Hot Chocolate
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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.