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# Absorption (chemistry)

CO2 inlet; 1b): H2O inlet; 2): outlet; 3): absorption column; 4): packing
Absorption in chemistry is a physical or chemical phenomenon or a process in which atoms molecules or ions enter some bulk phase - gas liquid or solid material This is a different process from adsorption since molecules undergoing absorption are taken up by the volume not by the surface (as in the case for adsorption) A more general term is sorption which covers adsorption absorption and ion exchange Absorption is basically where something takes in another substance
If absorption is a physical process not accompanied by any other physical or chemical process it usually follows the Nernst partition law:
"the ratio of concentrations of some solute species in two bulk phases in contact is constant for a given solute and bulk phases";
$frac\left\{\left[1\right]_\left\{1\right\}\right\}\left\{\left[2\right]_\left\{2\right\}\right\} = constant = K_\left\{N\left(x12\right)\right\}$

The value of constant KN depends on temperature and is called partition coefficient This equation is valid if concentrations are not too large and if the species "x" does not change its form in any of the two phases "1" or "2" If such molecule undergoes association or dissociation then this equation still describes the equilibrium between "x" in both phases but only for the same form - concentrations of all remaining forms must be calculated by taking into account all the other equlilibria
In the case of gas absorption one may calculate its concentration by using eg the Ideal gas law c = p/RT Alternatively one may use partial pressures instead of concentrations
In many technologically important processes the chemical absorption is used in place of the physical process eg absorption of carbon dioxide by sodium hydroxide - such processes do not follow the Nernst partition law
For some examples of this effect see liquid-liquid extraction it is possible to extract from one liquid phase to another a solute without a chemical reaction Examples of such solutes are noble gases and osmium tetroxide

## Other examples

An old method of Mercury #Gold and mining|gold mining] involves the absorption of gold into mercury
chemical bond is measured (McMurry2003)

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