C2

C2

1. natural and synthetic materials

   • all materials are made up of chemicals
     • chemicals are made up of atoms or groups of atoms bonded together
       • iron is a chemical element
         • made up of iron atoms
       • water is a chemical
         • made up of lots of water molecules
         • water molecules contain 2 chemical elements
           • hydrogen
           • oxygen
       • molecule is a group of atoms bonded together
     • some materials are mixtures of chemicals
       • a mixture contains different substances that are NOT chemically bonded together
         • for example, rock salt is a mixture of two compounds
           • salt and sand
   • some of these materials occur naturally
     • a lot of the materials we use come from other living things
       • materials from plants
         • wood and paper both made from trees
         • cotton comes from the cotton plant
       • materials from animals
         • wool comes from sheep
         • silk is made by the silkworm larva
         • leather comes from cows
   • others are synthetic
     • man-made
     • all rubber used to come from sap of the rubber tree
       • still get a lot of rubber this way (e.g. for car tyres)
       • can make rubber in a factory
         • pros: can control its properties
           • makes it suitable for different purposes
     • clothes
       • fabrics like nylon and polyester
       • properties can be controlled
     • most paints
       • ixture of man-made chemicals
       • pigment and stuff that holds it together are designed to be tough
     • raw materials used to make synthetic materials come from Earth's crust

2. materials and properties

   • different materials have different properties
   • MELTING POINT
     • most materials that are pure chemicals have a unique melting point
     • temperature where the solid turns to liquid
       • e.g. water's melting point is 0 degrees celcius
   • STRENGTH
     • strength is how good a material is at resisting force
     • can judge how strong it is by how much force is needed to either break it or deform it
     • two types of strength
       • tensile/tension strength
         • how much a material can resist a pulling force
         • things like ropes and cables need a high tensile strength or they'd snap
       • compressive strength
         • how much a material can resist a pushing force
         • building materials like bricks need good compressive strength or they'd be squashed by the weight of the bricks above them
   • STIFFNESS
     • a stiff material is good at not bending when a force is applied to it
     • different to strength
       • a bendy material can still be strong if a big force doesn't permenately deform it
     • materials like steel are very difficult to bend
       • they're very stiff
     • some kinds of rubber are very strong but they bend and stretch very easily
       • not very stiff
   • HARDNESS
     • hardness of a material is how difficult it is to cut into
     • hardest material found in nature is diamond
       • only material that can cut a diamond is another diamond
       • diamonds can cut most other materials
         • many industrial drills have diamond tips
   • density
     • a material's mass per unit volume
     • don't confuse with mass or weight
     • air is not very dense
     • gold is very dense
     • objects that are less dense than water will float
       • e.g. ice
     • objects that are more dense than water will sink

3. materials, properties and uses

   • the possible uses for a material depend on its properties
   • when choosing a material to use in a product, you need to think about its properties
     • PLASTICS
       • can be fairly hard, strong and stiff
       • some are fairly low density
         • good for lightweight goods