Using materials

Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting is an example of corrosion. Both air and water are necessary for iron to rust.

Corrosion can be prevented by applying a coating that acts as a barrier, such as greasing, painting or electroplating. Aluminium has an oxide coating that protects the metal from further corrosion.

Some coatings are reactive and contain a more reactive metal to provide sacrificial protection — for example, zinc is used to galvanise iron.

You should be able to describe experiments and interpret results to show that both air and water are necessary for rusting.

The standard experiment uses three test tubes, each with an iron nail:

• Tube 1: air and water — the nail is half in water and half in air. This is the control, and the nail rusts. • Tube 2: water but no air — the nail is covered with water that has been boiled to drive out dissolved air, with a layer of oil on top to keep air out. The nail does not rust. • Tube 3: air but no water — the nail sits in dry air over a drying agent such as anhydrous calcium chloride, with the tube stoppered. The nail does not rust.

Interpreting it: the nail rusts only in the tube where both are present, so neither air nor water alone is sufficient — both are necessary.

You should also be able to explain sacrificial protection in terms of relative reactivity.

The explanation: the coating metal is more reactive than iron, so it reacts with the air and water in preference to the iron. It corrodes instead of the iron — it is sacrificed — and the iron beneath is protected. Zinc galvanising works this way, and it keeps working even if the coating is scratched, because the zinc is still the more reactive metal in contact with the iron.

That is what separates sacrificial protection from a simple barrier. Paint or grease stops rusting only while the coating is unbroken; a scratch exposes the iron and rusting starts there.

Most metals in everyday use are alloys.

Bronze is an alloy of copper and tin. Brass is an alloy of copper and zinc.

Gold used as jewellery is usually an alloy with silver, copper and zinc. The proportion of gold in the alloy is measured in carats, 24 carat being 100% — pure gold — and 18 carat being 75% gold.

Steels are alloys of iron that contain specific amounts of carbon and other metals. High carbon steel is strong but brittle. Low carbon steel is softer and more easily shaped. Steels containing chromium and nickel — stainless steels — are hard and resistant to corrosion.

Aluminium alloys are low density.

You should be able to recall a use of each of the alloys specified, and to interpret and evaluate the composition and uses of alloys other than those specified given appropriate information.

When an unfamiliar alloy appears in a question, work from the properties in the data you are given to the use, and say which property decides it. That is the same reasoning as the table above: aircraft bodies need low density, cutlery needs corrosion resistance, blades need hardness.

Most of the glass we use is soda-lime glass, made by heating a mixture of sand, sodium carbonate and limestone. Borosilicate glass, made from sand and boron trioxide, melts at higher temperatures than soda-lime glass — which is why laboratory glassware, heated over a flame, is made from it.

Clay ceramics, including pottery and bricks, are made by shaping wet clay and then heating in a furnace.

The properties of polymers depend on what monomers they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced from ethene.

You should be able to explain how low density and high density poly(ethene) are both produced from ethene. They have the same monomer, so the difference is entirely in the conditions: LD poly(ethene) is made at a high temperature and pressure with a small amount of oxygen as a catalyst, giving branched chains that cannot pack closely, so the polymer is less dense and more flexible. HD poly(ethene) is made at a lower temperature and pressure with a different catalyst, giving straight chains that pack closely, so it is denser, stronger and more rigid.

Thermosoftening polymers melt when they are heated. Thermosetting polymers do not melt when they are heated.

You should be able to explain the difference between them in terms of their structures. A thermosoftening polymer consists of individual polymer chains held together only by weak intermolecular forces. Heating gives the chains enough energy to overcome those forces, so the polymer melts — and it can be melted and reshaped again and again.

A thermosetting polymer has cross-links, strong covalent bonds between the chains, holding them in a fixed position. Heating does not break those bonds, so the polymer does not melt; heated far enough, it chars instead.

Most composites are made of two materials: a matrix or binder surrounding and binding together fibres or fragments of the other material, which is called the reinforcement.

You should be able to recall some examples of composites.

Given appropriate information, you should be able to compare quantitatively the physical properties of glass and clay ceramics, polymers, composites and metals, and explain how the properties of materials are related to their uses and select appropriate materials.

Comparing quantitatively means using the numbers you are given — density, strength, melting point — rather than describing materials in words. When selecting a material, name the property that decides it and quote the figure from the table.