Chemical weathering reactions and processes are more complex, here we're just going to explain the processes in general.
Chemical weathering processes involve the reaction of particular minerals in a rock which results in rock decay usually in the form of soil or solution. While physical weathering mostly produce cracks or lines of weakness, chemical weathering further weathers the rock into residual soil through the racks created by physical processes. So, these two usually work hand in hand.
Rocks such as limestone or chalk contain a mineral called calcium carbonate. When rain or stream water containing carbon dioxide (acid water) comes into contact with the calcium carbonate, the mineral is converted into calcium-bicarbonate. The new mineral is soluble and is washed away in solution.
When oxygenated water comes into contact with rocks containing iron(Fe2+), the mineral is converted into (Fe3+) causing a red coloration (rust) and the eventual crumbling of the rock.
Minerals in rocks such as anhydrite expand upon absorbing water and shrink upon drying. The repeating process will eventually cause cracks to develop and or breaks down the rock. Hydration can be classified as physical weathering since it involves stress build up and the cracks produced can give way to other weathering process to take place.
Hydrogen in water reacts and combines with certain minerals in a rock. For a example when feldspar combines with hydrogen ( water) the reaction decomposes the feldspar into residual clay. Hydrolysis is probably the main chemical weathering since it completely breaks down rocks into fine regolith.
This can be classified as a stand alone biological weathering but is usually grouped with chemical weathering since it involves chemical reactions. Decayed matter such as plants and animals release considerable amounts of humic acids which can penetrate rocks through cracks and decompose it. This can usually contribute to deep weathering of underground rocks.