Question: What is the chemical reaction of the formation of a solid in a solution?
About the article "Chemical reaction", section "Precipitation".

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The chemical bath deposition involves two steps, nucleation and particle growth, and is based on the formation of a solid phase from a solution.
In the chemical bath deposition procedure, the substrate is immersed in an aqueous solution containing the precursors. 
Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.
Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.
In chemical engineering crystallization occurs in a crystallizer.
Crystallization is therefore an aspect of precipitation, obtained through a variation of the solubility conditions of the solute in the solvent, as compared to precipitation due to chemical reaction. 
Solubility equilibrium is a type of dynamic equilibrium.
It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound.
The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as acid or alkali.
Each type of equilibrium is characterized by a temperature-dependent equilibrium constant.
Solubility equilibria are important in pharmaceutical, environmental and many other scenarios. 
Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid.
When the reaction occurs in a liquid solution, the solid formed is called the precipitate.
The chemical that causes the solid to form is called the precipitant.
Without sufficient force of gravity (settling) to bring the solid particles together, the precipitate remains in suspension.
After sedimentation, especially when using a centrifuge to press it into a compact mass, the precipitate may be referred to as a pellet.
The precipitate-free liquid remaining above the solid is called the supernate or supernatant.
Powders derived from precipitation have also historically been known as flowers.
Precipitation may occur if the concentration of a compound exceeds its solubility (such as when mixing solvents or changing their temperature).
Precipitation may occur rapidly from a supersaturated solution.
In solids, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g.
rapid quenching orion implantation, and the temperature is high enough that diffusion can lead to segregation into precipitates.
Precipitation in solids is routinely used to synthesize nanoclusters.
An important stage of the precipitation process is the onset of nucleation.
The creation of a hypothetical solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution.
If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs. 
Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction.
It usually takes place when the concentration of dissolved ions exceeds the solubility limit and forms an insoluble salt.
This process can be assisted by adding a precipitating agent or by removal of the solvent.
Rapid precipitation results in an amorphous or microcrystalline residue and slow process can yield single crystals.
The latter can also be obtained by recrystallization from microcrystalline salts.