Melting Ice – Teacher guidance
This lesson is designed to exemplify an argumentation approach to practical work, using a ‘predict-observe-explain’ framework.
Overview
Students often think that some materials are intrinsically warm (wood, plastic, wool) while others are intrinsically cold (metals, glass, water). This lesson challenges these ideas by presenting observations which many will find counter-intuitive.
Through argumentation, students predict the outcome of an experiment; after observing the result, they discuss how scientific ideas about energy transfer can explain what they see.
The key features of the lesson are:
• predicting the outcome of a practical
• working in a small group to develop an argument to support the prediction
• observing the phenomenon
• discussing the scientific explanation for the observed outcome
• using scientific ideas about energy transfer to make further predictions.
Curriculum links
This lesson makes use of ideas about energy transfer, in particular conduction of heat. It could be used to consolidate these ideas, or (with some adaptation) as an introduction to conduction.
Age range and timing
Year 8 or 9 (Could be adapted for Year 10 or 11)
50 minutes
Learning outcomes
Skills-based
Students will be able to:
• work in small groups to generate and evaluate scientific arguments
• present a scientific argument using oral and visual methods.
Content-based
Students will be able to:
• describe how energy is transferred through a solid conductor from higher to lower temperature
• apply ideas about energy transfer by thermal conduction in unfamiliar situations.
Prior knowledge
This lesson can be used as an introduction to thermal conduction. Alternatively, it can be used as an opportunity for students to review and apply ideas about conduction which they have already learned.
Students should already know the following:
Ice melts at 0°C
Students may already know the following:
Energy must be supplied to make ice melt
Energy is transferred from higher to lower temperature
Energy is transferred through solids by conduction
Some materials are better conductors than others
They may also be familiar with the mechanisms of conduction.
Background information
To melt ice, energy must be supplied. Energy is transferred from hotter to colder places by conduction, convection and radiation; i.e. temperature difference results in energy transfer. Metals are better conductors than plastics.
Terminology
The terms which students need to understand and use in this lesson are:
melting – the change from solid to liquid; energy must be supplied to cause a solid to melt
conduction – the transfer of energy through a solid or liquid without the material itself moving
energy transfer – the movement of energy from one place to another
temperature – a measure of the hotness or coldness of an object
argument – the data that students use to articulate and justify claims or conclusions
claim – a conclusion, idea, proposition or assertion
evidence / data – the evidence and facts used to support the claim
Note that in this resource we have used the term energy throughout. Energy which is transferred because of a temperature difference is sometimes known as heat or heat energy (or even thermal energy, although this is not a standard term). Conduction is sometimes described as a thermal energy transfer. You will have to decide if any of these terms are appropriate to your own scheme of work.
For a discussion of the terms heat and temperature see www.nuffieldfoundation.org/practical-physics/heat-and-temperature.
Lesson outline
Step / Timing / Details / ResourcesSet the context / 5 min / Pass the metal and plastic blocks around so that students can feel them.
Explain that the lesson is about energy transfers and working together to develop scientific arguments. A good scientific argument uses evidence and scientific ideas to justify a point. At the end, students will be able to give better explanations of phenomena which involve conduction.
Put students in groups of three, and ask them to discuss and note down words that describe how the two blocks feel to the touch, concentrating on ‘similarities’ and ‘differences’. Encourage students to think about the materials the blocks are made of rather than the shape of the blocks.
Putting students in groups of three rather than pairs is more likely to result in debate in the next part of the lesson. / Metal and plastic blocks – see Practical guidance
Slides 2-3
Make predictions / 10 min / Explain that you are going to place an ice cube on each block. The question is: Which cube will melt first, and why?
Give students 1 min to discuss this question in their groups. Where one person in a group disagrees with the other two, they should be given priority to explain their ideas.
While this is going on give out the ‘predict, observe, explain’ A3 placemats (student sheet page 1).
After 1 min add some questions to prompt their thinking:
· Why does ice melt?
· Why would one ice cube melt more quickly than the other?
Initial explanations may include the idea that ‘metals are cold’. Asking why ice melts should prompt thoughts about energy transfer.
Give groups a further 2 – 3 min to finish their discussion and complete the ‘Predict’ section of the placemat. While this is going on, circulate and check that groups have thought about the questions and have given an explanation for their prediction. At the same time identify groups to report back in the next part of the lesson.
Most students are likely to suggest that the cube placed on plastic will melt first; the important thing is that they should attempt to justify their prediction with an explanation using scientific ideas. / Could show the ice cubes.
Student sheet page 1 (placemat) printed onto A3 paper (one per group).
Slide 4
Present predictions / 5 min / Find two groups to report, one proposing plastic, the other metal. Ask each to give a 30 - 60 second statement of their thinking to the class.
Then allow each group to say why they consider the other argument to be incorrect.
Ask the rest of the class to suggest whether they agree or disagree with the presented arguments and whether they can improve them.
Carry out practical activity / 5 min / Once the different ideas are clear give groups of students a pair of blocks and two ice cubes. Ask them to place the ice cubes simultaneously on the blocks, then leave them alone and observe what happens. (See Practical guidance). Alternatively this could be carried out as a teacher demonstration.
