Chemical Weathering

Chemical weathering is a type of weathering that primarily results in changes to the chemical composition of rocks and minerals. These chemical changes could also result in changes to the physical attributes of the weathered products.

Chemical weathering may take place alongside physical weathering. However, the rate of chemical weathering increases as a greater surface area of rocks and minerals is exposed through the actions of physical weathering. It therefore can be assumed that chemical weathering sets in after physical weathering has disintegrated the bulk rocks and minerals.

The chemical processes of weathering often results in increased solubility of minerals, release of plant nutrient ions into solution, structural changes to mineral structure and the formation of new minerals among others.

Liquid water is the commonest factor in chemical weathering, implying that chemical weathering is more evident in areas that experience greater precipitation.

The following are some of the processes of chemical weathering:

Solution

Solution is a process of chemical weathering simply refers to the dissolving (dissolution) of minerals in water. This is probably the commonest process of chemical weathering in nature. It is common in areas where liquid water is plentiful.

Being a universal solvent, water has the capacity to dissolve a large number of substances, including soil forming minerals. Solution occurs as water passes over or through rocks, dissolving and carrying with it the soluble portions of the rocks. While some minerals readily dissolve, others have a slower rate of dissolution.

As minerals dissolve in water, they are separated into the different ions that combined to form the mineral. For instance upon dissolution, table salt (Sodium Chloride (NaCl)), will be separated into the Sodium ion (Na⁺) and Chloride ion (Cl^-) components. Some of the separated ions will exist in their ionic state in solution, while some may combine with the water molecules to form a new aqueous solution. The aqueous solution, if formed, may have a greater capacity to dissolve minerals.

Hydrolysis

Under certain conditions, water will split into its respective ions – H⁺ (Hydrogen ion) and OH^- (Hydroxyl ion) – and these ions will move in to combine with other elements to form new compounds. This process is known as hydrolysis – one of the processes of chemical weathering.

The Hydrogen ion (H⁺) often replaces the positively charged cations in many compounds especially if the compounds are alkaline. On the other hand, the Hydroxyl radical (OH^-) combines with positively charged cations to form hydroxides and other new compounds. The new compounds formed are often more soluble than the original material.

The process of hydrolysis decomposes rocks and minerals by transforming less soluble materials into more soluble ones.

Carbonation

In carbonation, minerals are dissolved in carbonic acid (H₂CO₃) – a weak acid – instead of water. This acid is formed when gaseous carbon dioxide (CO₂) dissolves in water (H₂O).

CO_{2 (g)}   +    H₂O _(l)      à      H₂CO_{3 (aq)}

In the soil medium, carbon dioxide is generated through organic matter decomposition and the respiration of plant roots.

Carbonic acid is known to dissolve minerals that are only slightly soluble or not readily soluble in water e.g. calcium carbonate also known as calcite (CaCO₃), – a slightly soluble compound. The dissolution process of carbonation is similar to that of solution. However, carbonation is often an intermediary for solution as it often leads to the formation of new, readily soluble compounds. These new, readily soluble compounds can then dissolve in water (the process of solution). For example; the dissolution of calcium carbonate in carbonic acid will result in the formation of calcium bicarbonate (Ca(HCO₃)₂) – a readily soluble compound – which can then dissolve in water.

CaCO₃     +     H₂CO₃           à           Ca(HCO₃)₂

Hydration

Hydration is a process of chemical weathering, where water combines with a mineral or salt without actually decomposing or modifying the mineral. Water that reacts with minerals in this way is known as Hydration Water.

The combination of hydration water with a mineral often increases the volume of the mineral, resulting in changes to the mineral structure. These changes tend to expand the mineral crystal, leading to a softer and more porous crystal. In so doing, it hastens other processes of decomposition. The development of limonite (2Fe₂O₃.3H₂O) from hematite (2Fe₂O₃) is an example.

2Fe₂O₃    +    3H₂O      à      2Fe₂O₃.3H₂O

Due to varying weather conditions, the products of hydration may dry out, leading to dehydration – a situation where hydrated minerals return to their anhydrite states.

Notably, moisture (liquid water) plays an important and pivotal role in all processes of chemical weathering.

Related Articles: 
Weathering 
Physical weathering