paper in a wooden frame. A lamp behind the tube ensures that the tube is seen in silhouette.

You need enough mercury to fill the tube about up to 800 mm high, with some left in the reservoir.

Procedure

1. Set up the glass tube vertically with the lower end immersed in the mercury trough. The tube should be held in position by a clamp. Place the whole apparatus in a spill tray for safety.

2. Connect the top end of the tube to the vacuum pump by pressure tubing.

3. After pumping, measure the height of mercury in the tube above its level in the trough.

Teaching Notes

• Start by asking: "Would the same level difference be expected for a mercury filled U-tube with a vacuum on one side if the U-tube had arms of unequal size, one made of much wider tubing than the other."

• Draw a picture of several U-tubes: one with arms of equal width tubing; then one with one arm much wider than the other; the 'W' tube; and eventually an open dish and the tube.
• The glass tube dipping into an open dish of mercury is a U-tube with one arm very wide indeed. Dip a 1.5 m tall glass tube into a dish of mercury. Connect rubber tubing to the top of the tube and follow the procedure described above.
• Ask: "What makes the mercury go up? Why does it stop?" This is a barometer, which measures the pressure of the atmosphere in centimetres of mercury.

This experiment was safety-tested in July 2007

Demonstration

Setting up a mercury barometer.

Apparatus and Materials

• Barometer tubes with different diameters, 2 (see Technical note 1)
• Bottle of mercury
• Small funnel
• Mercury trough
• Mercury spill tray
• Retort stand, boss, and clamp
• Translucent screen and lamp (see Technical note 2)
• Length of stout iron wire
• Metre rule

Health & Safety and Technical Notes

It is advisable to wear latex or nitrile close-fitting gloves. The gloves must still give the wearer good grip of the glassware. The room must have good natural ventilation. The volume of the spill tray should be at least 110% that of the mercury being used. It should be made from a smooth-surfaced plastic tray, not metal. Fill the spill tray with a thin layer of water.

Watches/rings etc should be removed as the mercury forms an amalgam and can damage them.

Wash hands thoroughly after handling mercury, even if gloves have been worn.

Inexperienced teachers need to be shown how to do this experiment, to avoid a large spill of mercury.

Read our standard health & safety guidance

The barometer tubes need to have very different bores for this demonstration to be effective. 4 mm and 8 mm are recommended.

It is much easier for the students to see the mercury if the tube is placed in front of an illuminated sheet of translucent material. A suitable screen can be made of tracing paper in a wooden frame. A lamp behind the tube ensures that the tube is seen in silhouette.

Filling a barometer tube takes practice and needs a steady hand but it is well worth trying to master the art.

Procedure

1. Fill the barometer tube with mercury, holding it over the tray throughout. (An easy technique for this is to pour mercury into the tube, using the funnel and a 5 cm length of rubber tubing, until it is nearly full, all but 2 cm at the open end. Close that end with a finger. Tilt the tube to run the air bubble very slowly to the other end of the tube and back again, collecting up any small sticking bubbles on the way. Then fill the tube to the top.)

2. Hold a finger on the top and invert it into the trough. Do not remove the finger until the end of the tube is below the surface. Then ask students if air can get in. They should then watch what happens when the finger is taken away. Hold the barometer in a clamp and measure the height.

3. If the height does not remain the same, air has been included and the barometer must be refilled.
4. By securing the tube with a spiral of wire, it can be conveniently inclined to show the mercury level remaining the same height above the trough.

5. Set up two barometer tubes of different diameter side by side in the same trough. It will be seen that the mercury levels are the same in each tube.

Teaching Notes

• When the tube is full, before inverting it, check that the students realize that there is no air in it and that no air can get in when the tube is inverted.
• When the supporting finger is removed then the column of mercury falls to a height which the atmosphere can support. The gap at the top of the tube contains a vacuum, the Torricelli vacuum.
• When the vertical height, not the length along the tube, of the mercury column in the inclined tube is measured, it is shown to be the same as the height of the column in a vertical tube.

This experiment was safety-tested in July 2007

Demonstration

A barometer over 10 m tall is very memorable.

Apparatus and Materials

• Length of clear tubing (PVC or other plastic), 12 m
• Bucket
• Vacuum pump
• Trap (to prevent the water being sucked into the vacuum pump)
• Food colouring (OPTIONAL)

Health & Safety and Technical Notes

A trap should be used to prevent water being sucked into the vacuum pump.

Read our standard health & safety guidance

The water needs to be coloured, otherwise it is very difficult to see where the water column ends.

Procedure

1. Find a tall (10 m or more) teaching block.
2. Fix a suitable length (12 m) of clear plastic tubing to a vertical wall. Mark the tube with coloured tapes at 0.5 m intervals.
3. Dip the lower end of the tube into a bucket of water. Connect the upper end to the vacuum pump (preferably with the gas-ballast valve open), with a trap.
4. Pump the air out gently.
5. An alternative procedure is to fill the polythene tube with water and seal its ends. One end of the tube is then hauled up to a height of over 10 m and secured while the lower end is fixed below the water surface in the bucket. The bottom end of the tube is then opened. The water level will settle at a height equal to the atmospheric pressure measured in metres of water.

Teaching Notes

• Introducing the idea of a water barometer:
• "Suppose we didn’t have air all around us but had mercury instead, how high up would that mercury atmosphere extend from the ground to the top of the atmosphere if it were to make the same pressure that we feel here? What depth of mercury would make the same pressure? 76 cm, the same height as the mercury barometer."
• "How high would a water atmosphere be? Mercury is 13.5 times as dense as water so the water atmosphere would have to be 13.5 x 76 cm, about 10 m."
• At this point, show the water barometer.
• Extending the idea to determine the height of the atmosphere:
• "Now what about an atmosphere of air, assuming that the air does not vary its density at greater altitudes and there was nothing more above it?"
• "Mercury has a density of 13 600 g /litre and air has a density of 1.2 g /litre. This means that mercury is about 11 300 times denser than air and so the height of an air atmosphere would be 76 x 11 300 cm or about 8 500 m."
• "However the real atmosphere gets thinner and thinner and so it will be greater than this. Mount Everest is more than 8 300 m high and the atmosphere is thin up there – that is why mountaineers need to carry oxygen."
• The water will evaporate/boil at the top of the water column because there is a vacuum above it. This water vapour will depress the level of water in the water barometer. That is why mercury is used in a barometer: its vapour pressure is low and little evaporates into the vacuum. (The water barometer is also very cumbersome due to its height.)

This experiment was safety-tested in July 2007