Resource 6: Extend and challenge
Background information / subject knowledge for teacher
Extending the work on floating and sinking to provide an opportunity for differentiation in a real life problem-solving context
This resource provides an extension to Activity 3.
The context and problem
Recycling is important for saving precious resources when the item can’t be reused any more. Suppose you are trying to recycle waste plastics, how can you sort the plastics into different types so you can sell some of them to someone who can reuse them? (Plastic items have a recycling code on them which tells you what they are made of, but you can’t always see this code on a piece of waste plastic.)
One method that recycling companies use to sort mixed plastic waste is flotation: different polymers have different densities, so while some will float in a particular liquid (because the polymer is less dense than the liquid) others will sink (because the polymer is more dense than the liquid). If you use three different liquids – water, saturated salt solution and glycerol (propane-1,2,3-triol) – you can sort most polymers.
The common polymers and liquids (shown in bold) are listed here in order of increasing density:
PP (polypropylene), PE (polyethylene), water, ABS (acrylonitrile butadiene styrene), polystyrene, saturated salt solution, PMMA (polymethyl methacrylate, also called acrylic or perspex), PC (polycarbonate – density varies), glycerol, PC (polycarbonate – density varies), PET (polyethylene terephthalate), PVC (polyvinyl chloride).
How the process works
All the plastic waste is chopped up into small pieces before the batch is added to the first tank (water, the least dense of the three liquids). (This is important because the lid and bottle of many plastic bottles will be made from different materials, and because if you use a whole bottle, it has air in it so you are not looking at the density of the plastic but of the bottle.) All the bits that float are skimmed off, and all the bits that sink go into the next tank, and the process is repeated. (Notice that, depending on what was in the original mix, you might have one type of polymer or you might have two or even three polymers in each of the final, separated groups, and you might have to use other tests to work out what bits were a particular polymer: you can’t tell polyethylene and polypropylene apart by this method because they both float in water.)
Setting a differentiated task
You can control the amount of challenge by:
- the way you word the task
- the materials you provide and how you provide them
- the amount of information or guidance you provide.
Some different ways of setting the task
You will need to provide beakers or bowls for testing samples with each liquid. ‘Samples’ should be small pieces of clean plastic. You will also need to provide something to collect items that sink from the bottom of the container, and some cloths or paper towels to wipe up spills, and so students can wipe samples dry before putting them into the next liquid. It works best if you test samples one at a time, or you could leave students to find that out for themselves…
- Provide some identified samples and some ‘mystery’ samples, and ask students to identify the ‘mystery’ samples. This is a simple comparison or matching task.
- Provide some identified samples and ask students to explore which ones float and which sink in each liquid, then make an identification key. The more polymers, the harder the task.
- Provide unknown samples and the information about relative densities, and ask students to suggest what each sample is. The difficulty depends on what samples you provided and what you tell students about them. Deciding if a sample is x or y is easier than identifying with no possibilities suggested.
- Provide unknown samples and the information about relative densities, and ask students to suggest what each sample is and evaluate the method. This adds a different demand because students have to think about what the strengths and weaknesses are. (Was it easy to make a decision about which samples floated each time? What problems are there if you try to test several samples at once? Could they identify all the samples, or only say a sample ‘might be x or y’?)
Some things you could use as sources for different types of polymer
Note: These are suggestions based on what the polymers are usually used for. Try to find the recycling code on the object to confirm what the polymer is.
PP polypropylene: bottle tops, some cosmetics bottles, yoghurt pots, some food trays
PE polyethylene: bleach or detergent bottles, bottles for still drinks, some cosmetics bottles
PET polyethylene terephthalate: shampoo bottles, fizzy drinks bottles
PVC polyvinyl chloride: plastic pipes and cable sheaths
PS polystyrene: plastic cutlery, ‘foam’ food cartons, drinks cups
PMMApolymethyl methacrylate (acrylic, Perspex): plastic rulers, clear drinks cups.