Demonstration

An iron rod expands when it is heated.

Apparatus and Materials

• Iron rod, approximately 50 cm long, 5 mm in diameter
• Mass, ¹/ ₂ kg
• Mass, 1 kg
• Wooden blocks, 2
• Needle
• Drinking straw
• Bunsen burner
• Microscope slide
• Heat-proof mats to protect the bench
• Bimetallic strip (e.g. copper & iron), optional

Health & Safety and Technical Notes

This could be done as a class experiment if all members of the class can be trusted to place the burner flame on the rod, and not on other parts of the apparatus or each other.

Read our standard health & safety guidance

If the rod slips on the needle, hang a load on it near the needle. If the straw slips on the needle, run a short piece of fine copper wire through the eye of the needle, and wrap it round the straw.

Procedure

1. Rest the iron rod on the two supports as shown in the diagram. Prevent the rod from moving at one end by placing the kilogram mass on it. Let the other end roll on the needle resting on a small piece of glass (e.g. a microscope slide) placed on the block. Pierce the straw with the needle so that it acts as a pointer.
2. Heat the rod by moving a Bunsen flame along it. The straw will move with the rolling needle as the rod expands. The movement of the rod cannot be seen, but the magnification of this movement by the straw is impressive.

Teaching Notes

• Students can estimate, roughly, how much the rod has expanded by measuring the circumference of the needle. Note that the rod expands by twice the circumference of the needle when the straw rotates once, because the point of contact between the needle and the rod also moves across the glass. Do not complicate the explanation with students at this stage. An order of magnitude estimate is all that is needed.
• You can make an estimate of the rod's temperature by dropping a drop of water onto it. The ferocity of boiling gives some idea of how much higher than 100°C it is.
• Different metals expand by different amounts for the same temperature rise. You could show a bimetallic strip being heated. These are often made of copper and iron. Copper expands by about 1.5 times as much as iron and so the bimetallic strip must bend. Bimetallic strips are used in some heat-operated switches such as in an electric iron.

This experiment was safety-tested in March 2006

Demonstration

This model shows atoms making up a crystal held together by spring-like forces.

Apparatus and Materials

• Atom model

Health & Safety and Technical Notes

Read our standard health & safety guidance

The model recommended is large and wobbly. The more rigid types often used in chemistry departments are not suitable.

Apparatus manufacturers may stock the version needed. The most useful versions are those where the springs are weak enough for the model to wobble like a jelly. However, it must be able to stand up on its own.

Alternately, a model may be made from foamed polystyrene spheres (or even the more massive golf balls) and weak steel springs. The springs have loops on each end and are intended for stretching experiments. Each one should be pre-stretched a little so the coils are separated. Slots are made in the spheres (a very hot domestic table knife blade works well) and the loops on the springs glued into slots.

A cubical array of 27 spheres will do.

Procedure

1. The model is used as a prop to support a discussion of the way in which the particles in a solid are held together by spring-like forces.
2. Show the model to the class and demonstrate how vibrating it will shake the individual atoms, but that they can retain their place in the overall pattern.

Teaching Notes

• For this purpose, this is a better model of a small crystal than one made of spheres glued together. It helps students to think about the part played by the forces holding atoms together.
• The model solid is 'heated' by shaking it. This represents energy transferred to the solid so that it is stored thermally. When the model solid is 'heated', the individual balls start to move. This leads to the idea that, in a solid at a given temperature the atoms may be vibrating. Energy stored thermally is due to these vibrations.
• Heat the solid more and the vibrations become more violent. There are two consequences.
• First, the structure becomes bigger/takes up more space. This helps students to understand thermal expansion.
• Second, if enough energy is transferred, the vibrations become so violent that the structure breaks up. Melting is thus explained. Or is this really a picture of sublimation (or change directly from solid to gaseous state)?
• If the outer layer of atoms are vibrating violently the vibration will be communicated to the next layer of atoms. This shows how energy might be conducted through a solid material. The vibrations themselves increase in amplitude when more energy is transferred to the model, but the frequency remains the same. You can imagine the atoms in a solid elbowing each other a little further apart as they vibrate more and more.
• At advanced level you would go into details of the balance between the short-range attractive forces between atoms and the very short-range repulsive forces between the same atoms. These forces must maintain the system in equilibrium, changing their values when atomic vibrations increase because those vibrations carry individual atoms to different distances where they experience different forces. As a result of those changes of forces, the whole array takes up a different length and strength, again in equilibrium. This is too complicated a story to tell at an introductory level.
• All scientific models have limits and they do not behave like the real thing. It is important to stress the limits of the models in use.

This experiment was safety-tested in March 2006

Class practical

Comparing the resistances to cracking of soda glass and Pyrex (borosilicate) glass when heated.

