NCERT Notes: Chemical Weathering [Geography Notes For UPSC]

NCERT notes on important topics for the UPSC civil services exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about Chemical Weathering.

Chemical Weathering
  • A cluster of weathering processes namely solution, carbonation, hydration, oxidation, and reduction.
  • These processes act on rocks to decompose, dissolve or moderate them to a fine clastic state through chemical reactions by oxygen, surface/ soil water, and other acids.
  • Water and air along with heat must be present to speed up all chemical reactions.
Solution
  • When substances are dissolved in acids or water, then the water or acid with dissolved substances is called a solution.
  • This process includes the removal of solids in solution and depends upon the solubility of a mineral in weak acids or water.
  • Many solids disintegrate and mix up as a suspension in water as they come in contact with water.
  • Some of the soluble rock-forming minerals like sulphates, nitrates, and potassium, etc. are affected by this process.
  • Hence, these minerals are simply leached out without leaving any remains in rainy climates and accumulate in dry regions.
  • Minerals like calcium magnesium bicarbonate and calcium carbonate present in limestone are soluble in water containing carbonic acid and are transported away in water as a solution.
  • Carbon dioxide formed by decomposing organic matter along with soil water significantly assists in this reaction.
  • Sodium chloride is also a rock-forming mineral and is vulnerable to this process of solution.
  • Carbonation, oxidation and Hydration go hand in hand and accelerate the weathering process.
Carbonation
  • Carbonation is the reaction of bicarbonate and carbonate with minerals.
  • It is a general process helping the fragmentation of feldspars and carbonate minerals.
  • Carbon dioxide from the soil and atmospheric air is absorbed by water to form carbonic acid that acts like weak acid.
  • Magnesium carbonates and Calcium carbonates are dissolved in carbonic acid.
  • These are removed in a solution without leaving any residue resulting in cave formation.
Hydration
  • Hydration is the chemical addition of water.
  • Minerals take up water and enlarge.
  • This enlargement causes an increase in the volume of the material itself or rock.
  • This process is long and reversible, sustained recurrence of this process causes fatigue in the rocks.
  • This may lead to their disintegration of rocks.
Oxidation and Reduction
  • In weathering, oxidation denotes a mixture of a mineral with oxygen to form hydroxides or oxides.
  • Oxidation happens where there is ready access to the oxygenated waters and atmosphere.
  • The minerals commonly involved in this process are manganese, sulphur, iron, etc.
  • In the process of oxidation, rock fragmentation happens due to the disturbance caused by adding of oxygen.
  • Red colour of iron upon oxidation turns to yellow or brown.
  • When oxidised minerals are positioned in a situation where oxygen is absent, reduction occurs.
  • Such circumstances exist commonly below the water table, waterlogged ground and in areas of stagnant water.
  • Red colour of iron upon reduction turns to greenish or bluish grey.
  • These weathering processes are interconnected.

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