Properties of materials

Every material around you can be described with words. Glass is transparent and brittle. Rubber is flexible and waterproof. Steel is hard and strong. Each of these describing words is a property.

A property is a word that tells you what a material is like or how it behaves. Properties are how scientists compare materials and decide which one is best for a job.

It helps to sort properties into groups. For example:

• How it feels and looks — rough or smooth, shiny or dull, see-through or not.
• How it behaves — does it bend, snap, float, conduct heat?

The same material always has the same properties. Wood is always less dense than steel; glass is always brittle. Because properties are reliable, you can predict how a material will behave before you even use it.

Knowing the properties of materials is the first step to choosing the right one — which is what this whole topic is really about.

Some of the most useful properties are physical properties — ones you can test without changing the material into something new.

• Hardness — how well a material resists being scratched or dented. A diamond is extremely hard; chalk is soft.
• Strength — how much pulling, pushing or weight a material can take before it breaks. Steel is strong; dry spaghetti is weak.
• Flexibility — how easily a material bends without snapping. Rubber is flexible; glass is stiff and brittle.
• Density — how much mass is packed into a certain amount of space. Lead is dense and feels heavy for its size; foam is low in density and feels light.

A handy test for density: drop the material in water. Materials less dense than water (like wood and cork) float; materials denser than water (like a steel nail) sink.

These four properties come up again and again, so it is worth learning what each one means and a clear example of each.

Some materials let heat or electricity travel through them easily, and some do not.

• A conductor lets heat or electricity pass through it easily. Most metals — copper, steel, aluminium — are good conductors.
• An insulator does not let heat or electricity pass through easily. Plastic, wood, rubber and glass are good insulators.

This is why electrical wires have a copper core (the conductor) wrapped in a plastic coating (the insulator that keeps you safe). And it is why oven gloves are made of thick fabric — to stop heat reaching your hands.

Hold a magnet near different objects and you will find something surprising — most things are not pulled towards it.

A magnetic material is attracted to a magnet. A non-magnetic material is not. The surprising part is that only a few materials are magnetic:

• Iron, steel (which is mostly iron), nickel and cobalt are magnetic.
• Most other metals — copper, gold, aluminium — are not magnetic.
• Non-metals such as plastic, wood, glass and paper are never magnetic.

A common mistake is to think "all metals are magnetic". They are not — try a magnet on a copper coin and an aluminium can to prove it.

Being magnetic is a very useful property. It is used to separate iron and steel from rubbish at recycling centres, to make fridge doors close with a satisfying snap, and inside the speakers and motors in your home.

Now for the big idea: every object is made from materials chosen on purpose. Engineers ask, what does this part need to do? — then pick a material whose properties suit that job.

Look at a saucepan. The body is metal because metal conducts heat well, so the food cooks quickly. The handle is plastic because plastic is a heat insulator, so you can pick it up without burning your hand.

More examples:

• A window is glass — it is transparent so light passes through.
• A raincoat is plastic — it is waterproof and flexible.
• A bridge is steel — it is strong enough to hold heavy loads.

Understanding properties of materials helps you in many places ahead:

• Changes to materials — you will see how heating, mixing and dissolving change a material's properties.
• Electricity — conductors and insulators are the foundation of building electrical circuits.
• Forces and structures — strength, flexibility and density decide how bridges, buildings and vehicles are designed.
• Design and technology — every product you ever design will mean choosing materials on purpose.

Whenever you pick up an object, try asking: what is this made of, and why was that material chosen? That single question turns the world around you into a science lesson.