Class practical

This apparently trivial experiment can lead young students into interesting discussions.

Apparatus and Materials

For each student group

• Common salt
• Beaker of water (400 ml is convenient)
• Microscope (e.g. Junior type, x20), optional
• Evaporating basin
• Bunsen burner
• Tripod
• Pipe-clay triangle

Health & Safety and Technical Notes

Eye protection is required if the evaporation stage is done. (See CLEAPSS Laboratory Handbook for further information).

Read our standard health & safety guidance

Procedure

1. Students put some of the salt into the water and watch it dissolve. They then add more and more of the salt and watch it dissolving.
2. A faster group might do this experiment under a microscope. Some crystals of salt are put on a microscope slide and observed through the microscope. Then a few drops of water are added and the crystals are observed dissolving.

Teaching Notes

• Have students already seen the crystalline structure of common salt? If so, watching the salt dissolve should raise questions of what has happened to the crystals. If such questions do not come from the students, ask them what they think is happening to make the crystals appear to vanish.
• Analogies may be drawn with the washing away or removal of polystyrene spheres from a pyramid model.
• If it helps the discussion, put some water from one of the beakers in an evaporating dish. Heat it to drive off the water, revealing the salt that had been dissolved.

This experiment was safety-tested in June 2004

Class practical

This is a thought-provoking experiment with a surprising result.

Apparatus and Materials

• Volumetric flask, one-litre
• Common salt

Health & Safety and Technical Notes

Read our standard health & safety guidance

When sodium chloride dissolves in water to make a saturated solution there is a 2.5 per cent reduction in volume. One would never notice that in a beaker. Even in an ordinary flask it would be barely perceptible. However, if a volumetric flask is available from the chemistry laboratory, the volume change will be noticeable in the narrow neck. It is essential to remove all air bubbles from the salt that is to be dissolved. Therefore, it must be thoroughly wetted at the start of the demonstration.

The solubility of salt does not change much with temperature, so there is little profit in using hot water.

The salt should be in small crystals and not in rocks or very fine powder.

Procedure

1. Place 300 to 400 g of salt in the flask.
2. Pour in enough water to cover the dry salt, and swirl the water around in the flask to wet the salt and let air bubbles float up to the top. (This will not be enough water to dissolve more than a little of the salt; students will still see a lot of salt crystals.)
3. As soon as the air bubbles seem to have gone, fill the flask to the mark with water.
4. Label the water level clearly, with an OHP pen or some other marker. Point out that most of the salt is still there, as a solid unable to dissolve.
5. Shake the flask to hurry the dissolving until as much salt as will dissolve has done so.
6. Point out the consequent small contraction. Ask students why they think this has happened.

Teaching Notes

This gives students a lesson in the need always to be on the lookout for unexpected results. It also provides students with an opportunity to use their imagination to think of possible explanations. Both of these are much more important than knowing the real reason for the effect: that the sodium and chlorine atoms in their crystalline array take up more room than when they are separated. Indeed, there is no harm in not giving the answer now but, instead, undertaking to return to the problem later.

This experiment was safety-tested in June 2004

Class practical

This shows ready-made models, giving an opportunity to link the shapes of real crystals with an underlying structure.

Apparatus and Materials

• Adhesive
• Polystyrene spheres

Health & Safety and Technical Notes

If it is necessary to use solvents to prepare an adhesive, remember that some students may try to acquire them for sniffing!

Read our standard health & safety guidance

Prepare the models some time before the lesson by joining spheres together with a suitable adhesive (e.g. Copydex or a PVA adhesive). If conventional adhesives do not work, dissolve some sheet-foamed polystyrene (or one of the spheres) in propanone or, better, in amyl acetate. The adhesive should be very thick in consistency. Apply it with a wood splint to the spheres at the points of contact. Then press them firmly together. It will take 24 hours for the glue to harden.

You may prefer to use cocktail sticks or double-ended screws but gluing is easier.

Simple cubic lattice: Make four layers and place them one on top of the other. The layers can be fixed permanently together, left separate, or tied together with cotton.

Body-centred cubic lattice: This can be built up as illustrated. Put a second layer of nine spheres on top of the first layer of sixteen as shown. Put a layer of sixteen (already stuck together) similar to the first, on top of this layer. Then repeat the process.

Alternately, build a pyramid. Starting with a 4-sphere x 4-sphere base, then 9 in the second layer, 4 in the third, and 1 in the last.

Or make a large number of 4 x 4 layers and demonstrate both simple cubic and body-centred cubic by arranging the layers appropriately.

Hexagonal close-packed lattice: Build a close-packed, single-layer structure, as shown, with each sphere touching six others. Place a second layer, similarly constructed, on top of the first. Displace it slightly so that the spheres of the second layer fit into depressions in the first. Further layers can then be placed on top.

Procedure

The crystal models can be added to other exhibits of crystals.

Teaching Notes

• Although the different packings should be shown, it would probably be wise not to attempt to enlarge on detail and certainly not to give the long names for the different crystalline structures, except with advanced students.
• You might merely point out that these models appear to agree with the shapes and behaviour of crystals.

