(a) Explain the term chemical weathering of rocks. [2 marks] (b) Discuss four processes of chemical weathering of rocks [10 marks] (c) State four ways in which soil temperature is important for crop growth (4 marks)
Explanation
(a) of chemical weathering of rocks: Chemical weathering is the decomposition of rocks by chemical agents formed through the reaction of water with atmospheric gases such as air (oxygen and carbon dioxide) As some minerals in the rocks are dissolved and others change into new chemical products therefore disintegration of rocks occurs.
(b)Discussion of chemical weathering of rocks (Any four) Hydration Occurs when water combines with or binds to some minerals Intact water may bind to silicates, oxides of iron Hydrated compounds are soft and easily fragmented It does not usually affect the chemical composition of the hydrated product Example of hydration reaction are Fe203 + 3H20 . Fe203.3H20 Haematite Limonite/Hydrated Haematite (Yellow) (Red) OR CaSO4 + 2H20 CaSO4 .2H20 (Calcium Sulphate) (Gypsum) OR CuS04 + 5H20 CuS04. 5H20 (Anhydrous Copper Sulphate) Carbonation (Hydrated Copper II sulphate )/Blue stone/Carbonation This is brought about by the combination of carbonate (C02-3) or bicarbonte (HCO-3) ions with rock minerals The carbon dioxide released to the atmosphere from various sources combines with water/rainwater to form carbonic acid C02 + H20 H2C03 The carbonic acid attacks the minerals and C02 in the rock. In limestone or marbles the calcite present is dissolved. Oxidation/Reduction Atmospheric oxygen and free oxygen in rain water carry out this reaction Minerals containing iron (Ferrous form), manganese and sulphur are the ones most frequently affected by this reaction When they are exposed to air and water, the ferrous ion is oxidized to the ferric Fe2 + state. The change in valency from Fe ++ to Fe + destabilizes the molecule and leads to disintegration 4Fe C03 + 02 2Fe203 + 4C02 Iron (II) Oxygen Iron Oxide Carbon dioxide carbonate Hydrolysis A decomposition reaction in which silicate minerals are broken down Water molecule is split into H+ and OR ions
The H+ replaces the cation from the mineral structure while the mineral is released for plant uptake Example: CaSi03 + 2H20H2Si03 + Ca(OH)2 Calcium silicate water silica (Calcium Hydroxide) KAISi30g + H20 Hydrolysis HAISbOg + K+ + OH- (Microcline) (water) Acid Silicate Hydrolysis is the breaking of the chemical bond in the mineral by water Solution Water can dissolve any soluble mineral present in rocks and carry them from the place of reaction if the land is sloppy If the land is not sloppy, the products of the soil solution accumulate in that place and form particular types of soil. Example Stalactite and stalagmite
(c) Soil temperature It determines the rate of formation and decomposition of soil organic matter Low temperature causes a decrease in metabolic activities and reduction in enzymatic reactions in plants Extreme temperatures will impede the activities of micro-organisms It affects the level of soil moisture It affects the absorption of water and nutrients by roots Optimum temperature promotes seed germination Appropriate temperature facilitates root development It determines the population of soil microbes It determines maturity and ripening of fruits High temperature causes the wilting of crops High temperature leads to loss of soil nutrients through volatilization Optimum temperature promotes the activities of soil microorganisms High temperature may be harmful to crops by causing premature dropping of fruits
