Sorting plastics for recycling by floating and sinking

The idea

Common plastics have different densities, so liquids of known density sort them into groups, the same float–sink idea used to separate mixed plastics for recycling.

What you need

• Samples about 1 cm square, cut by the teacher, from items with resin codes: 1 PET (drink bottle), 2 HDPE (milk bottle), 3 PVC (pipe offcut or blister pack marked 3), 4 LDPE (squeeze bottle), 5 PP (takeaway container), 6 PS (clear disposable cup marked 6)
• Tap water; a salt solution made by dissolving table salt in water until 100.0 mL weighs 115 g (density 1.15 g/mL)
• 2 beakers, a glass stirring rod, 1 drop of detergent per beaker to release air bubbles
• Balance reading to 0.1 g and a 100 mL measuring cylinder
• Safety glasses

How to do it

1. Check the salt solution: weigh exactly 100.0 mL and adjust with salt or water until it weighs 115 g.
2. Drop one of each sample into the beaker of water with a drop of detergent, stir, and wait 1 minute.
3. Record which samples float and which sink.
4. Move the sinkers into the salt solution, stir, wait and record again.
5. Compare each result with the resin code and the density table.
6. Explain how a recycling plant could use two tanks to separate a mixed load of flakes, and which plastics it could not separate this way.

What you should see

In water (1.00 g/mL) PP (0.89–0.91 g/cm³), LDPE (0.91–0.93) and HDPE (0.94–0.96) float; PS (1.04–1.11), PVC (1.20–1.55) and PET (1.38–1.40) sink. In the 1.15 g/mL salt solution PS floats while PVC and PET sink, so two liquids sort the samples into three groups. Separating PP, LDPE and HDPE needs liquids between 0.89 and 0.96 g/mL; the Royal Society of Chemistry uses ethanol–water mixtures for that step, which are flammable and left out here. A sample with trapped bubbles floats wrongly until stirred, and plastics with mineral fillers can sink. The learner knows it worked when every result matches the density table for its resin code.

What changes

What you change

liquid density (1.00 or 1.15 g/mL)

What you measure

float or sink for each plastic

What you keep the same

• sample size about 1 cm²
• bubbles removed by stirring
• 1 minute before recording
• the same temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• All plastics float (several are denser than water).
• The recycling number says how recyclable an item is (it identifies the resin).
• A heavy piece sinks and a light piece floats (density, not size, decides).

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Technology 7–8 Syllabus (2023), NESA. Current: taught from 2026. Code read from the outcomes page on 2026-09-22.TE4-MSC-01TE4-SDP-01
• Technology Mandatory 7–8 Syllabus (2017), NESA. Outgoing: replaced by the Technology 7–8 Syllabus (2023) from 2026 and not available after December 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.TE4-9MA
• Australian Curriculum v9AC9TDE8K06

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/tas/technology-7-8-2023/outcomes
2. www.nsw.gov.au/education-and-training/nesa/curriculum/tas/technology-mandatory-7-8-2017
3. edu.rsc.org/experiments/identifying-polymers-by-density/385.article
4. education.nsw.gov.au/teaching-and-learning/curriculum/tas/planning-programming-and-assessing-tas-7-10/technology-7-8