HOT CHOCOLATESynopsis

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

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

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

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 isdivided between two lessons, unless a technician provides ready paintedapparatus.

Contents

Planning ahead

Teachers' notesIntroductionA suggested unit outlineLesson managementExtension workA full scale trial with Cadbury delivery vans

Technician sheet

Copy mastersOHPIOHP2

Delivering chocolate

OHP blank for the teacher, and a display blank for studentsto colour in their recommended schemes for a company fleet

SIS 1

SAG 1

Data sheet - a full scale trial with company vehicles

Interpreting data

(

OHP = Overhead Projector SlideSIS = Student Information Sheet

SAG = Student Activity Guide

,

Planning ahead

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

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

Desk lamps of the Anglepoise type, with a 60 or 100 watt bulb, are a simple andsafe way of reliably modelling the Sun as a source of radiation. A group of 3 or4 students will use one lamp. Availability of such lamps will need to bechecked when deciding which apparatus to provide for students. Alternativesare suggested in the Teachers' notes. Lamps should not be brought in fromhome or elsewhere unless they are subjected to the school's usual electricalchecks.

Choosing the paintsFour paints are included in the suggested practical work: black, white, purpleand 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 eitheraluminium or glass containers. No surface preparation is needed and onlyone coat is required. There are no problems in cleaning glassware paintedwith vinyl emulsions. Company colours for Cadbury, or any other company,can be closely matched in the paint sections of DIY stores. For example, forCadbury purple use Evening Iris W3-60 from the CROWN 'Expressions'range. The minimum order for custom paints is 1litre. Use a bright yellowpaint for the Cadbury 'gold'.

Ready-mix watercolour paints, available in school art departments or frombranches of the Early Learning Centre, can also be used. However, they havepoorer covering power and may require two coats. They work well on glass,but aluminium surfaces will need to be prepared with emery cloth. It isdifficult to obtain a good purple by mixing ready-mix watercolours. TheCadbury purple is very similar to the Violet POSTER COLOUR, made byWinsor & Newton, and available from shops selling artists supplies.

JIf a fictitious company and logo are used, paint that is left over from homedecorating might be useful.

Teachers' notesIntroductionPractical work involves painting identical apparatus (for example, aluminium cansas the model delivery vans) in four different colours and measuring the effect of colouron the absorption of radiation from the Sun. Lesson plans will depend onarrangements made for painting the apparatus. A suggested unit outline is shownbelow.

Teachers should select from the range of ideas and activities offered, to matchexisting schemes of work and the time available. Copy masters should be readalongside these notes.

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

A suggested unit outline

I' "" OptionalLesson 1 - 25 minutesOHPl,OHP2 r "'Introducing the problem to students Determining the

.....•• melting point ofFormulating a hypothesisPlanning investigations chocolate

\.. --IPainting apparatus

\.. ~

,r Lesson 2 - 45 minutes

""

Completing the investigationsInterpreting results

rComparing absorption "'

Making decisions ....•• of radiation at matt orDesigning a colour scheme for a gloss surfaces of thecompany fleet same colourA class display and the evaluation of \.. --Idifferent investigations, ./

r ""Interpreting the results- of a full scale trial with- company vehiclesSIS 1. Data sheetSAG 1. Interpreting data

\.. ~

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Lesson managementLesson 1 (25 minutes)Introducing the prob1em to students

The problem is that a company is receiving complaints that its chocolate is in poorcondition when delivered. The complaints arise in hot sunny weather when thechocolate is delivered in vans that are painted. in company colours, but not whenrented 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 toshade in the gold and deep purple colours for a simplified Cadbury logo.

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

Formulating a hypothesisStudents look for differences in the vans shown on OHP 1 (or OHP 2), in order toidentify possible causes of the problem. They should formulate a hypothesis aboutlight and dark colours and how the different colours could affect the chocolate on a hotsunny 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 purplebackground, 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 beintroduced at a later stage.

Planning investigationsTo encourage practical work on a manageable laboratory scale, students are told thatthe company is not prepared to bear the cost of taking vans off the road, to do a fullscale test, until the trouble-shooters can produce good evidence of causes and possiblesolutions. 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 extremesof 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 thatdarker colours are better absorbers of radiation and that lighter colours reflect awaythe 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 boilingtube, etc.)* 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)

Ji

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* 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, samedistance from lamp, etc.)

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

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

Different groups could carry out different investigations, so that the effectiveness ofdifferent techniques can be compared in a final class display of results. Some of thesuggested 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 finalclass display.

Model vans must be painted and left to dry before completing the investigation. Thering-pull on aluminium cans is useful for both holding the can and hanging it from apaper clip (hook) on a line to dry. It is suggested that there be a separate area for eachcolour of paint, with students moving to those points at appropriate times. There is noneed to paint the tops of cans.

