Page 17 - Curriculum Visions Dynamic Book

acidity: a general term for the strength of an acid in a solution.
oxide: a compound that includes oxygen and one other element.
solution: a mixture of a liquid and at least one other substance (e.g. salt water). Mixtures can be separated out by physical means, for example by evaporation and cooling.
 The reddish-coloured banding of this sandstone shows that these rocks were formed in hot conditions. The variation in colour shows that some materials were formed in more iron-rich environments than others. This is Delicate Arch, Arches National Park, Utah, USA.
Laterite
Most tropical soils have a deep red colour, which is the result of weathering of iron compounds in climates with high temperatures.
Laterite is the name given to a soil material that is found in some tropical regions with a long dry season followed by long wet season.
A laterite layer is a thick zone containing iron and aluminium oxides and very little else. This unusual material is so rich in iron compounds that it is mined as an iron ore.
Most laterites are very old.
It is believed that they result from the effect of large amounts of water washing through a tropical soil. Under these conditions the silica that makes up the body of sand and clay particles (normally not a soluble material) goes into solution, so that the iron and aluminium compounds remain behind and so become more concentrated.
While still in the soil, laterite layers are soft, but when they are exposed to the air, such as when a road cutting is made, they oxidise and quickly become rock hard.
 This laterite soil from Queensland in Australia is very deep and on exposure to air becomes very hard.
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