Demonstration

An exhibition of a wide variety of natural and synthetic materials for students to look at and handle: if possible over several weeks. It makes an ideal start to a course in physics at whatever age that takes place.

Apparatus and Materials

• Metal blocks such as iron, aluminium, brass, lead
• Examples of soft wood and hard wood, e.g. Formica and plywood
• Paraffin wax
• Foamed polystyrene, Perspex, or other polymers
• Glass of any type
• Building stone such as slate and marble
• Rubber, latex foam bloak, steel spring, bare copper wire
• Alum, sodium thiosulfate (hypo)
• Common salt, washing soda, silk, cotton, wool, curry powder, soap, vinegar, olive oil, gelatine, wheat, flour, greaseproof paper, paper
• Copper sulfate, calcite, cast bismuth
• Lead shot, Bakelite, granite, basalt, limestone, sandstone, schist, mica, flowers of sulfur, glass camphor, Milton (chlorine bleach), pitch, chalk, concrete, brick, ceramic, plaster of Paris, tungsten carbide tipped drill
• Glass bottles containing air and one from which some air has been evacuated (labelled AIR and VACUUM)
• Balloon full of air, one with hydrogen (or, if that is not available, natural gas), one with carbon dioxide
• Offering hand lenses might be useful.

Health & Safety and Technical Notes

Hands must be washed after handling lead. The general warning not to taste anything and to smell with care should be repeated.

Read our standard health & safety guidance

Science apparatus manufacturers including Philip Harris and Griffin Education sell elastic materials investigation kits and solid materials kits.

The bigger the size of samples, the more convenient is it for handling and viewing. However, if the samples can be of a similar size, relative densities will be more obvious. A suitable size would be 3 cm x 4 cm x 5 cm. Light plastic containers of the same volume (and shape) would be useful for holding liquids.

As far as possible, the substances exhibited should be common ones, of a domestic nature rather than special chemicals outside the students' experience.

The sense of smell is important which is why the list includes some items with strong smells,

Balloons should be filled immediately before display. Hydrogen, in particular, will diffuse quite quickly through the walls of the balloon. It is helpful to stretch a balloon before trying to inflate it. This softens the walls.

Procedure

1. Ask students to examine all the exhibits - without spending too long with each one. It will give their examination some focus if they are required to compile lists, say, hard or soft, or solid, liquid or gas.
2. If students do not seem to be thinking about why the materials differ ask a few leading questions such as "Why might solid iron be different from aluminium?" Or you could develop a game of 20 questions to identify a material.

Teaching Notes

• The aim of the exhibition is that students closely examine different materials and begin to wonder why there are these differences. They may also:
• Get a feeling for density;
• Wonder why materials have different properties.
• Draw students' attention to the basic properties of crystals - flat faces with the angles between these faces common to particular materials.
• If an inlet tube to the evacuated bottle can be opened under some coloured water, seeing the water rush into the bottle makes an unforgettable impression.

This experiment was safety-tested in January 2005

Class practical

An opportunity to look more extensively at the nature of crystals. Students are often fascinated by crystals especially if they can handle large ones.

Apparatus and Materials

• Alum crystal, large
• Calcite crystals
• Sodium thiosulfate crystals
• Caster sugar
• Sugar crystals, type used for coffeeHand lenses

Health & Safety and Technical Notes

Hands should be washed after handling crystals.

Read our standard health & safety guidance

Procedure

Ask students to handle the large crystals for themselves and closely examine the smaller ones.

Teaching Notes

• Students should learn that crystals are identified by flat faces with angles between them. The angles are characteristic of that material, regardless of their size. They could be encouraged to look for crystals at home.
• You may need to take precautions to avoid theft and breakage of the large crystals.

This experiment was safety-tested in May 2004

Class practical

As the crystal grows, its characteristic shape is seen.

Apparatus and Materials

• Sodium thiosulfate crystals
• Test-tube (e.g. 100 by 12 mm)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Some hours before the lesson, preferably the day before, put sodium thiosulfate crystals into each test-tube to a depth of about 5 cm. Melt the crystals by gentle warming. They will melt in their own water of crystallisation. However, if they do not do that easily, add a drop of water to the stock of crystals so they are damp.

Allow the tubes of melted sodium thiosulfate to cool down to room temperature. If the tubes are left a long time after cooling, the sodium thiosulfate may recrystallise in some of them. That is one reason for some spares to be prepared.

Recrystallisation may be started by dust, so it is advisable to cover the mouths of the tubes until they are needed. It is also advisable to avoid jarring the tubes. This may also start recrystallisation.

Procedure

Give each student a test-tube with the cool liquid sodium thiosulfate in it. They are to drop one crystal of sodium thiosulfate into the liquid and watch very carefully what happens.

Teaching Notes

• If students work in pairs, it is useful to get one to do the experiment whilst both watch. Then everyone gets a second look at what is a fascinating sight.
• This experiment will reinforce the idea that crystals are identified by flat faces with angles between them characteristic of that material, regardless of their size.
• As the crystals form, heat is given off which will make the test-tubes get warm (allow it to touch a cheek). It is probably best not to tell students to look out for this. Students who comment on it might be told to make a note for later. Considering where this energy comes from - particles losing their kinetic energy as they are held in the crystal structure - could better be discussed later.

