Hot Chocolate

Synopsis

Chocolate is vulnerable to heat damage. Complaints of products being in poor condition can arise during sunny summer weather, especially if the chocolate is delivered in vans that are painted in dark colours.

The painting of company colours onto delivery vans, and the style of company logos, can therefore affect the amount of radiation absorbed from the Sun and, consequently, the temperature inside the van. In this unit, students solve problems of heat damage to chocolate by identifying the most suitable colour scheme for a company delivery fleet. A simple negative of a company logo, as well as other solutions, are considered.

This unit is based on the experience of Cadbury Limited and the deep purple and gold colours used by the company. If use of a company name is likely to be a sensitive issue, teachers could substitute a fictitious name for a chocolate manufacturer.

What the students do * Formulate a hypothesis * Model the problem on a small scale * Investigate the effect of colour on absorption of radiation * Evaluate different investigations * Decide if empirical evidence justifies the cost of a full scale trial * Produce display material * Recommend a suitable colour scheme * Evaluate alternative solutions * Interpret the results of a full scale trial with company vehicles

Key ideas * Absorption of radiation * Heat transfer and heat capacity * Planning and carrying out investigations t Timing Drying time for painted surfaces is needed. A time allocation of 70 minutes is divided between two lessons, unless a technician provides ready painted apparatus. Contents

Planning ahead

Teachers' notes Introduction A suggested unit outline Lesson management Extension work A full scale trial with Cadbury delivery vans

Technician sheet

Copy masters

OHPI Delivering chocolate OHP2 OHP blank for the teacher, and a display blank for students to colour in their recommended schemes for a company fleet

SIS 1 Data sheet - a full scale trial with company vehicles

SAG 1 Interpreting data

(

OHP = Overhead Projector Slide SAG = Student Activity Guide SIS = Student Information Sheet , Planning ahead

Resources Small (150 ml) aluminium drinks cans can be used to model the delivery vans. These cans give excellent results and are much easier to fit with thermometers than other cans, such as those used for baked beans or baby food. Students could be asked to collect the aluminium cans. They are used for fruit juices, mixers and mineral waters. The ring-pulls should not be detached - they are useful as handles when painting the cans. (The use of ml on drinks cans leads to inconsistencies in the units of volume in this unit.)

Good results are also obtained using boiling tubes or other standard laboratory glassware.

Desk lamps of the Anglepoise type, with a 60 or 100 watt bulb, are a simple and safe way of reliably modelling the Sun as a source of radiation. A group of 3 or 4 students will use one lamp. Availability of such lamps will need to be checked when deciding which apparatus to provide for students. Alternatives are suggested in the Teachers' notes. Lamps should not be brought in from home or elsewhere unless they are subjected to the school's usual electrical checks. Choosing the paints Four paints are included in the suggested practical work: black, white, purple and gold (use yellow). A class of 30 will use less than 50 cm'' of each paint. Supplies of black and white may not need to be bought specially.

Vinyl matt emulsion paints are recommended. They work well on either aluminium or glass containers. No surface preparation is needed and only one coat is required. There are no problems in cleaning glassware painted with vinyl emulsions. Company colours for Cadbury, or any other company, can be closely matched in the paint sections of DIY stores. For example, for Cadbury purple use Evening Iris W3-60 from the CROWN 'Expressions' range. The minimum order for custom paints is 1litre. Use a bright yellow paint for the Cadbury 'gold'.

Ready-mix watercolour paints, available in school art departments or from branches of the Early Learning Centre, can also be used. However, they have poorer covering power and may require two coats. They work well on glass, but aluminium surfaces will need to be prepared with emery cloth. It is difficult to obtain a good purple by mixing ready-mix watercolours. The Cadbury purple is very similar to the Violet POSTER COLOUR, made by Winsor & Newton, and available from shops selling artists supplies. J If a fictitious company and logo are used, paint that is left over from home decorating might be useful. Teachers' notes

Introduction Practical work involves painting identical apparatus (for example, aluminium cans as the model delivery vans) in four different colours and measuring the effect of colour on the absorption of radiation from the Sun. Lesson plans will depend on arrangements made for painting the apparatus. A suggested unit outline is shown below.

