Question 12
Analyse the information in the table to determine whether polyvinyl chloride (PVC) or polyethylene is the most suitable material for wastewater pipes in the home. Justify your response with four properties from the table.
| Properties | PVC | Polyethylene |
|---|---|---|
| Mechanical | resistant to abrasion, impact and weathering rigid — limited flexibility, can withstand high pressure | resistant to cracking, impact and environmental stress; strong; durable malleable — highly flexible |
| Young’s modulus | 2.41–4.14 GPa | 1.08 GPa |
| Yield strength | 40.7–44.8 MPa | 26.2–33.1 MPa |
| Tensile strength | 40.7–51.7 MPa | 22.1–31.0 MPa |
| Chemical | excellent chemical resistance (including corrosive chemicals) | good chemical resistance |
| Thermal | good heat resistance | very good heat resistance |
| Manufacturing cost | low | high |
Table adapted by the QCAA from Callister, WD Jr and Rethwisch, DG 2014, Materials Science and Engineering: An introduction, 9th edn, John Wiley and Sons, and Petron Thermoplast 2023, ‘What are the differences between PVC and HDPE?’.
[5 marks]
Six of the seven rows favour PVC, and the seventh does not matter in a home
This is QCAA’s table. Each row is checked in turn and the material it favours is shaded. Watch the counters under the two columns.
Analysing the table means weighing every row against the job. Polyethylene wins on heat, but a home drain never carries anything hotter than boiling water, so PVC is the answer.
Choose four rows, write each one, then make the decision
Tap a row of the table to use it, then pick how you would write it. The checklist shows which marks your answer earns.
Your sentences appear here as you write them.
Work through the solution one mark at a time
Try each step yourself before you reveal it. Four marks are for properties described from the table and one is for the decision.
Start with the row that matters most for a drain. PVC is excellent, polyethylene only good, and the table names corrosive chemicals.
“PVC is resistant to corrosive chemicals, making it more suitable for use in wastewater pipes in the home where it is likely to come into contact with corrosive chemicals, such as bleach or drain cleaner.”
Marker: describes properties from the table appropriately using wording indicating that PVC has greater chemical resistance than polyethylene
Polyethylene wins this row. Say so, then say why it does not decide the case.
“Although polyethylene has greater heat resistance than PVC, the pipes won’t be exposed to anything hotter than boiling water in the home so PVC will be good enough.”
The notes say properties that favour polyethylene “may be awarded marks as this shows the information in the table has been analysed”.
Marker: describes properties from the table appropriately using wording indicating that polyethylene has better thermal properties so is more heat resistant than PVC, but this is not an important property for this application
The Young’s modulus row and the mechanical row say the same thing: PVC is the stiffer material. Use the numbers.
$$\begin{aligned}E_{\text{PVC}}&=2.41\text{ to }4.14\text{ GPa}\ >\ E_{\text{PE}}\\&=1.08\text{ GPa}\end{aligned}$$
“PVC has a higher Young’s modulus than polyethylene, which means it will be more rigid/less flexible, making it suitable for long lengths of pipe under the sink, floor or ground.”
Because both rows are rigidity, the marking guide has one dot point for them. Quoting both counts once. The notes also list PVC’s higher yield strength or tensile strength, and values for Young’s modulus, yield strength or tensile strength, as appropriate descriptions, so those rows work as a fourth property too.
Marker: describes properties from the table appropriately using wording indicating that PVC is more rigid or less flexible than polyethylene
The last row is the easiest mark on the page. Low against high.
“PVC is cheaper than polyethylene as the manufacturing cost is lower.”
Marker: describes properties from the table appropriately using wording indicating that PVC is cheaper than polyethylene
State the decision in words. Implying it is not enough.
“The combination of these properties makes PVC a more suitable material for use in the home.”
“… more properties favour PVC. Therefore, the response must determine that PVC is most suitable to be awarded that mark.”
Marker: determines that PVC would be most suitable for wastewater pipes in the home
Each phrase of the question gave you something
The 2024 subject report says effective responses determined that PVC was most suitable and described four properties identified from the table. It praised the chosen excerpt for working out which properties in the table were most relevant to the application.
QCAA did not publish a band for this question. It reads as simple familiar because the table supplies every property and six of the seven rows point the same way. PVC and polyethylene are no longer named Unit 4 plastics for 2026, so you are not expected to know their properties. The method transfers unchanged: a 2026 table comparing ABS, PC or PA6 is answered the same way, four comparative properties plus the decision.
If I had to guess where you might lose marks, it would be writing each property about PVC alone, such as “PVC has excellent chemical resistance”, with no comparison to polyethylene. Every dot point in the guide is a comparison. The other loss is spending two of your four properties on rigidity by quoting both the mechanical row and the Young’s modulus row.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording, any figures and marking-guide steps are from the 2024 QCAA Engineering external assessment, © State of Queensland (QCAA) 2024, 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.
Spot a mistake on this page? Report it