Choose a material in two passes. First remove every material that fails a must-have criterion, then rank the ones left on the preferred criteria.
This lesson starts the set on materials and technology through practical evidence. All materials here are fictional, so the table is your only source of facts.
Why two passes?
A quick look at a table tempts you to pick the material with the best-looking numbers. That can give you a material that fails the one requirement the job cannot do without.
Two passes prevent this. The first is pass or fail. The second is better or worse.
Worked example: a pan handle
An original question supplies this table. The handle will be used on a pan that reaches 220°C. The handle must not become too hot to hold, and lower mass is preferred.
| Material | Highest safe temperature | Conducts heat | Mass of a 100 cm³ piece |
|---|---|---|---|
| Alloy R | 400°C | Very well | 450 g |
| Polymer S | 150°C | Poorly | 120 g |
| Ceramic T | 1 200°C | Poorly | 300 g |
Pass 1: must-have criteria.
- Must survive 220°C. Alloy R (400°C) passes. Polymer S (150°C) fails, because 150°C is below 220°C. Ceramic T (1 200°C) passes.
- Must conduct heat poorly. Alloy R conducts very well, so the handle would carry heat to the hand, and it fails. Ceramic T passes.
Only Ceramic T remains.
Pass 2: preferred criteria. With one material left, there is nothing to rank. If two had passed, the lower mass would decide.
Answer: Choose Ceramic T. It survives 220°C and conducts heat poorly, while Polymer S is unsafe above 150°C and Alloy R would carry heat to the hand.
Assumption: the table gives the properties of the same quality of each material, and the pan does not exceed 220°C.
The mistake that costs marks
The slip is to pick Polymer S because it has the lowest mass, without noticing that it fails the temperature requirement.
| Wrong | Right |
|---|---|
| “Polymer S is the lightest, so choose it” | “Polymer S is lightest, but its safe temperature of 150°C is below 220°C, so it fails” |
| “Alloy R is the strongest” | “Alloy R conducts heat very well, so it fails the handle requirement” |
| “Ceramic T is best” | “Ceramic T passes both must-haves, so it is chosen” |
The repair is to write the pass or fail verdict for each material before mentioning any advantage.
Check yourself
An original table describes three covers for an electric cable. The cover must be an electrical insulator, must bend without breaking, and must survive 80°C. Lower cost is preferred.
| Material | Insulator | Bends | Safe temperature | Cost per metre |
|---|---|---|---|---|
| U | Yes | Yes | 60°C | RM1 |
| V | Yes | Yes | 105°C | RM2 |
| W | Yes | No | 300°C | RM3 |
Choose a material and give the reason.
Answer
Pass 1: U fails because 60°C is below 80°C. W fails because it does not bend. V passes all three must-haves: insulator, bends and 105°C is above 80°C.
Only V remains, so choose V. The cost of RM2 per metre is acceptable, and no ranking is needed. Assumption: the cable never exceeds 80°C.
What to study next
The next lesson, explaining a trade-off without declaring one material best for every use, covers the case where two materials pass and the decision is close. The graph evidence and fair-comparison lab offers more supplied data to practise on.
To rehearse two-pass answers with a teacher, see online one-to-one Science tuition.