Teachers should select from the range of ideas and activities offered, to match existing schemes of work and the time available. Copy masters should be read alongside these notes.

The unit could also accompany work on changes of state and a determination of the melting point of chocolate.

A suggested unit outline

I' "" Optional Lesson 1 - 25 minutes r OHPl,OHP2 Determining the "' Introducing the problem to students .....•• Formulating a hypothesis melting point of Planning investigations chocolate \.. --I Painting apparatus \.. ~ , r "" Lesson 2 - 45 minutes Completing the investigations r Interpreting results Comparing absorption "' Making decisions ....•• of radiation at matt or Designing a colour scheme for a gloss surfaces of the company fleet same colour A class display and the evaluation of \.. --I different investigations , ./

r "" Interpreting the results of a full scale trial with - company vehicles SIS 1. Data sheet SAG 1. Interpreting data \.. ~

1 Lesson management Lesson 1 (25 minutes) Introducing the prob1em to students The problem is that a company is receiving complaints that its chocolate is in poor condition when delivered. The complaints arise in hot sunny weather when the chocolate is delivered in vans that are painted. in company colours, but not when rented vans are used. OHP 1 could be displayed on screen to illustrate the problem, using the first two diagrams only. OHP 2 could be used instead if teachers prefer to shade in the gold and deep purple colours for a simplified Cadbury logo.

Students are told that company experts recognise that the poor condition of the chocolate is due to heat damage. The company wonders if the logo and colours have anything to do with the problem. Scientists are required to find out if there is a connection and, if so, how the problem can be avoided. This is the task for students, acting as trouble-shooters for the company.

Formulating a hypothesis Students look for differences in the vans shown on OHP 1 (or OHP 2), in order to identify possible causes of the problem. They should formulate a hypothesis about light and dark colours and how the different colours could affect the chocolate on a hot sunny day. Students then plan investigations to test their ideas.

The idea of reversing the logo (from the positive form of gold lettering on a deep purple background, to the negative form of deep purple lettering on a gold background), shown on OHP 1 or coloured in on OHP 2, could be included as a prompt or could be introduced at a later stage.

Planning investigations To encourage practical work on a manageable laboratory scale, students are told that the company is not prepared to bear the cost of taking vans off the road, to do a full scale test, until the trouble-shooters can produce good evidence of causes and possible solutions. Students must therefore devise simple laboratory tests for their hypotheses.

Groups of four are suggested. It is useful to include black and white, as the extremes of dark and light colours, in all investigations. A total of four colours is tested - black, white, purple and gold (use yellow). The intended outcome is that students learn that darker colours are better absorbers of radiation and that lighter colours reflect away the radiation.

In planning investigations, students will need to answer questions such as: * What can be used to model the Sun as the heat source? (desk lamp, central heating radiator, Sun if a hot sunny day, etc.) Electric radiant heaters, Bunsens, electric hotplate surfaces, etc. are less safe. * What can be used to model the closed van ? (150 ml aluminium drinks can, small steel baked bean can, stoppered boiling J tube, etc.) i * What will the model van be filled with? (air, water, sand, iron filings, chocolate, etc.) * How will the temperature inside the van be measured? (thermometer, heat sensitive paper, the melting of chocolate)

2 * How will the test be made fair? (same dimensions of model van for each paint, same positioning of thermometer, same thickness of paint, same type of paint, same exposure time to lamp, same distance from lamp, etc.)

Students could be asked to select from a limited range of apparatus the equipment that they think will produce the most convincing results. Some classes might be asked to provide for themselves the materials required to model the van. An element of competition, in producing the most convincing and reproducible results, might be included.

