Question 2
Waste water drainage pipe is commonly manufactured using
Not quite. Nylon is strong and wear resistant, which is why it makes gears and bearings. In a drain it absorbs water and swells, and it costs far more than a drain pipe needs. In 2022, 4.5% of Queensland students chose (A).
Not quite. Polypropylene resists chemicals and water, and it is used for some waste fittings, so this is the strongest distractor. PVC is much more rigid, so it holds its line over long runs, and it is the pipe commonly used. In 2022, 23.2% of Queensland students chose (B).
Not quite. Polylactic acid is biodegradable. A drain pipe has to last for decades in water, so a plastic designed to break down is the wrong choice. In 2022, 3.6% of Queensland students chose (C).
That is it. Step through the hints below to see why. In 2022, 68.2% of Queensland students chose (D).
A drain pipe carries waste water from sinks, showers and toilets for decades. It runs in long lengths that have to hold a steady fall, and it meets bleach and drain cleaner.
So it needs to be rigid, chemical resistant and unaffected by water. QCAA’s 2024 marking guide chose PVC for wastewater pipes in the home on chemical resistance and rigidity.
Polylactic acid is biodegradable, so it fails the demand to last. That rules out (C).
Nylon absorbs water and swells, and it is an expensive engineering plastic meant for gears and bearings. That rules out (A).
Both resist chemicals and water. The difference is stiffness. QCAA’s 2024 table gives PVC a Young’s modulus of 2.41 to 4.14 GPa against 1.08 GPa for polyethylene, and polypropylene sits much nearer polyethylene.
Rigid PVC holds its line over long runs, and it is cheap to make. That is (D).
Waste water drainage pipe has to be rigid, resist household chemicals and last for decades in water. PVC does all three and is cheap to make, which is why QCAA chose it again for wastewater pipes in 2024 Question 12. The 2019 syllabus that 2022 students sat listed PVC for “water/waste drainage pipe”.
The most common wrong answer was (B), chosen by 23.2%. Polypropylene shares PVC’s chemical resistance, so a student who stopped at that demand could not separate them. Stiffness is the demand that does.
QCAA did not publish a band for this question. It reads as simple familiar because it is one recall step from a household job to its plastic, with no stimulus to analyse. For 2026, PVC, polypropylene and polylactic acid are no longer named in Unit 4, and PLA is not in the syllabus at all. Nylon stays as PA6, with machine parts among its uses. What transfers is the reasoning QCAA rewarded in 2024 Question 12: name what the pipe needs, then pick the material whose properties meet it.
If I had to guess where you might lose marks, it would be stopping at chemical resistance. Polypropylene passes that test too, and almost a quarter of Queensland students chose it. Test every demand before you commit.
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 2022 QCAA Engineering external assessment, © State of Queensland (QCAA) 2022, 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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