7.3 Materials, Properties & Recycling

Properties: The Words Scientists Use to Describe Materials

A material's properties are the ways it behaves, and choosing a material for a job is really choosing a set of properties. This theme runs through the whole ICAS Science range - the properties of different materials is a named topic on the Introductory paper - and it keeps returning in more applied forms on the later papers. Here are the properties you should be able to name and recognise:

• Strength - how much force a material can take before breaking
• Flexibility - how easily it bends without snapping
• Thermal conductivity - how well it carries heat (a metal pot versus its wooden spoon)
• Electrical conductivity - how well it carries electricity (the copper inside a wire versus its plastic coating)
• Transparency - whether light passes through it (window glass)
• Magnetism - whether a magnet attracts it (steel, but not aluminium or plastic)
• Absorbency - how well it soaks up liquid (a cotton towel)
• Density - how much mass is packed into a given volume, which explains why some things float and others sink

Materials that let heat or electricity pass through easily are called conductors; materials that block them are insulators. Most good designs need both: a saucepan has a metal base to conduct heat and a plastic handle to insulate your hand from it.

Natural and synthetic materials

Natural materials come from plants, animals or the ground - wood, cotton, wool, stone and natural rubber. Synthetic materials are made by people through chemical processes - plastics such as polyethylene, and fabrics such as nylon and polyester. Some materials sit in between: glass is made by melting sand, and steel is refined from iron ore, so they start natural but need heavy processing.

Comparing common materials

The golden rule is that no material is simply better than another - each one is better for a particular job. An ICAS stimulus will usually describe a job and then ask which property matters most or which material fits best.

Recycling: Giving Materials Another Life

Recycling - a named topic on Papers A and B - means collecting used materials and processing them into new products instead of burying them in landfill. In Australia the yellow-lidded bin typically accepts:

• paper and cardboard
• glass bottles and jars
• steel and aluminium cans
• many hard plastics (the number inside the triangle symbol tells you the type)
• food and garden scraps, which are recycled by composting

Why recycling matters

Recycling saves raw materials and reduces landfill, but its biggest win is energy. Making aluminium from its ore, bauxite, takes enormous amounts of electricity. Melting down a used can takes only about five per cent of that energy, so every recycled can saves roughly 95 per cent of the energy cost. Better still, aluminium can be recycled again and again without losing quality - a can returned today can be back on a shelf as a brand-new can within weeks.

Plastics are trickier. There are many different types that cannot simply be melted together, so they must be sorted first. Even then, recycled plastic is usually made into lower-grade products - a drink bottle might become carpet fibre or a park bench rather than a new bottle. This is called downcycling, and it is why reducing and reusing plastic matters even more than recycling it. The contrast makes a tidy exam comparison: the aluminium loop stays at the same quality forever, while the plastic loop usually steps down in quality each time around.

Choosing materials for a design brief

ICAS loves applied questions in which you must pick a material for a job and justify the choice. Try these three:

• A raincoat must be waterproof, flexible and light. A plastic-coated synthetic fabric such as polyester fits all three. Wool fails on absorbency - it soaks up water and grows heavy - and glass or metal obviously fail on flexibility.
• A lunchbox must be light, strong, washable and waterproof. Plastic wins: glass is too heavy and brittle for a school bag, wood rots when washed, and metal dents and costs more.
• A power line must conduct electricity extremely well, and be strong yet light enough to hang between poles. Aluminium and copper are the standouts; rubber, glass and plastic are insulators and would stop the current completely.

Notice the routine: list the properties the job demands, then match a material to them, then check the rejected options against the property they fail. That three-step routine is exactly how to answer the stimulus questions on the paper.