Depending on the materials available and the previous knowledge of students, the teacher could offer prompts about materials that heat up quickly (because of low heat capacity) and materials that are good conductors. The best choice of materials and the best design of investigation will produce sufficiently high temperature increases inside the cans (vans) to show clearly any effects that might be due to different coloured surfaces. Some actual results are shown on pages 4 and 5.

Different groups could carry out different investigations, so that the effectiveness of different techniques can be compared in a final class display of results. Some of the suggested answers on page 2 are less realistic or less likely to give convincing results, but choosing them should not affect the credit given to students for their plans. Reasons behind the best choices may emerge when groups compare results in a final class display.

Model vans must be painted and left to dry before completing the investigation. The ring-pull on aluminium cans is useful for both holding the can and hanging it from a paper clip (hook) on a line to dry. It is suggested that there be a separate area for each colour of paint, with students moving to those points at appropriate times. There is no need to paint the tops of cans.

If boiling tubes are painted, the top 2 em, where stoppers are fitted, need not be painted. NB. For watercolour paints used on glass tubes, holding the dry painted tubes up to the light and looking down the open end will show if a second coat is required (if so, streaks of transmitted light will be visible on the inside walls).

Lesson 2 (45 minutes) Completing the investigations Three possible arrangements of apparatus for carrying out this investigation are shown on pages 4 and 5. There are many other possibilities. Actual results are shown alongside the diagrams.

In each case the following factors were common: * the heat source was an Anglepoise lamp with 60 watt bulb * exposure time was 5 minutes * model vans (cans or tubes) were tested in pairs * each can or tube was equidistant (5 em) from the lamp * identical cans or tubes were used ~,, * all paints were vinyl matt emulsions. Careful attention must be paid to the hazards of any heat sources used in these investigations.

3 1. Measuring the increase in temperature of air inside painted aluminium cans

Can Temperature °C

start after 5 mins increase 150ml Black 20.0 30.0 10.0 aluminium Air drinks can Purple 19.5 28.0 8.5 (painted) Gold 20.5 26.5 6.0

White 19.5 23.5 4.0

Hints: Thermometer Small cans use less paint. 20-40°C range Desk lamp Aluminium cans are better than steel cans. must be visible Cans must be dry inside. Use Blu-Tack to fix the can onto the thermometer. Clamp Clamp the thermometer not the can. Cans with fitted thermometers are unstable in the upright position. Test two cans at once, as shown below.

There is a noticeable difference in temperature, felt by the hand, when dark and light coloured cans are handled immediately after exposure to the lamp.

2. Measuring the increase in temperature of air inside painted boiling tubes

Can Temperature °C Desk lamp start after 5 mins increase

Black 20.5 30.0 9.5

Purple 20.5 28.5 8.0

Gold 20.5 26.0 5.5

White 21.0 24.0 3.0

Hints: Use thin-walled boiling tubes. Do not use a mixture of thick and thin- Thermometer walled tubes. 20-40 °c range Tubes should be dry inside. must be visible Clamp the thermometer not the tube. Before beginning the test, adjust the clamps so that thermometers are easy to read. Test two tubes at once. Clamp

4 3. Measuring the increase in temperature of water inside painted boiling tubes

Can Temperature °C

start after 5 mins increase

Black 20.5 24.0 3.5

Purple 20.5 23.0 2.5 Thermometer 20-40°C range Gold 20.5 22.5 2.0 must be visible White 20.5 21.5 1.0 Desk lamp

Hints: Use thin-walled boiling tubes. 3 Add only 40 em of water to each boiling Water tube, to allow room for expansion. level Keep the outside of the tubes dry. Clamp the tube not the thermometer. Before beginning the test, adjust the clamps so that thermometers are easy to read. Boiling Test two tubes at once. tube

Some practical details and sources of errors These comments are intended for the teacher only, for use if students have questions about the validity of their ideas or problems in obtaining reproducible results.

