Materials are classified by identifying whether their properties describe inherent physical characteristics, reactions with other substances, or responses to applied forces. These are called physical properties, chemical properties, and mechanical properties, respectively.
In A3.1 Material classification and properties, classification means arranging properties into groups according to shared characteristics. Designers use these groups to compare materials and select the most suitable material for a product.
| Property category | What it describes | Examples |
|---|---|---|
| Physical | Characteristics that can be observed or measured without changing the material's chemical composition | Density, thermal conductivity, electrical conductivity, thermal expansion, magnetism, and transparency |
| Chemical | How a material reacts with substances or environmental conditions, potentially changing its chemical composition | Corrosion resistance, oxidation resistance, chemical resistance, flammability, and resistance to ultraviolet degradation |
| Mechanical | How a material behaves when forces or loads are applied | Tensile strength, compressive strength, stiffness, hardness, toughness, elasticity, and ductility |
For example, aluminium has relatively low density, which is a physical property. Its formation of a protective oxide layer gives it corrosion resistance, a chemical property. Its ability to deform plastically before fracture demonstrates ductility, a mechanical property.
These classifications support material selection. A cooking-pan handle requires low thermal conductivity, while a bridge component requires high tensile or compressive strength and resistance to environmental degradation. Designers must consider several property categories because one property rarely determines suitability by itself.
A common misconception is that physical properties include every measurable property. Mechanical properties can also be measured, but they specifically describe behaviour under forces. Another misconception is confusing strength with stiffness: strength concerns resistance to failure, whereas stiffness concerns resistance to elastic deformation.
For an IB exam response, classify each named property correctly and then explain how it affects performance in context. When asked to justify a material choice, connect a specific property to the product's function, environment, manufacturing requirements, or expected load.