This experiment was safety-tested in June 2004

Class practical

Students pile marbles into a cardboard tray and so find a convenient way of packing them together.

Apparatus and Materials

For each student group

• Marbles, 1.5cm diameter, 55
• Card, thin, approx 13cm square

Health & Safety and Technical Notes

A lively class will need careful instruction to avoid spilling marbles all over the floor.

Read our standard health & safety guidance

Improvise the trays on which the marbles are to be stacked from the sheets of card. These are marked with lines about 2 cm in from the edges as shown in the diagram. Most important is that when the edges are folded up along these lines, a row of 5 of your marbles can snugly but comfortably fit between them. Make cuts as shown, and fold and staple the edges to form a square tray.

The base of the tray must hold a layer of 25 marbles. Marbles tend to vary a bit in size.

Procedure

Students pour in marbles to form a layer of 25 marbles. On top of this they add layers of 16, 9, 4 and 1 marbles to form a pyramid.

Teaching Notes

• Students should notice the shape of the model crystal they are building and the angles between its faces. (They may need reminding that their model is of a tiny number of particles/atoms compared with that making up a real crystal.)
• You could ask students if they can identify real crystals with the same shape or angles between faces as their model. Alum would be a good example.
• You can draw students' attention to the fact that even if a marble is (carefully!) removed, the shape of the crystal remains.

This experiment was safety-tested in September 2004

Demonstration

This model shows crystal growing by the addition of more polystyrene spheres.

Apparatus and Materials

• Foamed polystyrene spheres (3.8cm diameter), 55 approx
• Wooden base with ridges
• Large alum crystal, if possible

Health & Safety and Technical Notes

Read our standard health & safety guidance

The tray needs to be made as in the diagram.

The outer walls must be able to contain rows of 5 spheres.

The inner walls must not only be able to contain rows of three spheres, but also be thin enough to allow the outer layer of spheres in the final model to touch the spheres inside.

Procedure

1. Form a square of 9 balls inside the inner walls. Then put 4 balls on this base and 1 on top to form a pyramid.
2. Now let the crystal grow by the addition of more balls, one at a time. Add balls carefully until there is a pyramid with a 4 x 4 base.
3. Then add more balls to make a 5 x 5 base.

Teaching Notes

• At each stage, get the students to observe the shape and to look at the angles between the faces.
• You might ask students if they can identify any real crystals with the same shape or angles between faces as their model. A large alum crystal could be shown, if available.
• You could also use this model to draw students' attention to the possible planes that should be used to cut a slice through the model crystal, in readiness for cleaving a real crystal.

This experiment was safety-tested in September 2004

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

Apparatus and Materials

• Students

Health & Safety and Technical Notes

Some laboratories may not have sufficient space to do this safely but a school hall could be used.

Read our standard health & safety guidance

Procedure

1. The students sit or stand in a regular array with arms stretched out to hold the shoulders of neighbours.
2. They should start at absolute zero and they should be told to warm up by vibrating to and fro.
3. If the model is allowed to go to higher and higher temperatures, the solid will eventually melt as the crystal comes to pieces.
4. If a model of a liquid is needed, students should stand close together with arms folded, moving about as a fluid crowd.

Teaching Notes

• There are occasions when the mood of a class makes this activity worth trying. If it can be done, it is useful because it is unforgettable!
• The full story of heating a solid is more complicated than the simple one of just making the amplitude grow as you warm up the model atoms, because there is a quantum restriction on the way in which energy is transferred. The energy of atoms is quantized. The restriction does not make itself felt in ordinary measurements, such as thermal capacity, at room temperature. However, at low temperatures the currency restrictions of quantum rules make themselves felt and specific heats drop to unexpectedly low values. This behaviour could not be explained on classical theory and itself helped to point to the quantum restrictions.

This experiment was safety-tested in August 2007

Demonstration

This compares pouring marbles, peas, sand and water from one container to another.

Apparatus and Materials

• Perspex containers, 5 or 6, or hard plastic beakers
• Marbles, dried peas, sand and water

Health & Safety and Technical Notes

Read our standard health & safety guidance

The containers should be large enough to hold marbles or dried peas. They need to be transparent but not glass as the marbles may break them.

One container should be empty, the others three-quarters full.

Procedure

1. Pour the marbles into the empty container so that the individual impacts can be clearly heard.
2. Repeat with the dried peas.
3. Then try the sand. (Although this is obviously discrete it sounds much more like a continuous fluid.)
4. Finally pour the water.

Teaching Notes

• Ask, 'How big are atoms?' Lead students to understand that if atoms were so big they could be easily seen you would certainly notice them and you might even hear them. 'Suppose atoms were as big as these marbles. Listen as they are poured into an empty container.'
• This experiment gives further evidence for atoms being very small.
• If alumina powder is available, it is fun to put some in a small milk jug. If some is poured into a tea cup, it can look quite like milk.

This experiment was safety-tested in June 2004