Which of these substances plays the most important part in chemical weathering?

The substance that plays most important part in chemical weathering is water.

Chemical weathering is caused by rain water reacting with the minerals grains in the rocks to form new minerals and soluble salts. The reactions occur particularly when the water is slightly acidic. These chemical processes need water, and occur more rapidly at higher temperature, so warm, damp climates are the best conditions. Chemical weathering (mostly hydrolysis and oxidation) is the first stage in the production of soils.