difference between precipitation and crystallisation?

Answer 1

Precipitation is the formation of a solid in a solution during a chemical reaction. The formation of a precipitate is a sign of a chemical change. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension).

Crystallisation, on the other hand, is the (natural or artificial) process of formation of solid crystals from a uniform solution. Crystallization is also a chemical solid-liquid separation technique. Has 2 stages: 1) Nucleation and 2) Crystal formation.

Answer 2

precipitation is breaking down the solid compounds of the liquid like putting an HCl to a milk. while crystallisation is turning the solution into crystals like a fructose solution when mixed with a certain chemical. crystallisation takes more time than precipitation.

Answer 3

Precipitation is the formation of a solid in a solution during a chemical reaction. When the chemical reaction occurs the solid formed is called the precipitate. This can occur when an insoluble substance, the precipitate, is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. The formation of a precipitate is a sign of a chemical change. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension).

A use for this type of reaction is when making paints.

This effect is useful in many industrial and scientific applications whereby a chemical reaction may produce a solid that can be collected from the solution by various methods (e.g. filtration, decanting, centrifuging). Precipitation from a solid solution is also a useful way to strengthen alloys, this process is known as solid solution strengthening.

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.

An example of a precipitation reaction: Aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl) and the precipitation of a white solid, silver chloride is observed. (Zumdahl, 2005)

AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq)

The silver chloride(AgCl) has formed a solid, which is observed as a precipitate.

This reaction can be written emphasizing the dissociated ions in a combined solution

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) → AgCl(solid) + K+(aq) + NO3-(aq)

A final way to represent a precipitate reaction is known as a net ionic reaction. In this case, any spectator ions (those which do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following:

Ag+(aq) + Cl-(aq) → AgCl(s)
Crystallization is the (natural or artificial) process of formation of solid crystals from a uniform solution. Crystallization is also a chemical solid-liquid separation technique.
Contents
[hide]

* 1 Process
* 2 Crystallization in nature
* 3 Artificial methods
o 3.1 Crystal production
o 3.2 Purification
* 4 Thermodynamic view
* 5 References
* 6 External links
* 7 See also

[edit] Process

The crystallization process consists of two major events, nucleation and crystal growth.

Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that become stable under the current operating conditions. These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, supersaturation, irregularities, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure — note that "crystal structure" is a special term that refers to the internal arrangement of the atoms, not the macroscopic properties of the crystal: size and shape.

The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (Control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for pharmaceuticals). Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again.

[edit] Crystallization in nature
Snow flakes are a very well known example, where subtle differences in crystal growth conditions result in different geometries.
Snow flakes are a very well known example, where subtle differences in crystal growth conditions result in different geometries.

There are many examples of natural process that involve crystallization.

Geological time scale process examples include:

* Natural (mineral) crystal formation (see also gemstone);
* Stalactite/stalagmite, rings formation.

Usual time scale process examples include:

* Snow flakes formation (see also Koch snowflake);
* Honey crystallization (nearly all types of honey crystallize).

[edit] Artificial methods

For crystallization to occur the solution must be supersaturated. This means that the solution has to contain more solute entities (molecules or ions) dissolved than it would contain under the equilibrium (saturated solution). This can be achieved by various methods, with 1) solution cooling, 2) addition of a second solvent to reduce the solubility of the solute (technique known as anti-solvent or drown-out), 3) chemical reaction and 4) change in pH being the most common methods used in industrial practice. Other methods, such as solvent evaporation, can also be used.

Applications:

There are two major groups of applications for the artificial crystallization process: crystal production and purification.

[edit] Crystal production

From a material industry perspective:

* Macroscopic crystal production, for supply the demand of natural-like crystals with methods that "accelerate time-scale" for massive production and/or perfection:
o ionic crystal production;
o covalent crystal production.
* Tiny size crystals:
o Powder, sand and smaller sizes: using methods for powder and controlled (nanotechnology fruits) forms.
+ Mass-production: on chemical industry, like salt-powder production.
+ Sample production: small production of tiny crystals for material characterization. Controlled recrystallization is an important method to supply unusual crystals, that are needed to reveal the molecular structure and nuclear forces inside a typical molecule of a crystal. Many techniques, like X-ray crystallography and NMR spectroscopy, are widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds and bio-macromolecules.
o Thin film production.

Massive production examples:

* "Powder salt for food" industry;
* Silicon crystal wafer production.