The ice on metal melts first. Once this is apparent, ask: who is surprised? Students should write their observations on the A3 placements. / See Practical guidance
Slide 4
Develop explanations / 15 min / Give students 2 min to discuss in their groups what they have seen and to decide whether their prediction was correct. If their prediction was incorrect, ask them to try and explain what they saw.
After 2 min, challenge groups to devise an agreed explanation of what they have seen. They will need to develop an argument for why they think their explanation is correct. This means they need to support their explanation with evidence (i.e. what they saw happen). Add some questions to prompt their thinking.
· What is the temperature of ice?
· What is needed to melt the ice?
· Where does it come from?
· How and why does it go into the ice?
Students may already be thinking in terms of energy. You may need to remind some students that metals are conductors and plastics are insulators, and how energy is transferred through solids. More able students can also try to explain why the ‘plastic is warmer’ idea is not supported by their observations.
Students write their explanations (using key words) into the placemat, which also has space for a diagram to show how the energy moves. While this is going on, circulate and check that groups have thought about the questions and have justified their explanations by referring to their observations.
An example of a diagram is provided on slide 8. It could be enhanced by including smaller arrows showing small amounts of energy being transferred from the air to the ice.
Challenge students to justify their explanations by offering up counter-claims. e.g. ‘So why don’t you think it was the temperature of the metal which made the ice melt faster?’. For more able students challenge them to also explain why metal feels colder to the touch than plastic. See diagram on slide 9.
Get one or two groups to share their explanations. Ask other groups where they disagree, or where they feel they have something to add. You could give students coloured cards to hold up to indicate if they disagree (red card) or if they have something to add (orange card). / Slides 5-9
Present explanations / 15 min / Give groups the model answer (student sheet page 2) and ask them to compare it to their own answer. Ask them to identify what they have done well and how they could improve. Give groups the opportunity to improve their answers.
Ask students what makes a good argument and how they went about developing their own arguments. They should identify the importance of evidence to support a claim. / Student sheet page 2
Assessing learning / Home-work / Ask students to complete either the true or false questions (lower ability) or the plastic vs glass questions (higher ability). These are an opportunity to apply what they have learnt about conduction. / Student sheet pages 3-4
Model answer
Ice is at a temperature of 0°C; the surroundings are at about 20°C. In order for ice to melt, it must gain energy from the surroundings.
Energy can be transferred (move) from the surroundings to the ice by conduction through the metal or plastic. Metal is a better thermal conductor than plastic, so energy is transferred more quickly through the metal. This is why we saw the ice-on-metal melt more quickly.
(Note that a small amount of energy may enter the ice from the air, but this is a small effect compared to conduction through the metal/plastic because air has very little mass.)
More able students may also add:
Metal feels colder to the touch than plastic. This is because (in the lab) the metal and plastic are at a lower temperature than our fingers, which are at 37°C (or perhaps a few degrees less than this).
When we touch the metal, energy is conducted from our fingers into the metal. This loss of energy cools our fingers and our nerves signal that our fingers are cold. We interpret this as an indication that the metal is cold.
When we touch plastic, energy is conducted away from our fingers only very slowly, so our fingers do not cool and the plastic ‘feels’ warm. (The area round where we touch the plastic will have been warmed up to the temperature of our fingers.)
Differentiation / optional extra activities
• Differentiation: You could provide a description and explanation of the experiment as a series of statements on separate cards, for students to sequence.
• Pass round samples of other materials (e.g. wood, glass, acrylic, expanded polystyrene, copper) similar to the plastic and metal blocks and ask ‘On which of these would ice melt quickly, and on which more slowly?’ If time allows, try it out.
• Further Questions – see below
Taking it further
• You could repeat the practical as a demonstration with temperature probes attached to the blocks. Can students predict how the readings will change? There is a video of this here: www.nationalstemcentre.org.uk/elibrary/resource/2087/thermal-conductivity
Links to related practical activities on Practical Physics
• Comparing the thermal conductivities of different materials www.nuffieldfoundation.org/practical-physics/comparing-thermal-conductivities-different-materials
Further Questions
1 Ice melts more quickly on a metal block than on a plastic one. Would the result be different if ice cubes were placed in a metal box and a plastic box, so that they were completely enclosed?
2 You are supplied with samples of several different materials. How could you adapt this experiment to put them in order, from best conductor to worst conductor?
3 Why do some materials feel warmer to the touch than others? Diamonds are sometimes referred to as ‘ice’ because, if you touch a diamond on your upper lip, it feels cold. What does this tell you about diamond?
Answers
1 The results would be the same, because the metal box would conduct energy in from the surroundings more quickly than the plastic box.
2 Place an ice cube on a sample of each material and time them melting. The best conductor will result in the fastest melting ice cube.
3 When you touch a material which is a good conductor, energy escapes from your finger. This cools the skin and your nerves detect the cold. Diamond shows this effect and so must be a good conductor – its thermal conductivity is higher than any metal.
Assessing learning: Answers
True or false
true / false1 A block of ice will melt when energy escapes from it. / false
Correction: is transferred into
2 Plastics are good conductors of heat energy. / false
Correction: poor/bad (or change conductors to insulators)
3 Conduction is one way that energy is transferred from place to place. / true
Correction:
4 Energy can travel through a solid material by convection. / false
Correction: conduction
5 Energy is conducted from a place where the temperature is higher to a place where the temperature is lower. / true
Correction:
Plastic vs glass