This experiment was safety-tested in May 2004

Class practical

Apparatus and Materials

• Hand lenses
• Microsope, with illuminants and power supply
• Things to look at

Health & Safety and Technical Notes

Microscopes should never be used in a manner which could result in the Sun being imaged; if daylight is used as illumination, ensure the window faces north or away from the Sun.

Read our standard health & safety guidance

There should be one hand lens per student and, if possible, a microscope for every four students. Long queues will spoil students' enjoyment of this activity.

Things to look at might include some of the following: sand, salt, blotting-paper, talcum powder, a hair, students' own handwriting, a fingerprint, rocks such as granite, salt dissolving. Prepared slides of blood and a fibrous mineral could also be included if they are available.

Procedure

1. Encourage students to look at anything they want to.
2. Get students to begin with their magnifying glasses. It may help to get them to bring the hand lens right up to their eye with one hand. Then they use the other hand to bring the object to be viewed towards the eye. The hand lens could also be supported in a clamp stand.
3. Produce the microscopes later. Teach the students to start by looking at the objective lens, from the side, as it is lowered to be near the object to be viewed. Then they look through the eyepiece as the microscope tube is raised, until a clear image is seen. (Students should never lower the microscope as they are bringing the image into focus. The objective lens may crash through the slide!)

Teaching Notes

• A chief aim of this exercise is for students to become familiar with microscopes so as to be able to use them later just as a tool, rather than a distracting novelty.
• It may also enable students to compare the benefits and disadvantages of using microscopes. They will then be able to decide in other contexts between an adequate magnifying glass and a time-consuming and cumbersome microscope.

This experiment was safety-tested in January 2005

Class practical

This is a fascinating experiment. The sight of the edges of the crystal moving across the slide will never be forgotten.

Apparatus and Materials

• Microscope, with illuminants and power supply
• Microsope slide
• Beaker, 400ml
• Common salt, saturated solution
• Bunsen burner
• Salol
• Sodium thoisulfate

Health & Safety and Technical Notes

Students should be shown how to gently and safely heat the microscope slide over the Bunsen flame. They need to know that glass does conduct, even if poorly.

Also, that if a cool slide is heated too fiercely it may crack and shatter.

Do not use the sub-stage mirror to reflect the Sun through the microscope.

Read our standard health & safety guidance

If possible have a microscope per four students and a microscope slide per pupil.

There should be one Bunsen burner and beaker for every microscope.

Prepare an almost saturated solution of common salt the day before. Decant this into the beakers.

Salol is often easier to use than common salt to achieve good crystals. Sodium thiosulfate also provides good crystals to view.

Procedure

1. Adjust the microscope so that it is focused on a plane just above the slide. It may be helpful to focus on some mark on a piece of paper placed on top of the slide.
2. Warm the slide over a low Bunsen flame, place a drop of the salt solution on the slide and then immediately put the slide under the objective - without moving the objective. The slide must be warm enough to evaporate the liquid rapidly, but not so hot as to boil the liquid.

Teaching Notes

Students should see the straight edges and angles typical of common salt crystals. They should also realize that the solid grows because of what comes out of the liquid.

This experiment was safety-tested in January 2005

Class practical

Looking for crystalline structures in each specimen.

Apparatus and Materials

• Hand lenses
• Microscope, with illuminants and power supply
• Granit specimens, with unweathered surfaces
• Cast bismuth specimens
• Granulated sugar
• Demerara sugar
• Caster sugar
• Icing sugar

Health & Safety and Technical Notes

Do not use the sub-stage mirror on a microscope to reflect the Sun through the instrument.

Read our standard health & safety guidance

It might be worth looking also at flour or talc to see something which is not visibly crystalline.

Broken slabs of cast zinc can also be tried, but they are not as effective as bismuth.

Procedure

1. Ask the students to look for crystalline structures in each specimen: flat faces with angles between them characteristic of that material, regardless of their size. Ask students also to note similarities and differences between the crystals they see.
2. All specimens should be examined first with the naked eye, then with a hand lens and finally, in some cases, with a microscope.
3. Point out that strong illumination at a low angle to the surface is helpful.

Teaching Notes

• Sugars: Demerara sugar is seen to be crystalline even to the unaided eye. Granulated sugar is clearly seen to be crystalline when viewed with the hand lens, the caster sugar with the microscope. It will probably not be possible to see the crystalline nature of icing sugar even with a microscope, though this will depend on both the grade and the microscope.
• Students should learn that the basic shapes of sugar crystals appear regardless of the size of the crystal. They should realize that crystals do not just appear separately but sometimes as a fused mass and sometimes with others.
• Other specimens: Specimens of granite may be examined with hand lenses. Three main constituent materials will be observed: clear, glass-like quartz, pink or white, opaque feldspar, and shiny, flaky mica. Traces of other minerals can often be distinguished. It may be possible to observe the perfect cleavage lines of the mica crystals, less perfect on the feldspar and none on the quartz.
• Cast bismuth can be examined either with the naked eye or with a hand lens. The crystal structure is clearly visible.
• This is a chance to enjoy using the microscope and wall displays of crystals. Exotic ones like diamond and snowflakes, and common ones such as table salt, are helpful for discussion.

