The big idea: Selection starts at the requirement, never at the material.
Each requirement names a property, each property belongs to a family — physical, chemical or mechanical — and the property has a number you can look up or measure.
Requirements narrow the field. Step through and watch the candidate list shrink as each one is applied.
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| Requirement on the stool | Property needed | Family |
|---|---|---|
| The seat must not bend permanently under a heavy student | Stiffness, and yield strength | Mechanical |
| It must survive a student dropping onto it | Impact strength — toughness | Mechanical |
| The frame is mopped daily and splashed with dilute acid | Corrosion resistance | Chemical |
| A student must be able to carry two of them | Density, and therefore mass for that stiffness | Physical |
| It must not conduct if a live wire touches the frame | Electrical conductivity | Physical |
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How a selection actually runs
Turn each requirement into a property with a number
"Strong enough" is not a requirement. "No permanent bend under 120 kg applied at the seat edge" is, and it names stiffness and yield.
Apply the ESSENTIAL properties as a filter
Anything that fails one is out, however attractive it is elsewhere. That usually removes most of the list in one pass.
Rank the survivors on what is left
Mass, cost, availability, sustainability and aesthetics all act here — on a shortlist, not on everything.
Check the property is the right one
Strength is the load before it breaks; stiffness is how little it bends first. A stool that bends visibly and never breaks still fails the user.
The pair students get wrong here: Strength and stiffness are not the same thing.
A seat that flexes alarmingly under a student has a stiffness problem, and swapping to a stronger material with the same modulus will not fix it. Section shape and depth fix stiffness; material choice alone often does not.
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| Candidate | Meets the essentials? | Where it wins | Where it loses |
|---|---|---|---|
| Birch plywood seat | Yes | Stiff for its mass, warm to sit on, repairable in the workshop | Swells if water gets into an unsealed edge |
| Polypropylene seat | Yes | Tough, cheap in volume, coloured right through | Flexible unless ribbed, and it creeps under a long load |
| Mild steel frame | Only if coated | Cheap, available in every section, easy to weld | Corrodes where the coating is chipped by trolleys |
| Stainless steel frame | Yes | Survives daily mopping and acid splashes with no coating | Dearer per metre, and harder to cut in a school workshop |
Answer in the language of the property: "Plywood is good for the seat" scores nothing.
"Birch plywood is stiff for its mass, so a 15 mm seat carries a 120 kg student with no visible deflection while the stool stays light enough to carry two at once" names the property, the number and the consequence.
Everyday materials and the properties that earn their place
PET: drinks bottles, food boxes
Lightweight, transparent, food-safe, waterproof, tough when dropped, easy to mould and recyclable.
Aluminium alloy: aircraft, bike frames
Low density with good strength, so parts are light (less fuel), and it resists corrosion outdoors.
Steel: car bodies, frames
Strong, stiff and cheap, but HEAVY (more fuel to move) and it RUSTS unless coated.
Thermoset: pan handles, kettle bases
Sets once and never softens again, so it keeps its shape near heat and insulates the hand.
Cast iron: hand-plane bodies, machine beds
Hard, dense (heavy, so it sits steady and damps vibration) and keeps a flat, wear-resistant surface.
Rubber: soles, seals, grips
Flexible, waterproof, grippy and durable; it can be moulded and coloured.
Glass: desk tops, shelves
Hard and easy to wipe, but brittle: it chips or shatters, and it shows every fingerprint and smear.
How this is tested — identifying appropriate materials from physical, chemical and mechanical properties. It comes up two ways:
Paper 1 — multiple choice
- Match a stated requirement to the property family it belongs to.
- Identify which candidate fails a stated essential property.
Paper 2 — analysing a product
- Identify appropriate materials for parts of a named product.
- Explain why a property rules a candidate material out.
The trap: Naming a material with no property behind it. Every mark here needs requirement, property and consequence — the material name on its own is worth nothing.
A folding stool for a school science lab needs a seat, a frame and feet. Identify an appropriate material for each, referring to the properties required.
Model answer plan
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