Chemical Weathering
Water, oxygen and acid can all weather rocks away. They breakdown rocks by dissolving or reacting with some of the minerals in the rock. Water itself can cause chemical reactions with
some rocks. Water attacks one of the minerals in granite,
for example. The granite break down into tiny particles of clay. These
then get washed away. Oxygen
gas in the air can attack some rocks,
particularly those containing iron.
The rusty brown streaks in some rocks are due to iron reacting with oxygen.
This leads to the formation of iron compounds which causes weakness in the rocks, hence
the rocks wear away.
Chemical
weathering - mainly
occurs in warm and wet climates.
Water
attack
Oxygen
attack
Chemical Weatherimg: Acid Rain
Chemical weathering is not just due to "acid rain" caused by pollution, it is also instigated by ordinary rain. You probably think that rain-water is pure. Actually it is slightly acidic. This is because carbon dioxide gas dissolves in rain as it falls. The resulting weak acid (carbonic acid) attacks rocks, mainly those containing calcium carbonate.
Calcium carbonate, in limestone, reacts with carbonic acid:
Calcium carbonate
+ carbonic acid 
calcium hydrogencarbonate
CaCo3(s) + H2CO3(aq) Ca(HCO3)2(aq)
Soluble calcium hydrogencarbonate is formed. So the limestone is slowly worn away.
Tags: Erosion, Weathering, Biological weathering, Acid attack, Water attack, Oxygen attack, How do weathering and erosion work together, what does weathering mean, chemical weathering acid rain