Only the top layer of the earth is used to obtain the chemicals we need. This layer is called the Earth's crust and its thickness varies from about 5km to about 50km. From this layer we get the soil we need for growing crops, the ores from which metals are extracted, the rocks we need for building, and the fossil fuels we use.

Rocks can be used for building and for the extraction of useful chemicals other than metals. The most useful of these is Limestone (CaCO3).

Limestone is an important resource from which a useful range of compounds can be made. The picture next to this text shows some important uses of limestone and the related compounds: quicklime and slaked lime. The reactions involved in producing these compounds can be imitated in the laboratory.

A piece of calcium carbonate can be heated strongly for some time to produce lime (Ca + O2 --> CaO).

The piece of lime is allowed to cool and then a few drops of water are added. The lime which was produced before turns into "slaked lime". This reaction is strongly exothermic. If more water is added, an alkaline solution (limewater, Ca(OH)₂) is obtained.

If we were to add CO2 instead of water the result would be Limestone (CaO + CO2 --> CaCO3)