some of the original primary minerals in the rock to secondary minerals, remove other substances as solutes, and leave the most stable minerals as a chemically unchanged resistate. In effect, chemical weathering changes the original set of minerals in the rock into a new set of minerals that is in closer equilibrium with surface conditions. However, true equilibrium is rarely reached, because weathering is a slow process, and leaching carries away solutes produced by weathering reactions before they can accumulate to equilibrium levels. This is particularly true in tropical environments. Water is the principal agent of chemical weathering, converting many primary minerals to clay minerals or hydrated oxides via reactions collectively described as hydrolysis. Oxygen is also important, acting to oxidize many minerals, as is carbon dioxide, whose weathering reactions are described as carbonation. The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca2+ and other ions into surface waters. Dissolution Dissolution (also called simple solution or congruent dissolution) is the process in which a mineral dissolves completely without producing any new solid substance. Rainwater easily dissolves soluble minerals, such as halite or gypsum, but can also dissolve highly resistant minerals such as quartz, given sufficient time. Water breaks the bonds between atoms in the crystal: The overall reaction for dissolution of quartz is The dissolved quartz takes the form of silicic acid. A particularly important form of dissolution is carbonate dissolution, in which atmospheric carbon dioxide enhances solution weathering. Carbonate dissolution affects rocks containing calcium carbonate, such as limestone and chalk. It takes place when rainwater combines with carbon dioxide to form carbonic acid, a weak acid, which dissolves calcium carbonate (limestone) and forms soluble calcium bicarbonate. Despite a slower reaction kinetics, this process is thermodynamically favored at low temperature, because colder water holds more dissolved carbon dioxide gas (due to the retrograde solubility of gases). Carbonate dissolution is therefore an important feature of glacial weathering. Carbonate dissolution involves the following steps: CO2 + H2O → H2CO3 carbon dioxide + water → carbonic acid H2CO3 + CaCO3 → Ca(HCO3)2 carbonic acid + calcium carbonate → calcium bicarbonate Carbonate dissolution on the surface of well-jointed limestone produces a dissected limestone pavement. This process is most effective along the joints, widening and deepening them. In unpolluted environments, the pH of rainwater due