Materials Science

Coursework and Investigations for KS3 and KS4

May 2007Contents

3Ammonia Diffusion

7Enthalpy of Crystalisation

9Viscous Goo Challenge

13Reactivity Series and Corrosion

15To determine a reactivity series for a range of metals using a voltmeter

17Passivity Demo / A level

19An Electrifying Experience

21Tensile Testing

33Hardness Testing

37Creep

43Ferrofluids

These experiments are intended as a framework to be adapted for the specific needs of a class.

All teachers should carry out their own safety assessment for all experiments.Ammonia Diffusion TC "Ammonia Diffusion" This Sc1 should follow a demonstration of the diffusion of ammonia and hydrogen chloride along a tube. The position and shape of the smoke ring so produced indicates

Different gases diffuse at different speeds. This links in to the rate of diffusion being related to molar mass.

The shape of the ring shows ammonia to be less dense than hydrogen chloride, since it slants in such a way as to indicate that ammonia floats over the top of the hydrogen chloride. Leaving the ring to develop illustrates this clearly.

Also, observing the ring closely shows small convection currents which can be stimulated by holding two fingers below the ring and waiting for heat to diffuse through the glass.

This gives pupils imagination to tackle the Sc1

Does Ammonia gas diffuse equally fast in all directions?

The question prompts a discussion on the density of gases brighter pupils can calculate that ammonia is less dense than air (approximates to nitrogen or oxygen nitrogen mix?) Others can look up data in a book, or simply be told.

Diffusion works well in boiling tubes, typically taking 5 to 15 minutes to cover the distance from the neck to base, depending on conditions.

Progress of the gas is measured by using thin, pink litmus sensors. (Long litmus strips give a diffuse boundary which makes taking measurements harder.)

Diffusion (boiling) tubes can be set up along Cartesian axes, and brighter pupils may want to further divide the angles. This should not be encouraged too much, especially initially.

There is scope for a reasonable, simple theory and prediction. Complicated or Simple plans to suit pupils abilities are soon ready allowing lots of results and graphs to be produced for examiners. As differences are random and not directional, there is plenty of scope for evaluations, explanations and suggested improvements.

Some considerations for key factors include, amounts of ammonia solution used, water inside the tube reabsorbing ammonia gas, holding the tubes and making one warmer than another, ammonia already in the air

In spite of the difficulties, this Sc1 provides a short simple assessment that even the most challenged can attempt and the brightest can score maximum marks in 2 3 weeks. It also teaches that experiments do not always work the way you expect, but for GCSE that does not matter providing it is realized that the experiment has not worked as expected ideally with an explanation.

Pupils will assume the ammonia advances down the tube with a flat diffusion front perpendicular to the direction of diffusion. This is clearly not so. When difficulties arise, a demo of dropping a few cm3 of milk into a 250 cm3 of still water illustrates the misconception and answers some of the more confusing anomalous results.The following represents a distillation from a few bright (A* grade) pupils - for your ideas.Does Ammonia diffuse equally in all directions?

Theory

Gases diffuse because their particles are moving very fast and are at very large distances apart compared to their actual size. As a result particles of one gas can move between and mingle with particles of another gas.

The particles spread out evenly throughout a container over time.

It is known that gases of low formula mass diffuse faster than gases of large molecular mass, as demonstrated by allowing ammonia

Ammonia, NH3 is a gas at room temperature with a molecular mass of 17 g mole-1.

Air is a mixture of gasses at room temperature. For the purpose of this investigation it is assumed that the composition of air is 80% N2 and 20% O2.

Therefore average molar mass of air=0.8 x MN + 0.2 x MO

Where MN and MO represent formula masses of nitrogen and oxygen respectively,

=0.8 x 28 + 0.2 x 32

=28.8 g mole-1.

Therefore the molar mass of ammonia is much less than the average molar mass of air.

The molar volume of any gas

=24.0 dm3 at room temp. and press. (25(C)

Therefore molar density of ammonia=17.0 g mole-1 / 24.0 dm3 mole-1.

=0.71 g dm-1.

Therefore molar density of air

=28.8 g mole-1 / 24.0 dm3 mole-1.

=1.2 g dm-1.

Because ammonia gas is less dense than the surrounding air, I predict that flotation will assist diffusion in the vertically upward direction, oppose diffusion in the vertically downward direction and have no noticeable effect on the horizontal directions.

That is I predict diffusion upward will be faster than diffusion downward and that horizontal diffusion will be an intermediate speed.

Method - Plan

Ammonia can be detected using damp, pink litmus paper that turns blue on contact with ammonia.

