Cement and concrete
Thermal decomposition is a reaction where one substance breaks down on heating to give at least two new substances.
Calcium carbonate (limestone) thermally decomposes when heated;
calcium carbonate= calcium oxide = carbon dioxide
Cement is made when limestone is heated with clay
Concrete is made by mixing cement with sand and small stones with water.
reinforced concrete is a composite material which has steel rods or meshes running through it. Composites contain at least two materials that can still be distinguished.
More on reinforced concrete
Concrete is strong under compression (squashing force) but weak under tension (pulling force).
if heavy loads are applied to a beam, the concrete will bend. This creates tension and compression. The tension cracks the concrete.
Reinforced concrete is harder and more flexible than concrete, so is a better construction material.
Impure copper can be purified in the laboratory using an electrolysis cell.
Advantages of recycling copper are that;
-it has a fairly low melting point so the energy cost to melt is low; - it reduces the need for mining, saving reserves and the enviromental problems caused by mining;- it keeps the cost of copper down.
-the small amounts used in electrical equipment are difficult to seperate; -valuable 'pure' copper scrap must not be mixed with less pure scrap, such as solder; - the actual seperating process may produce pollution; - a lot of copper is thrown away as it is difficult to pursuade people to recycle it.
in the purification of copper by electrolysis, an electrolyte of copper (ii) sulfate sollution is used.
The impure copper is the anode and a sheet of pure copper is used for the cathode;
-the positive anode loses mass as the copper dissolves.
-the negative cathode gains mass as the copper dissolves.
The concentration of the copper (ii) sulfate electrolyte stays the same because as the impure copper anode dissolves, pure copper is plated on to the cathode at the same rate.
At the anode, the Cu atoms lose elctrons to form Cu ions. This is called oxidation. The electron half equation is;
At the cathode,copper is plated when Cu ions gain electrons. This is reduction. The electron half equation is;
Alloys are mixtures containing at least one metal.
Alloys have different properties and we match the alloy to the job we use it for. For example;
-amalgam, which contains mercury ( this is used for filling teeth)
-brass, which contains copper and zinc.
-solder, which contains lead and tin,
Some smart alloys return to their original shape after being heated to a certain temperature.
Nitinol (nickel-titanium) is a smart alloy which is used to make spectacle frames, it returns to its original shape after being bent if it is put in hot water. Smart alloys are becoming more important as new ways to use them are found.
Rusting and corrosion
Only iron and steel rust. Other materials corrode.
Acid rain and salt water accelerate rusting.
Rusting is an oxidation reaction because iron reacts with oxygen forming an oxide.
The word equation for rusting is;
iron+oxygen+water=hydrated iron(iii) oxide
Aluminium does not corrode in moist air because it has a protective layer of aluminium oxide which, unlike rust, does not flake off the surface.
Different metals corrode at different rates.
Materials used in cars
Different materials are used in cars because they have different properties.
Alloys are mixtures of elements containing at least one metal, and often have different and more useful properties than the metals they are made from, for example;
-steel is harder and stronger than iron
-steel is less likely to corrode than iron
Car bodies can be built from aluminium or steel and there are advantages and siadvantages with each. Aluminium is lighter and more resistant to corrosion than steel but steel costs less and is stronger.
A lighter aluminium body means that fuel economy is improved. Because aluminium corrodes slowly, the car body will also last longer.
Advantages of recycling the materials from old cars include;
-less mining saves finite resources needed to make materials; - less crude oil is needed to make new plastics; - less waste means less landfill; - fewer toxic materials, such as lead from batteries, are dumped.
Disadvantages of recycling the materials from old cars include;
- fewer mines are built and fewer mining jobs created; - difficult to seperate the different materials; - some seperating techniques produce pollution; - some recycling processes are very expensive.
There are laws which specify that a minimum percentage of all materials used to manufacture cars must be recyclable to help protects the enviroment.