Apparatus and Materials

• Piece of window glass (approximately 100 mm square)
• Tripod
• Clear silica tubes 125 mm (if available), 2
• Safety screens, 2
For each group of students
• Soda glass tube or length of tubing 125 mm
• Pyrex (borosilicate) glass tube or length of tubing 125 mm
• Bunsen burner
• Beaker

Health & Safety and Technical Notes

Whenever a demonstration is performed with glass which might shatter and hurt someone, you must use a pair of large safety screens of Perspex. The screen between the teacher and the apparatus should be approximately 1 m high by 60 cm wide. This is narrow enough so that you can reach round and manipulate the apparatus and high enough to shield the face. The screen between the apparatus and the class should be approximately 1 m square.

These screens (of 3 mm Perspex or thicker) should not be framed. That would spoil the feeling of full transparency. They can be supported by pairs of slotted bases.

The fractured glass tube must be disposed of safely.

Used engine oil contains carcinogens and must NOT be brought into a school laboratory. New and unused oil is safe to use.

Read our standard health & safety guidance

Procedure

1. Place the piece of window glass on a tripod. Heat it near one corner with a bare flame. When the pane cracks, ask students why this happens. Do not give an explanation yet.
2. Give each group of students a soda glass tube. Ask them to heat it in a Bunsen flame. Then plunge it into water in the beaker. Tell them to repeat the experiment with the Pyrex tubing.
3. Repeat the experiment again, as a demonstration, with the silica tube if it is available.

Teaching Notes

• The window glass cracks because it is not a good thermal conductor. The portion in the flame expands so that the strain between that portion and the cool, unheated part fractures the glass.
• The difference between the Pyrex glass and the soda glass is probably due to the lower expansion of the Pyrex glass. The strain it experiences is lower than soda glass for the same temperature change.

• Students may suggest that the difference in cracking between soda glass and Pyrex is that Pyrex is able to stand a greater strain. You can demonstrate that this is not the case. Put two lengths (1 m or 1.5 m soda glass and Pyrex of equal size) between two stools. Depress the centre of each one in turn with a half-metre rule. Note the position when each breaks, which should be very similar. The greater the depression, the greater the strain. Normally the tubes break with the same amount of bending; the same strain.
• The silica tubing withstands high temperatures very well and does not crack. This is why Nichrome wire is threaded through silica glass tubing to make the element in a bar fire.

This experiment was safety-tested in March 2006

Demonstration

Two different demonstrations on how to crack glass through thermal conduction. See also this experiment:

Cracking glass

Apparatus and Materials

• Glass bottle
• Glass cutter (e.g. wheel or diamond)
• Unused car-engine oil
• Large glass pipe
• Non-electric soldering iron or poker
• Nichrome wire

Health & Safety and Technical Notes

Whenever a demonstration is performed with glass which might shatter and hurt someone, you must use a pair of large safety screens of Perspex. The screen between the teacher and the apparatus should be approximately 1 m high by 60 cm wide. This is narrow enough so that you can reach round and manipulate the apparatus and high enough to shield the face. The screen between the apparatus and the class should be approximately 1 m square.

These screens (of 3 mm Perspex or thicker) should not be framed. That would spoil the feeling of full transparency. They can be supported by pairs of slotted bases.

The fractured glass tube must be disposed of safely.

Used engine oil contains carcinogens and must NOT be brought into a school laboratory. New and unused oil is safe to use.

Read our standard health & safety guidance

Procedure

Demonstration 1 An entertaining (as well as useful) technique is to half fill a bottle with unused car engine oil.

1. Heat a large non-electric soldering iron or poker in a Bunsen flame.
2. Plunge it into the oil.
3. The bottle will crack cleanly at the oil level. This is because the glass above the oil level is not being heated. Therefore it is not expanding as is the glass below the oil level.
4. Turning the bottle into a 'beaker and a funnel' is entertaining.

Demonstration 2 A further demonstration is to use a Nichrome wire heated electrically to crack a glass pipe.

1. Start the crack in your glass pipe with a wheel or diamond.
2. Wrap the wire over the crack and heat it to 'red heat'. The glass pipe will break cleanly at the wire.
3. This is useful technique for cutting glass tubes.

Teaching Notes

This experiment was safety-tested in March 2006

Class practical

Observing the expansion of water, and comparing the expansion of soda glass and Pyrex.

Apparatus and Materials

For each group of students

• Thermometer -10°C to 110°C
• Tripod
• Aluminium container or water bath
• Bunsen burner
• Soda glass test-tube, 75 mm x 12 mm or similar
• Pyrex glass test-tube, 75 mm x 12 mm or similar
• Bungs with capillary tubing to fit test-tubes
• Test-tube holder
• Bottle of ink (washable)
• Heatproof gloves

Health & Safety and Technical Notes

The tripod and water bath will take time to cool sufficiently before clearing away. If time is short, heat-proof gloves or suitable clothes should be provided.