Ammonia can be controlled conveniently by using 0.1M ammonia solution on cotton wool. The gas escapes from the solution and diffuses through air to the litmus paper turning it blue. The time taken for the litmus to turn blue is recorded. By placing at least five papers at measured intervals in a boiling tube, the speed (or rate) of diffusion can be measured along the tube.

The ammonia is made to diffuse upwards, downwards and horizontally in separate experiments, each experiment being repeated to check measurements and to obtain an average value.

Key Factors to Control

The experiment was tested several times to determine the optimum conditions. Based on preliminary tests, the following key factors to be controlled were noted.

Temperature should be the same for each test, since diffusion is due to the kinetic motion of particles and the speed of the particles is related to temperature.

The amount of ammonia solution used (4 drops from a dropper), as more solution means more ammonia evaporating per second and hence a higher rate of diffusion. (Diffusion from a point is concentration or partial pressure dependant, hence the need to control the amount of ammonia.)

Parallax, hence the need to stick a scale as close to the ammonia as possible.

Need to use dry boiling tubes, as ammonia dissolves readily in water. If the ammonia dissolves in water drops inside the tube, the measured rate of diffusion will be affected. (This may be discounted providing the tubes are ALL equally damp at the start.

Use thin strips of litmus, as litmus paper changes through several shades of blue if diffusion is slow. Thinner strips make the change sharper.

Handling of the tube needs to be minimal, as heat from the hand can cause localised warming and hence change the rate of diffusion.

Method Procedure

A boiling tube was cleaned with a paper towel to make sure it was dry.

Thin pieces of damp, pink litmus were placed at approximately 1.5 to 2.0 cm intervals inside the boiling tube using a glass rod.

The tube was clamped in position and four drops of ammonia solution were placed on a piece of cotton wool that had been glued to a rubber stopper. The stopper was placed firmly into the boiling tube.

As soon as the first litmus turned blue, the stopwatch was started and the times taken for other papers to turn blue were recorded.

This method was repeated for diffusion in vertically up, vertically down and horizontal directions. Measurements for each direction were taken three times each and an average was calculated.

A graph of distance versus average time was plotted.

Results

NOTE: The diffusion of any gas is not so simple. Some pupils will achieve results that agree with their predictions by chance, most will not. A simple way of illustrating the point for pupils is to show a large tube of still water in a measuring cylinder. The cylinder represents the air in the boiling tube. By adding a few cm3 of milk, to represent the ammonia, the milk will be seen to swirl as it travels down the tube. Sometimes it will move down one side of the cylinder before returning up the opposite side. Pupils often get analogous results to this with the ammonia apparently diffusing the wrong way. This illustration of turbulent flow is enough for pupils to explain any anomalous results. The turbulence is random and can be influenced by many factors, such as holding the tube in one place, setting up convection currents, moisture inside the tube (including the moist litmus paper) absorbing ammonia from the air. This gives good opportunities for the student to suggest improvements and explain anomalous results. It also has the potential to develop into a discussion about the differences between a model of a situation and the reality, with the need to refine models to obtain more accurate predictions.

Enthalpy of Crystallisation TC "Enthalpy of Crystalisation" Thermodynamic terms can be difficult for sixth formers to understand, especially when they are not directly measurable. This experiment does not provide an accurate method for determining the enthalpy of crystallisation of sugar, but does offer an (edible) illustration of it.

Boiled sweets are essentially amorphous sugar and very soluble in water with an accompanying increase in temperature. Granulated sugar, being crystalline requires that enthalpy of crystallisation is added to the system to enable dissolution. Consequently, granulated sugar dissolves endothermically in water.

Method

Using a measuring cylinder, measure 100cm3 of water and leave it to reach an equilibrium temperature.

Crush about three Foxs Glacier mints in a mortar and pestle. Take care as large pieces take longer to dissolve and too much crushing may compact the mint into the base of the mortar.

Weigh an empty polystyrene cup.

Add the crushed mint to the cup.

Reweigh the polystyrene cup and obtain the mass of the mint used.

Take the temperature of the water using a 0 to 50(C thermometer (easily readable to 0.2(C).

Add the water to the mint, in the polystyrene cup, and stir vigorously to dissolve the mint. Take care not to let any solution splash out of the cup.

Record the highest temperature reached.

Repeat the experiment using granulated sugar.

For the purpose of approximation, assume that both materials are chemically identical and the mint is both anhydrous and completely amorphous.

Questions

1. Explain the observed difference in the change of temperature.

2. If left exposed to the atmosphere, the mint becomes sticky on the surface, but granulated sugar does not. Explain this observation.