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

Lesson 2 (45minutes)Completing the investigationsThree possible arrangements of apparatus for carrying out this investigation areshown on pages 4 and 5. There are many other possibilities. Actual results areshown 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 theseinvestigations.

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

150mlaluminium Airdrinks can(painted)

Can Temperature °C

start after 5 mins increase

Black 20.0 30.0 10.0

Purple 19.5 28.0 8.5

Gold 20.5 26.5 6.0

White 19.5 23.5 4.0

Hints:Small cans use less paint.Aluminium cans are better than steel cans.Cans must be dry inside.Use Blu-Tack to fix the can onto thethermometer.Clamp the thermometer not the can.Cans with fitted thermometers are unstablein the upright position.Test two cans at once, as shown below.

Thermometer20-40°C rangemust be visible

Clamp

Desk lamp

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

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

Can Temperature °C

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

Desk lamp

Hints:Use thin-walled boiling tubes.Do not use a mixture of thick and thin-walled tubes.Tubes should be dry inside.Clamp the thermometer not the tube.Before beginning the test, adjust the clampsso that thermometers are easy to read.Test two tubes at once.

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Thermometer20-40 °c rangemust be visible

Clamp

3. Measuring the increase in temperature ofwater 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

Gold 20.5 22.5 2.0

White 20.5 21.5 1.0

Thermometer20-40°C rangemust be visible

Desk lamp

Hints:Use thin-walled boiling tubes.Add only 40 em3 of water to each boilingtube, to allow room for expansion.Keep the outside of the tubes dry.Clamp the tube not the thermometer.Before beginning the test, adjust the clampsso that thermometers are easy to read.Test two tubes at once.

Waterlevel

Boilingtube

Some practical details and sources of errorsThese comments are intended for the teacher only, for use if students have questionsabout 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 whenstudents read from scales on inverted thermometers. Students could record thetemperature in each can (or tube) every minute for five minutes.

2. When pairs of cans (or tubes) are substituted, the lamp should be temporarilyswitched off. However, it will still be hot and will continue to radiate until it hascooled down. Consequently, errors can arise if substitute cans are placed inposition long before the lamp is switched on again. Thermometers will also needtime to return to room temperature. Students must not assume that all cans (ortubes) start at the same temperature.

3. Separation of cans (or tubes) from the lamp can be kept constant if the cans arefitted onto the thermometers, rather than removing thermometers from the clampsand fitting the thermometers into the cans.

4. Thermometer bulbs should be in the same position in all cans (or tubes) because ofpossible convection currents inside the apparatus. Beware that the required regionof the temperature scale on the thermometer is not obscured by Blu-Tack or arubber stopper, or by enclosure within the can or tube.

5. To check that cans or tubes are equidistant from the lamp, view apparatus fromboth in front and behind the bench.

6. To get a wider spread of results, reduce the 5 em separation of cans (or tubes) andlamp. Alternatively, use a 100 watt bulb.

7. If the two cans (or tubes) are too close together, radiation from the hotter one willaffect the temperature of the cooler one.

8. Unsatisfactory results may occur with glass tubes if the paint coat is not complete.

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

Interpreting results and making decisionsWhen the investigations are complete, groups could be asked to attempt some or all ofthese 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 radiationfrom the Sun.* Decide whether further investigations need to be carried out.* Decide whether the evidence obtained is good enough to justify the planning andcost 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 forthe Cadbury fleet. (Bear in mind that the company will be concerned with theadvertising impact of the vans as well as the protection of chocolate from heatdamage.)* Identify other solutions to the problem and their advantages and disadvantages.

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

From the colours investigated, the most protective logo from the point of view ofminimum 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 visualimpact. Students must make their own decisions.

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

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

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

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Extension work1. The investigations could be extended to consider the effect of surface texture, that ismatt 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 ismilk chocolate? What would you do to find out?

3. Students could investigate whether the temperature inside model vans of differentcolours will eventually reach the same value, if the investigation is allowed tocontinue for longer. The idea of an equilibrium may emerge from thisinvestigation - with the model vans each reaching a temperature at which theyemit radiation (from dark-side surfaces) at the same rate at which the light-sidesurfaces absorb radiation from the lamp.

4. Students could investigate whether the colour of paintwork is more important thanthe material from which the body of the van itself is made.

5. Could the same apparatus be used to investigate emission of radiation rather thanabsorption? Rates of cooling would need to be measured.

6. A collection of chocolate wrappers and packaging may be useful to show the use ofa single logo in different forms and colours.