1. Temperature is best measured to the nearest 0.5 °C. Errors may be made when students read from scales on inverted thermometers. Students could record the temperature in each can (or tube) every minute for five minutes. 2. When pairs of cans (or tubes) are substituted, the lamp should be temporarily switched off. However, it will still be hot and will continue to radiate until it has cooled down. Consequently, errors can arise if substitute cans are placed in position long before the lamp is switched on again. Thermometers will also need time to return to room temperature. Students must not assume that all cans (or tubes) start at the same temperature. 3. Separation of cans (or tubes) from the lamp can be kept constant if the cans are fitted onto the thermometers, rather than removing thermometers from the clamps and fitting the thermometers into the cans. 4. Thermometer bulbs should be in the same position in all cans (or tubes) because of possible convection currents inside the apparatus. Beware that the required region of the temperature scale on the thermometer is not obscured by Blu-Tack or a rubber stopper, or by enclosure within the can or tube. 5. To check that cans or tubes are equidistant from the lamp, view apparatus from both in front and behind the bench. 6. To get a wider spread of results, reduce the 5 em separation of cans (or tubes) and lamp. Alternatively, use a 100 watt bulb. 7. If the two cans (or tubes) are too close together, radiation from the hotter one will affect the temperature of the cooler one. 8. Unsatisfactory results may occur with glass tubes if the paint coat is not complete.

5 9. Time is unlikely to permit the use of proper controls. For example, the position of each of the pair of cans or tubes in front of the lamp should be reversed.

Interpreting results and making decisions When the investigations are complete, groups could be asked to attempt some or all of these tasks, giving reasons for their decisions: * Interpret their results and look for patterns. * Display results in the form of a line graph (temperature versus time) or a bar chart. * Prepare a clear statement on the effect of colour on the absorption of radiation from the Sun. * Decide whether further investigations need to be carried out. * Decide whether the evidence obtained is good enough to justify the planning and cost of a full scale test involving the repainting of company vehicles. * Colour in a paper copy of OHP 2 to show their recommended colour scheme for the Cadbury fleet. (Bear in mind that the company will be concerned with the advertising impact of the vans as well as the protection of chocolate from heat damage.) * Identify other solutions to the problem and their advantages and disadvantages.

A class display Results of investigations and suggested colour schemes could be displayed. Students could compare results of the different investigations during a class discussion. For example, students might learn that aluminium cans containing air give the better results (more exaggerated temperature differences) than water-filled glass tubes because aluminium is a better conductor of heat than glass and because gases heat up faster (lower thermal capacity) than liquids or solids.

From the colours investigated, the most protective logo from the point of view of minimum heat transfer would be gold lettering on a white background. However, this colour scheme may be unacceptable to students because it is too weak in visual impact. Students must make their own decisions.

If positive and negative forms of a logo have been considered, students should reach the conclusion that, for Cadbury colours, the negative form offers much better protection against heat damage. This may not be the case with colours used by other companies. Students may have observed the use of positive and negative forms of their own school logo on various printed documents.

Some students may recommend altering the relative area of lettering compared to coloured background.

Other solutions include the use of refrigerated vans or vans insulated with expanded polystyrene. The effect of these solutions on the cost of vehicles, and the amount of products that can be carried, would need to be considered. i, Cleaning up Paints are easy to wash from glass tubes after a short soak in hot soapy water. Use a sink tidy to trap paint residues, so that the sink drains are not blocked.

