Class Demonstration Place a piece of chalk into some dilute hydrochloric acid. The chalk dissolves, and bubbles of carbon dioxide gas are released. This is similar to what happens to limestone in rain, only it happens much faster in acid. Carry out the same experiment with a sample of limestone.
(b) Hydration
This is the chemical combination of rock minerals with water. Some minerals in rocks are able to chemically combine with water. As these minerals are hydrated they expand, causing stress and strain within a rock and shattering it. This may seem similar to the mechanical process of crystallisation of salts; however
hydration is a chemical reaction and hydrated minerals are different to their unhydrated parents. Hydration affects shale.
ACTIVITY
Hydration in everyday life Hydration is used to provide nappies with their water-absorbing properties. The liquid-absorbing substance in nappies undergoes hydration. Water-holding gels for houseplants also work by using hydration. Pour some water onto a nappy or water-holding gel and watch it slowly expand.
(c) Oxidation
When a rock mineral chemically combines with oxygen in water or air, oxidation is said to occur. Rusting iron is a well-known oxidation process. Rocks that contain iron are easily weathered in this way. Iron oxides in rock appear as reddish, orange-yellow streaks seeping out of a rock. Oxidised soils may also appear reddish in colour, e.g. latosols and terra rossa soils.
Fig. 9 Iron oxide being leached from rock layers in cliffs at Ballybunion, County Kerry
Fig. 8 Bubbles of carbon dioxide gas being released from the reaction of hydrochloric acid on limestone.