A full scale trial with Cadbury delivery vansStudents may be interested in the outcome of trials with Cadbury delivery vanspainted in different styles, but using the familiar company colours. Data for bothpurple and white livery are tabulated and displayed on sheets SIS 1 and SAG Irespectively. As a homework exercise, students could be challenged to interpret thethe graph on SAG 1. Alternatively, if OHPs are made of the relevant sections of SIS 1and SAG 1, the results of the full scale trial could be used as the basis for classdiscussion. 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 withcontainer box painted in white, with company name and text in purple. Note that thecab remains purple, as this will not affect the temperature in the storage section of thevan, 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 thetemperature 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 andtemperature inside the vans is not particularly good and some students willhave good reasons to state that the pattern is not clear.

Those students who make a more detailed analysis of the data may report thatnot all days of long hours of sunshine have resulted in high maximumtemperatures inside the vans. For example, on days 218 and 224 the maximumtemperature inside the purple van is 25°C on each occasion, despite the largedifference in the sunshine record of those days. Some possible reasons foranomalous or unconvincing results are included in answers to question 4. It is

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

Q4. Unexpected results occur on days 226 and 233, when the temperature is higher inthe white van. Students could reasonably add that unexpected results shouldinclude the small differences in temperature on days such as 218 and 223. As a goodabsorber of radiant heat, the purple van would be expected, on sunny days, always tobe 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 theweather is very different* whether or not the load is collected from a refrigerated store affects thetemperature inside the van

* errors have been made in recording results.

Q5. Drivers should be instructed to record additional details, as indicated by Q4, sothat there is sufficient information to interpret results correctly.

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

The illustration of white livery on the front cover could also be compared to designsproduced by students during the activities in the earlier part of this unit. The criteriaagainst 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.

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HOT CHOCOLATETechnician sheet

Requirements for this lesson must be checked with the teacher. The followingapparatus 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 30will 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 ofthese 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 thethermometers 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 paintsVinyl matt emulsion paints are recommended. They work well on either aluminium orglass containers. No surface preparation is needed and only one coat is required. Thereare no problems in cleaning glassware painted with vinyl emulsions. Supplies of blackand white may not need to be bought specially. The Cadbury purple can be closelymatched in the paint sections of DIY stores. For example, use Evening Iris W3-60, fromthe CROWN 'Expressions' range. The minimum order for custom paints is 1 litre. Usea bright yellow paint for the Cadbury 'gold'.

Ready-mix watercolour paints, available in school art departments or from branchesof the Early Learning Centre, can also be used. However, they have poorer coveringpower and may require two coats. They work well on glass, but aluminium surfaceswill need to be prepared with emery cloth. It is difficult to obtain a good purple bymixing ready-mix watercolours. The Cadbury purple is very similar to the VioletPOSTER COLOUR, made by Winsor & Newton, and available from shops sellingartists supplies.

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Delivering chocolate

Van Hire

CADBURY

OHPl

CADBURY logo is goldlettering on a deeppurple background.

Complaints.The chocolate is heatdamaged duringsummer months.

No complaints.The chocolate isalways delivered ingood condition.

Avoiding complaints.Will reversing the logo,to purple on gold, makeany difference ?

OHP2

SIS 1

Data sheetA full scale trial with company vehicles

The data below are the maximum temperatures inside two Cadbury delivery vanson 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 theweather in Birmingham on those days is also shown.

Maximum temperature inside

delivery van °C

Day Purple livery White livery

218 25.0 24.5

219 41.5 27.5

220 26.0 21.5

221 19.0 18.0

222 28.0 22.0

223 25.5 25.0

224 25.0 21.0

225 23.0 22.0

226 18.0 19.5

227 33.5 24.5

228 33.5 25.0

229 31.0 23.0

230 28.5 24.5

231 28.5 24.5

232 33.0 27.5

233 23.0 25.0

234 26.0 23.5

235 21.0 18.0

236 22.5 17.0

Weather conditions

in Birmingham

Max temp °C Hours of sunshine

21.7 2.2

20.6 12.8

19.7 0.1

18.2 0

20.6 1.5

21.1 7.0

19.6 7.9

18.7 6.8

15.8 1.4

18.4 11.8

21.4 7.6

18.4 6.8

19.8 8.5

19.8 7.1

20.5 10.5

20.5 0.2

20.7 3.8

18.3 0.8

17.0 5.7

SAG 1

Interpreting data

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

I 0 Purple livery -- White livery

45

40

35

30Maxtemp

25OC

20

15

10218 220 222 224 226 228 230 232

Days outof365 (5th-23rd August 1992)234 236

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 hightemperatures 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 dothey occur? How could these results be explained?

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