The Earth Contents

3. Weathering

Acid hydrolysis neutralises the acid in rock waters

Whatever the source of acid, the process of chemical weathering by acid hydrolysis neutralises the acid. Carbonate rock minerals such as limestone are particularly reactive towards acids, especially when compared to silicate rock minerals, as is demonstrated by a comparison of the waters draining from igneous rocks with those derived from limestone areas. The resulting waters found reflect the characteristics of each rock type. On igneous rocks the silicate minerals containing calcium, magnesium, sodium and potassium are attacked only slowly by acid hydrolysis, with the result that the waters have only low concentrations of dissolved solids and their natural acidity is not neutralised. By contrast the rapidly reacting carbonate rocks produce waters with high concentrations of dissolved solids whose acidity has been effectively consumed by weathering.

  Igneous rock Carbonate rocks
Description Soft Hard
Dissolved Solids Low High
pH 6 to 8 7 to 9
Cation Composition Na, K, Ca, Mg Ca, Mg

Give some examples of neutralised and non-neutralised rock waters.