Year 9 Science · Unit 2 · Lesson 02

Physical Properties of Materials

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Learning Goals

Match each physical property to its unit and how it is measured.
Calculate density and use it to identify a metal.
Use thresholds in a data table to shortlist materials and justify a final choice.

Warm Up

Match each property to how it is measured

Write the matching letter in the middle column. Each letter is used once.

Property	Letter	Unit or test
Density		A. Unit W/m·K: how fast heat travels through a sample.
Hardness		B. Unit °C: the temperature at which the solid turns to liquid.
Thermal conductivity		C. Unit g/cm³: mass from a balance divided by volume.
Electrical conductivity		D. Unit MPa: the pulling stress at which a sample breaks.
Tensile strength		E. Mohs scale or Vickers test: scratching or denting the surface.
Melting point		F. Unit S/m: how easily an electric current flows through it.

1. Define thermal conductivity and name one material with a high value and one with a low value.

Recall 2 marks

Your Turn

2. A window pane must let light through and must not be scratched by fingernails or coins. Name the two physical properties that matter here, and explain how glass meets each one.

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Table 1. Physical properties of six materials

Material	Density (g/cm³)	Thermal conductivity (W/m·K)	Melting point (°C)
Aluminium	2.70	237	660
Copper	8.96	401	1085
HDPE plastic	0.95	0.5	130
Magnesium	1.74	156	650
Stainless steel	8.0	16	about 1400
Titanium	4.51	22	1668

Source: typical values at room temperature (CRC Handbook; HDPE from supplier data).

3. A solid cube of an unknown metal has sides of 5.0 cm and a mass of 1120 g. Calculate its density, then use Table 1 to identify the metal.

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Density g/cm³

Metal

Go Deeper

Scenario: choosing a piston

A car engine piston moves up and down thousands of times a minute beside burning fuel. The engineer’s thresholds are: density below 3.0 g/cm³ (light), thermal conductivity above 100 W/m·K (carries heat away) and melting point above 600 °C (stays solid near the flame).

4. Use the three thresholds to cross out materials in Table 1. Which materials pass all three? For each material you crossed out, name one threshold it fails.

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Show What You Know

5. Table 1 alone cannot decide between the materials that pass. Recommend one of them for the piston. Use numbers from Table 1, say what extra information you would want before deciding, and state one trade-off.

Analyse 5 marks