This experiment was safety-tested in May 2004

Class practical

Here students have the satisfaction of growing their own crystals. This is a wonderful experience.

Apparatus and Materials

• Jars, small, e.g. jam jars, one per student
• Alum or copper sulfate
• Plastic bucket with lid

Health & Safety and Technical Notes

Strong solutions of copper sulfate are HARMFUL.

Read our standard health & safety guidance

Label the jars carefully: otherwise the best crystal always appears to belong to everyone!

The saturated solution

• Several days before, a solution of the salt which is saturated at room temperature must be prepared. This is best done by allowing a supersaturated solution to deposit its excess solid as explained below.
• To prepare a working solution, dissolve the salt in warm water (about 50°C) at a rate of 40 g per 100 ml for potassium alum. Seven litres is sufficient for a class of 32 using jam jars. Pour this solution into the bucket, close the lid, and allow to cool to room temperature. This solution is now supersaturated.
• Seed this solution with a pinch of tiny crystals and leave it in the closed vessel for two to three days, shaking occasionally, to become saturated at room temperature. Pour off the clear saturated solution into another vessel which is closed with a lid.

Seed crystals

• One method of producing the seed crystals is to dip a length of thread into the saturated solution and then to hang it up to dry. Small crystals will appear on it. The whole thread is then hung in the solution. After one or two days, examine the crystals that have developed and break off all but the best. These will then form the seeds for the next stage.
• Another method is to place about 50 ml of the prepared solution in an open beaker and allow it to evaporate overnight. Sort and dry the resulting seeds, retaining those which are perfect in shape and about 3 mm long.
• If you have many students growing crystals, you may need to collect the jam jars from students. Be warned - jars must be very clean as crystals do not grow well in a solution of raspberry jam!
• You may have storage problems. One solution is to use tapered glass tumblers that can be stacked. Wherever they are stored, remember that their temperature must remain even.

Procedure

First method

1. Suspend the thread with its perfect seed crystal from, say, a pencil laid across the top of the jar.
2. Fill the jar with the saturated solution until the seed is completely covered to a depth of at least two centimetres.
3. Cover the jar with a piece of thin cotton cloth (for example muslin or cheesecloth) which is held in place with an elastic band.

Second method

1. Tie the seed to a short length of cotton or thread. This is not an easy process. Students might be advised to prepare a slip knot and insert the crystal, rather than trying to tie a reef or granny knot around the seed.
2. Suspend the thread with its perfect seed crystal from, say, a pencil laid across the top of the jar.
3. Fill the jar with the saturated solution until the seed is completely covered to a depth of at least two centimetres.
4. Cover the jar with a piece of thin cotton cloth (for example muslin or cheesecloth) which is held in place with an elastic band.

In either case, the jar is then left undisturbed and at an even temperature for several days.

Teaching Notes

• Getting a crystal to grow provides a great sense of achievement.
• Label the jars carefully - otherwise everyone claims the best crystals belong to them.
• The most common problem encountered is that the temperature at which the jars are stored does not remain constant, day and night. Increasing temperature increases the solubility and the precious crystals disappear.
• Colourful pictures of crystals can make a good classroom display. Do not forget snowflake crystals.
• This activity could also be done at home. If you choose this option, encourage students to grow crystals of such common substances as sugar, common salt, Epsom salts and washing soda. You could also provide them with plenty of alum to take home to try. Students would be well-advised to keep their jar in the refrigerator. This activity usually gets parents involved - this can be beneficial as long as they do not take over.

This experiment was safety-tested in May 2004

Class practical

This impressive demonstration shows how easily the crystal comes apart if the blade is accurately aimed between planes of atoms.

Apparatus and Materials

• Calcite crystal, large
• Single-edged razor blade or sharp knife
• Hammer, light

Health & Safety and Technical Notes

Razor blades must be handled with care and disposed of so that the sharp edge cannot harm anyone. The demonstrator should wear eye protection.

Private practice develops the skill needed to ensure success in front of students as well as helping to make the demonstration safer for the teacher!

Read our standard health & safety guidance

If obtainable, single-edge razor blades are particularly good because they usually fail to break the crystal if held at the wrong angle. A fine chisel will also work.

The less brute force that is used, the more impressive the demonstration.

Procedure

1. Although not essential, most teachers prefer to place the crystal on a bed of Blu-tac.
2. As shown in the diagram, the plane of the blade needs to be parallel to a face of the crystal.
3. Give the blade a light tap with the small hammer.
4. If a student asks why the right orientation of the blade is so important, show what happens if the orientation is wrong - no clean cleavage is obtained.

Teaching Notes

• If a ready-made polystyrene sphere model of a crystal is available, this could be used to show what is happening with the calcite.
• Seeing this demonstration should help students appreciate the skills of those who cleave gem diamonds!

This experiment was safety-tested in May 2004