Read our standard health & safety guidance

The soda and pyrex glass test-tubes should be similar sizes for this experiment to work.

Procedure

1. Heat water in the water bath until the temperature is between 60°C and 80°C. Dip the thermometer into the hot water and watch carefully.
2. Fill a soda glass tube with coloured water (a little ink is suitable for this). Insert the bung so that no air bubbles remain in the tube, and the coloured liquid extends to about 20 to 50 mm above the top of the bung.
3. Grip the test tube in the holder and plunge the tube into hot water. Observe the behaviour of the water level in the tube.
4. Repeat the experiment using a Pyrex test-tube.

Teaching Notes

• Students may observe the initial fall in the thermometer liquid. This fall is due to the glass expanding before the water in the tube. It takes time for energy to be conducted through the glass to the water in the tube.
• When the Pyrex tube is used, the initial dip of the water level is very much less. This is because the expansion of Pyrex is less than soda glass.
• A change in the volume of a liquid or solid indicates a clear change in the average spacing between its molecules.
• Students should discuss the similarities between the thermometer and the test-tube plus capillary tube.

This experiment was safety-tested in March 2006

Demonstration

The expansion of water when it freezes breaks an iron bottle.

Apparatus and Materials

• Iron flask for freezing
• Freezing mixture of ice and salt (or solid carbon dioxide)
• Bucket
• Cloth, piece of
• Tongs or heat-proof gloves (if using solid carbon dioxide)
• Water

Health & Safety and Technical Notes

While the flask is in the freezing mixture, cover the bucket with a cloth. This will ensure that no debris is ejected.

If solid carbon dioxide is used, it must be handled with tongs or heat-proof gloves to prevent injury to skin.

Read our standard health & safety guidance

To avoid a long wait and uncertain timing, cool the flask beforehand.

Before the demonstration, boil the water that will be used in the flask to make it air-free.

Procedure

1. Fill the iron flask with water.
2. Remove all air bubbles and screw the plug in.
3. Bury the bottle in the freezing mixture and cover it with a cloth. (If solid carbon dioxide is available, the iron flask can be embedded in that.)

Teaching Notes

• The breaking of the flask will be clearly heard. Students are not surprised when glass breaks, but the strong iron flask is another matter.
• When water freezes it swells. The volume of the ice is about 10% greater than the volume of the water. Water is unusual in expanding when freezing, but it is not the only substance which does so. Iron also expands on freezing and this is why it can be cast.
• As the water becomes solid, the molecules take up fixed positions in relation to each other. Average spacings between molecules are larger than when the water is in a liquid state.

This experiment was safety-tested in May 2006

Demonstration

Measuring the volume of vapour produced when a small drop of hydrocarbon solvent is vaporized.

Apparatus and Materials

• Measuring cylinder, Pyrex, 100 cm³ capacity
• Bung to fit cylinder carrying two short glass tubes
• Tall-form beaker, 2000 cm³
• Plastic tubing
• Hypodermic syringe
• Petroleum ether, 60-80°C boiling fraction
• Rubber cap
• Beaker, 400cm³
• Electric kettle to provide hot water

Health & Safety and Technical Notes

Petrol as used in cars may NOT be brought into a school laboratory because it contains benzene. Petroleum ether can be regarded as petrol without the hazardous carcinogen. It is still highly flammable and must not be used in the same room as naked flames.

Read our standard health & safety guidance

Here is a way to fill the tubes with water. Fill the measuring cylinder and the two rubber tubes on either side of the stopper with hot water (at a comfortable temperature to work with) before inserting the bung loosely into the measuring cylinder. The water will siphon over until there is about a depth of 2 cm in the small beaker. The measuring cylinder is topped up with water and the bung is secured. The measuring cylinder is then put into the large beaker and boiling water is poured around it.

Procedure

1. Fill the measuring cylinder and the flexible tube leading through the bung with water, as if a siphon were being made. The second glass tube through the bung is closed with a rubber cap on the inside of the bung.
2. Immerse the whole cylinder in hot water (at about 90°C) in the tall-form beaker.
3. Inject about 0.1 cm³ of "petrol" into the cylinder through the rubber cap tube. It will evaporate and occupy some 80 cm³.
4. As the water cools, the vapour will condense again and the expelled water will return to the cylinder.

Teaching Notes

The volume expansion when "petrol" turns into "petrol vapour" is an increase of about 800 times. When water turns into water vapour it is a volume change of about 1600 times.

This experiment was safety-tested in March 2006