Deep within the Earth’s crust minerals are protected and remain stable, but when exposed to atmospheric conditions on the surface and in the hydrosphere, minerals undergo chemical changes. When this happens, the rock changes into a new substance.
The rusting of iron is an example of oxidation. In this case iron is exposed to water and air and changes to form rust (iron oxide). When feldspar is uplifted to the surface if forms clay.
Chemical weathering usually requires water to bring about mineral changes. Chemical weathering takes place most rapidly in warm and moist climates.
Some minerals resist chemical weathering. Quartz (found in sand) resists most chemicals. However olivine quickly weathers to clay when exposed to the atmosphere.
Limestone is a hard rock that usually withstands physical weathering, however it is mostly made up of calcite, that when exposed to water decomposes and creates organic acid, while the rest of the rock becomes soluble in water and is carried away. Water containing acid will only speed up the process.