Physical Properties of Materials
In 2024, NASA's Orion capsule used aluminium alloy 2219, just 2.7 g/cm³, to survive re-entry at 11 km/s without burning up.
Printable Worksheets
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Q1 · Think about a metal spoon and a wooden spoon, what differences would you notice if you left both in a pot of boiling soup?
Q2 · Why do you think engineers need to know the exact physical properties of a material before they use it to build something like a bridge or an aircraft wing?
● Know
- The six key physical properties of materials
- How each property is measured
- Common materials and their physical property values
● Understand
- Why density is mass per volume, not total mass
- How thermal and electrical conductivity are related to structure
- Why physical properties determine material applications
● Can do
- Define and measure physical properties
- Match materials to applications based on their properties
- Identify the misconception that denser materials are always heavier
Hold a golf ball in one hand and a ping-pong ball in the other: the golf ball feels much heavier even though both are about the same size, because more mass is packed into the same volume. Density is the mass of a material per unit of volume, calculated using $\rho = m/V$. It is measured in g/cm³ or kg/m³. To measure density in the lab: measure mass on a balance, then find volume either by calculation (length × width × height for regular shapes) or by water displacement for irregular objects. A material is denser when the same space contains more mass, osmium (22.6 g/cm³) vs water (1.0 g/cm³).
Hardness is resistance to scratching or indentation. The Mohs scale ranks hardness from 1 (talc, scratched by a fingernail) to 10 (diamond, scratches everything). In the lab, a simple scratch test compares samples, the material that is scratched has lower hardness. Industrial hardness is measured with the Vickers or Rockwell tests, which press a diamond indenter into the surface under a known load and measure the dent size.
A 100 g rock sample displaces 40 cm³ of water in a measuring cylinder. Density = 100 ÷ 40 = 2.5 g/cm³. Quartz has a density of 2.65 g/cm³ and Mohs hardness of 7, it can scratch glass (hardness ~5.5) but not a steel file (hardness ~6.5).
Geologists at the CSIRO Australian Resources Research Centre in Perth use density measurements to identify ore minerals in rock cores drilled from Western Australian mine sites, high-density zones signal iron, gold, or nickel deposits worth billions of dollars.
Thermal conductivity measures how readily a material transfers heat energy. It is measured in watts per metre per kelvin (W/m·K). Metals have high thermal conductivity (copper: 401 W/m·K; steel: ~50 W/m·K) because their delocalised electrons carry kinetic energy through the lattice. Non-metals and polymers have low thermal conductivity (wood: ~0.15 W/m·K; air: 0.026 W/m·K), this is why they are used as insulators.
Electrical conductivity measures how easily electric current flows through a material. Again, metals conduct well because of free electrons. Silver (6.3 × 10⁷ S/m) conducts better than copper (5.96 × 10⁷ S/m), but copper is used for most wiring because it is far cheaper. Ceramics and most plastics have near-zero electrical conductivity, they are insulators, critical for safely covering wires and components.
A ceramic oven dish (thermal conductivity ≈ 1 W/m·K) heats food slowly and evenly; a copper base saucepan (401 W/m·K) heats almost 400× faster. Both are used in kitchens but for entirely different cooking tasks requiring different thermal properties.
Australian homes in NSW use bulk insulation (glass wool or polyester batts) with thermal conductivity of about 0.04 W/m·K in wall cavities to reduce heat transfer, cutting heating and cooling energy costs by up to 40% according to the NSW Department of Planning.
Engineers use materials data tables to compare candidates systematically. Each row is a material; each column is a property (density, hardness, melting point, conductivity, cost). To use a data table: (1) identify which properties are critical for your application, (2) set a minimum or maximum threshold for each property, and (3) eliminate any material that fails a threshold. What remains are the viable candidates.
Reading across rows reveals trade-offs: titanium is lighter than steel (density 4.5 vs 7.9 g/cm³) and equally strong, but costs 20× more. Reading down columns reveals which material is best for a single property. The skill is combining multiple columns to identify the material with the best overall profile, rarely is any single column decisive on its own.
Choosing a heat sink for a computer processor: candidates must have density < 5 g/cm³ (lightweight), thermal conductivity > 150 W/m·K, and cost < $10/kg. From a standard data table, aluminium (2.7 g/cm³, 237 W/m·K, ~$2/kg) satisfies all three criteria; copper fails on density and cost.
CSIRO's Granta Design database lists over 3000 materials with up to 60 properties each. Australian engineering university students from UNSW to the University of Melbourne use this tool in their first-year materials courses to practise data-driven selection.
In a materials data table, each row is a and each column is a property such as density or hardness. To use the table, first identify which properties are for your application. Next, set a minimum or maximum for each critical property. Then eliminate any material that to meet a threshold, leaving only the viable candidates. Reading across a row reveals for example, titanium is lighter than steel but costs far more.
At the start of this lesson, you heard about a 1 cm cube of osmium, the densest natural element at 22.6 g/cm³, being so dense it could punch through concrete if dropped from a great height, while a penny would just flutter down harmlessly. That dramatic difference comes entirely from one physical property: density.
Now that you've worked through the lesson, how has your understanding of density, conductivity, and hardness changed? Could you now explain why osmium is so dangerous compared to a penny, and name at least two other physical properties that matter just as much in engineering?
Q1. Define density and explain why a small piece of lead feels heavier than a large piece of styrofoam even if the styrofoam has more mass.
Q2. A saucepan has a metal base and a plastic handle. Using physical properties, explain why each material is appropriate for its specific part of the saucepan.
Q3. Compare the physical properties of copper and rubber. Explain how each material's properties make it suitable for different parts of an electrical cable.