A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids.  

• Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions.  
• Over and above the carbon dioxide present in the air, decomposition of plants and animals increases the quantity of carbon dioxide underground.  

• These chemical reactions on various minerals are very much similar to the chemical reactions in a laboratory. 
• There are five major types of mechanical weathering: thermal expansion, frost weathering, exfoliation, abrasion, and salt crystal growth. 

Source: NCERT – XI Fundamentals of Physical Geography