Agricultural Science Paper 2, May/June 2010  
Questions: 1 2 3 4 5 6 7 8 9 10   Main
General Comments

Question 3

(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 ]


This question was poorly attempted by the candidates. In question 3(a), majority of the candidates could not explain chemical weathering of rocks. Also, in question 3(b), many candidates were unable to discuss the processes of chemical weathering of rocks. Further to this, most candidates could not state ways in which soil temperature is important for crop growth.
The expected answers include:

Explanation 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.

Discussion of chemical weathering of rocks (Any four)
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)
CaSO4  + 2H20     CaSO4 .2H20
  (Calcium Sulphate)             (Gypsum)
OR CuS04  + 5H20                             CuS04. 5H20
(Anhydrous Copper Sulphate) Carbonation                     (Hydrated Copper II sulphate )/Blue stone/
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.
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
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
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.
Stalactite and stalagmite

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
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