Year 9 Science · Unit 2 · Lesson 1
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Learning Goals
Odd one out
Circle the odd one out in each group. Then give your reason in the last column.
| # | Group | Odd one out and reason |
|---|---|---|
| 1 | density · melting point · flammability · hardness | |
| 2 | reacts with acid · rusts in moist air · burns in oxygen · conducts heat | |
| 3 | tensile strength · reacts with water · electrical conductivity · colour | |
| 4 | wool · timber · cotton · nylon |
1. Explain why “burns in air” is a chemical property of wood, but “floats on water” is not.
Order the steps
These four notes come from an engineer choosing the material for the face grille of a cricket helmet. Number them 1 to 4 in the order of the material selection process.
| Order | Engineer’s note |
|---|---|
| Compare steel, titanium and aluminium alloy grilles against the list of needs. | |
| Decide what the grille must do: stop a cricket ball (about 160 g) from reaching the batter’s face. | |
| Check the price and supply: titanium grilles cost several times more than steel ones. | |
| List the properties needed: high strength, low density, and no corrosion from sweat and rain. |
Scenario: racing bicycle frames
A bike maker is comparing three alloys for a racing frame. The strength-to-weight ratio is the tensile strength divided by the density: the higher it is, the stronger a frame can be for its mass.
| Metal | Density (g/cm³) | Tensile strength (MPa) | Strength-to-weight ratio |
|---|---|---|---|
| Steel (4130) | 7.85 | 670 | 85 |
| Aluminium (6061) | 2.70 | 310 | |
| Titanium (Ti-6Al-4V) | 4.43 | 950 | 214 |
Source: typical published values for these alloys. Ratios rounded to whole numbers.
2. Calculate the strength-to-weight ratio of the aluminium alloy. Write it in Table 1.
Answer MPa per g/cm³
3. Which alloy in Table 1 gives the lightest frame for the same strength? Most racing bikes are still made of aluminium alloy or carbon fibre instead. Use step 4 of the selection process to explain why, and name the trade-off the buyer accepts.
4. Screws that hold a broken bone together must stay in the body for years. They could be made from stainless steel (8.0 g/cm³) or titanium alloy (4.43 g/cm³). Recommend one, using the four steps of material selection.