6 Extension work 1. The investigations could be extended to consider the effect of surface texture, that is matt or gloss surfaces of the same colour, on absorption of radiation. 2. Are plain chocolate and white chocolate just as vulnerable to heat damage as is milk chocolate? What would you do to find out? 3. Students could investigate whether the temperature inside model vans of different colours will eventually reach the same value, if the investigation is allowed to continue for longer. The idea of an equilibrium may emerge from this investigation - with the model vans each reaching a temperature at which they emit radiation (from dark-side surfaces) at the same rate at which the light-side surfaces absorb radiation from the lamp. 4. Students could investigate whether the colour of paintwork is more important than the material from which the body of the van itself is made. 5. Could the same apparatus be used to investigate emission of radiation rather than absorption? Rates of cooling would need to be measured. 6. A collection of chocolate wrappers and packaging may be useful to show the use of a single logo in different forms and colours.

A full scale trial with Cadbury delivery vans Students may be interested in the outcome of trials with Cadbury delivery vans painted in different styles, but using the familiar company colours. Data for both purple and white livery are tabulated and displayed on sheets SIS 1 and SAG I respectively. As a homework exercise, students could be challenged to interpret the the graph on SAG 1. Alternatively, if OHPs are made of the relevant sections of SIS 1 and SAG 1, the results of the full scale trial could be used as the basis for class discussion. The word 'livery' may need to be explained to students.

Purple livery refers to a van painted in purple with company name and text in white.

White livery is illustrated on the front cover and refers to an identical van with container box painted in white, with company name and text in purple. Note that the cab remains purple, as this will not affect the temperature in the storage section of the van, but will improve the visual and advertising impact of the van.

Answers to questions on SAG 1, and relevant points, are as follows: Q1. a). The temperature inside the purple van is usually higher. However, some results do not fit this pattern. b). Heat damage was most likely on days 219, 227, 228 and 232.

Q2. White livery would best protect the chocolate against heat damage, because the temperature inside the white van is generally lower.

Q3. a). Long hours of sunshine result in the highest temperatures inside the vans. b). Students should be given some credit if they report that the pattern is clear. However, for the data supplied, the correlation between hours of sunshine and temperature inside the vans is not particularly good and some students will have good reasons to state that the pattern is not clear.

Those students who make a more detailed analysis of the data may report that not all days of long hours of sunshine have resulted in high maximum temperatures inside the vans. For example, on days 218 and 224 the maximum temperature inside the purple van is 25°C on each occasion, despite the large difference in the sunshine record of those days. Some possible reasons for anomalous or unconvincing results are included in answers to question 4. It is

7 indeed long hours of sunshine, rather than high air temperatures, that result in heat damage to chocolate in delivery vans.

Q4. Unexpected results occur on days 226 and 233, when the temperature is higher in the white van. Students could reasonably add that unexpected results should include the small differences in temperature on days such as 218 and 223. As a good absorber of radiant heat, the purple van would be expected, on sunny days, always to be significantly warmer inside than the white van.

Unexpected results may be explained by one of the following: * vans have been parked in the shade * vans have travelled outside the Birmingham area to destinations where the weather is very different * whether or not the load is collected from a refrigerated store affects the temperature inside the van * errors have been made in recording results. Q5. Drivers should be instructed to record additional details, as indicated by Q4, so that there is sufficient information to interpret results correctly.

Drivers could be instructed to record the increase in temperature inside each van, in addition to the maximum temperature.

The illustration of white livery on the front cover could also be compared to designs produced by students during the activities in the earlier part of this unit. The criteria against which students' designs are judged could be: * the potential of the colours, and their relative areas, to protect chocolate from heat damage in summer by way of minimum absorption of radiant heat * visual and advertising impact.

8 HOT CHOCOLATE

Technician sheet

Requirements for this lesson must be checked with the teacher. The following apparatus may be used with a class of 30 students working in groups of three or four:

The teacher will require: * OHP and colour felt pens, including purple and yellow * Four paints: black, white, gold (use yellow), purple - details below. A class of 30 will use less than 50 cm3 of each paint. * 8 small paint brushes (0.25 - 1.0 cm wide) * Newspapers to protect bench surfaces * Detergent and washing up bowl * Supply of paper clips or wire to bend into simple hooks * String to use as a line to hang painted cans from * Emery cloth.

