Question 5
What is a suitable application for polylactic acid (PLA)?
Not quite. A drainage pipe must last for decades in waste water. A biodegradable pipe would break down and leak. QCAA keyed drainage pipe to PVC in 2022 Question 2. In 2025, 6.6% of Queensland students chose (A).
Not quite. Cable insulation has to last as long as the cable. Insulation that breaks down would expose the conductor. QCAA keyed cable insulation to polyethylene in 2020 Question 4. In 2025, 6.0% of Queensland students chose (B).
That is it. Step through the hints below to see why. In 2025, 79.7% of Queensland students chose (C).
Not quite. Laboratory equipment is washed, heated and reused, so it has to last. PLA is designed to break down. QCAA keyed the laboratory beaker to polypropylene in 2023 Question 1. In 2025, 7.4% of Queensland students chose (D).
QCAA’s 2020 marking guide gives PLA’s properties as biodegradable, high tensile strength, non-toxic in solid form and resistant to chemicals.
Biodegradable is the one that sets PLA apart. Ask of each option whether breaking down is a fault or a feature.
Drainage pipes, cable insulation and laboratory equipment all have to last for years. A plastic that breaks down is a fault in every one of them. That rules out (A), (B) and (D).
An implant such as a bone screw is only needed while the body heals. A PLA implant is absorbed after that, so the patient avoids a second operation.
Non-toxic and strong are the other two properties the job needs. That is (C).
Biodegradability is a fault in a pipe, a cable or a beaker, and a feature in an implant that the body absorbs once it has healed. The three wrong options are the jobs QCAA keyed to PVC in 2022, polyethylene in 2020 and polypropylene in 2023. The 2019 syllabus that 2025 students sat listed PLA for “automobile interiors and medical implants”.
The most common wrong answer was (D), chosen by 7.4%, and the three wrong answers drew almost equal shares. Each one is a job that needs a plastic to last.
QCAA did not publish a band for this question. It reads as simple familiar because it asks for one listed application of a named plastic. For 2026, polylactic acid is not in the syllabus at all, and medical implants now sit on the PA6 list. Every option here recycles an earlier paper: drainage pipes from 2022, cable insulation from 2020, laboratory equipment from 2023 and implants from 2020 and 2024. QCAA reuses its own contexts, so I expect the jobs on the 2026 lists, such as gears, helmets, headlamps and implants, to come back as options.
If I had to guess where you might lose marks, it would be reading the 2026 lists as if PLA were still on them. In 2026, medical implants belong to PA6.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2025 QCAA Engineering external assessment, © State of Queensland (QCAA) 2025, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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