3. Of the two forms of sucrose, which has the highest energy level?

4. Make an estimation of the difference in energy of the two forms of sugar.

5. You may make very precise measurements in this experiment, but it cannot be accurate. Explain the meaning of the terms accuracy and precision in this context.

6. Briefly discuss the accuracy of this experiment.

Assume specific heat capacity, s, of the solution is 4.3 J g-1 (C-1.

Thermal energy change = ms(T2-T1),

Where m = mass of solution and (T2-T1) = Temperature change of the system.

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Viscous Goo Challenge TC "Viscous Goo Challenge" \f C \l "1" Notes

This simple experiment is fun for everyone and is often used as entertainment in science clubs, Open Days or end of year activities. However in this format it is used as a science investigation by linking observation to numerical values.

This is not intended to be a thorough piece of investigative work, but is used to illustrate scientific principles in an entertaining way. There are several levels at which this can be approached depending on the ability of the class that will carry out the investigation.

Very young / low ability: This can be simply a bit of fun tied in with data collection

More able KS3 / KS4: The experiment can be used to reinforce ideas about

Solid, liquids and gases and their basic properties

Polymers, their properties and cross linking

Viscosity

Discussions about accuracy and precision of data and what level is appropriate

Top GCSE and A level

Hydrogen bonding

Curing (properties will change with time)

Creep (and an analogy to convection in the mantle of the Earth or metals)

and more besides.

PVA adhesive is non toxic, but is not edible.

Borax is poisonous if swallowed and may cause skin irritation where students have sensitive skin or an allergy. Borax has been used as a domestic water softener a mild antiseptic and domestic cleaner, though, today, applications are generally superseded by more commercial products.

Viscous Goo Challenge - Teachers Notes

PVA adhesive (wood glue / white paper glue) will vary in composition depending on its source, so it is necessary to try quantities for yourself. White paper glue mixed with water in a 1 :2 ratio (glue to water) has been found to give consistent results. PVA could stand for Poly Vinyl Alcohol or Poly Vinyl Acetate. In fact glues are probably a mixture of the two, but since both are polar and can interact with borax, this is not important for the experiment.

PVA is non toxic; Borax would be poisonous if consumed but (other than occasional skin irritation) is harmless. Uses include mild anti-septic, fridge and kitchen surface cleaner, water softener. Until recently borax was readily available from chemist shops. The solution used is made as a saturated (4%) solution prior to use.

The behaviour of Goo is useful as a discussion on solids and liquids (stretches if pulled slowly, snaps if pulled quickly). The flow of goo on a bench surface illustrates creep. Creep is common in polymers and metals at about 1/3 mpt (lead is a good example). Also convection currents in the solid mantle of the earth can be describes as creep in a ceramic material. Although not the same process, there are similarities with bonds being broken and reforming in a different position. Whereas Goo will creep by several cm in a few seconds, lead will take centuries and the mantle takes millennia.

Experiments can be aimed at KS2 to A-level, from purely fun activities to simple experiments illustrating changes in physical properties of polymers due to cross linking and intermolecular forces (Hydrogen Bonds).

It is possible to develop investigations based on the above experiments, particularly the sinking coin by linking this to the amount of borax used on a fixed amount of PVA.

Viscous Goo ChallengePlastics (polymers) are large chains of repeating molecules. We can sometimes change the properties of polymers and make them into more useful materials by joining up the separate chains using chemical bonds. This is called cross-linking.

Chains of P.V.A. (Poly Vinyl Alcohol) can be (loosely) cross-linked by adding borax solution. This results in the P.V.A. changing from a liquid to a rubbery slime.

Polymer chains can slide past each other under the effect of a force. Liquid polymers are viscous (thick, not very runny liquids) because of forces between the chains. We can change the viscosity by changing the way that the chains slide past each other effectively changing the friction between them.

More force/friction = harder for chains to slide past = more viscous liquid.

PVA molecules and how they interact

PVA molecules with Borax

Your challenge:

Design and test:

The slime with the greatest bounce.

The slime that can stretch the most without breaking. Investigate the viscosity of slime as a function of the amount of borax usedYour time limit is 30 minutes.

5 minutes to make a plan. 10 minutes to carry out your plan. 10 minutes to carry out your tests. 5 minutes to clean up the mess.The basic recipe for the slime is given below.

Chemical and Apparatus Lists 250 ml bottle of 25% P.V.A. solution (shared).

250ml beaker of saturated Borax solution (shared). Borax is a >>>- ( POISON ( -