Students will use some, but not necessarily all, of the following: * 16 thermometers (range -10 to 110°C). Check with the teacher how many of these thermometers should be fitted into rubber bungs to fit into boiling tubes. Check that the range 20 to 40°C is not obscured by the bung. The bulbs of the thermometers should be in the same position in each tube. * A collection of identical (150 ml) aluminium cans or alternative. * Boiling tubes or 100 em3 conical flasks * Desk lamps of the Anglepoise type, or alternative sources of radiation * Stands and clamps * Blu-Tack * 50 cm3 measuring cylinders.

Choosing suitable paints Vinyl matt emulsion paints are recommended. They work well on either aluminium or glass containers. No surface preparation is needed and only one coat is required. There are no problems in cleaning glassware painted with vinyl emulsions. Supplies of black and white may not need to be bought specially. The Cadbury purple can be closely matched in the paint sections of DIY stores. For example, use Evening Iris W3-60, from the CROWN 'Expressions' range. The minimum order for custom paints is 1 litre. Use a bright yellow paint for the Cadbury 'gold'.

9 OHPl

Delivering chocolate

CADBURY logo is gold lettering on a deep purple background. Complaints. The chocolate is heat damaged during summer months.

Van Hire No complaints. The chocolate is always delivered in good condition.

CADBURY Avoiding complaints. Will reversing the logo, to purple on gold, make any difference ? OHP2 SIS 1

Data sheet A full scale trial with company vehicles

The data below are the maximum temperatures inside two Cadbury delivery vans on each day between 5th August (day 218) and 23rd August (day 236) 1992. One van was painted in purple livery, the other was painted in white livery.

The vans were based at the Oldbury depot in Birmingham. Details of the weather in Birmingham on those days is also shown.

Maximum temperature inside Weather conditions delivery van °C in Birmingham

Day Purple livery White livery Max temp °C Hours of sunshine

218 25.0 24.5 21.7 2.2

219 41.5 27.5 20.6 12.8

220 26.0 21.5 19.7 0.1

221 19.0 18.0 18.2 0

222 28.0 22.0 20.6 1.5

223 25.5 25.0 21.1 7.0

224 25.0 21.0 19.6 7.9

225 23.0 22.0 18.7 6.8

226 18.0 19.5 15.8 1.4

227 33.5 24.5 18.4 11.8

228 33.5 25.0 21.4 7.6

229 31.0 23.0 18.4 6.8

230 28.5 24.5 19.8 8.5

231 28.5 24.5 19.8 7.1

232 33.0 27.5 20.5 10.5

233 23.0 25.0 20.5 0.2

234 26.0 23.5 20.7 3.8

235 21.0 18.0 18.3 0.8

236 22.5 17.0 17.0 5.7 SAG 1

Interpreting data

The graph below compares the maximum temperatures inside two Cadbury delivery vans on each day between 5th August (day 218) and 23rd August (day 236) 1992. One van was painted in purple livery, the other was painted in white livery.

I 0 Purple livery -- White livery

30 Max temp 25 OC 20

10 218 220 222 224 226 228 230 232 234 236 Days outof365 (5th-23rd August 1992)

Study the graph and then answer these questions:

Q1. a) What difference does colour make to the temperature inside the vans? b) On which days might the chocolate have been heat-damaged?

Q2. Which livery would best protect the chocolate against heat damage? Explain your answer.

Q3. Use the weather data on SIS 1 to look for patterns that could explain the high temperatures inside the vans. a) What type of weather is most likely to result in high temperatures inside a van? b) Is the pattern clear? Explain your answer.

Q4. There are several unexpected results on the graph above. On which days do they occur? How could these results be explained?

Q5. Data on SIS 1 was recorded by the drivers. What instructions would you give to drivers, to make sure that the results of future trials are easier to interpret?