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What is the main difference between precipitation and crystallization. Is precipitation a chemical change and crystallization a physical change. How can we say that the resulting solute is precipitate or a crystal? Is it necessary that when we want precipitates to form then its ksp should be less than its Qsp or the precipitates form even after mixing two unsaturated aqueous salt solutions?

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The two words are often used interchangeably (perhaps incorrectly), though there is a slight nuance.

IUPAC defines the following:

Crystallisation: The formation of a crystalline solid from a solution, melt vapour or a different solid phase, generally by the lowering of the temperature or by evaporation of a solvent.

Precipitation: The sedimentation of a solid material (a precipitate) from a liquid solution in which the material is present in amounts greater than its solubility in the liquid.

In essence, a crystallisation is the formation of a crystalline solid (i.e. not amorphous), for instance leaving a beaker of $\ce{NaCl}$ in water out for a long time will eventually grow crystals as the water evaporates.

Precipitation can be thought of a something 'crashing out', for instance if you have certain kinds of amines, you can add $\ce{HCl}$ in diethyl ether to precipitate out the $\ce{HCl}$ salt of the amine as they are not soluble.

There are of course in-between cases. Attempts at 'crystallisation' can yield amorphous solids if the conditions are not right, and, equally, precipitation can sometimes give crystalline solids.

The choice of words is therefore always ambiguous, but in general the IUPAC nomenclature should be followed where appropriate (eg. you can never crystallise to give an amorphous solid).

To answer the other part of your question about chemical vs. physical changes, you are correct that crystallisation is always a physical change (different isomorphs of the same material: amorphous, crystalline needles etc). Precipitation can be either, for instance in the $\ce{PPh3}$ case described by Jan, or in the $\ce{HCl}$ amine salt I previously described.

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  • $\begingroup$ I don't think the distinction between chemical and physical is quite right. Yes, sometimes we add HCl to cause a salt to crystallise, but this is a chemical change followed by crystallisation which we do because some types of chemical (e.g. salts of amines) are easier to crystallise than the raw base. $\endgroup$ – matt_black Jul 23 '17 at 10:13
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Both are physical changes. In neither does the substance crystallising/precipitating truely change, in both it is more or less the same after as before. (Technically, ions in solution are something totally different than ions in a crystal lattice, but especially for organic chemicals the difference is neglegible if present at all.)

The terms are not really clearly defined and there is a big grey area in the middle where both can apply. But in general, one would speak of crystallisation if it is a slow process that mainly happens at the bottom of the solution and yields something with a defined crystal structure. Precipitation would be a process which can happen throughout the entire solution rather quickly and tends to result in something cloudy, ill-defined.

So in the case of adding a chloride solution to one of silver nitrate, when silver chloride is formed immediately and almost quantitatively as a cloundy substance everywhere where there are chloride ions, one would not call it crystallisation but precipitation. On the other hand, if I dissolve copper sulphate in water and let it stand at cold temperatures for a long time to generate large single crystals, I would speak of crystallisation and deem precipitation to be wrong.

An example for the grey area would be the removal of triphenyl phosphane from a solution in dichloromethane or ether by adding hexanes. The precipitate forms rather fast, especially if the more polar solvent is evaporated, but at the same time it is rather crystalline looking (I have no clue if they are actually crystals or just look nice). I have heard both crystallisation and precipitation in this context.

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This is not a definition, but mostly we use it as follows:

  • Crystallization - Product forms as nice crystals, generally because you reduce the solubility slowly.
  • Precipitation - Your product forms microcrystals, because the solubility was reduced too fast and crystals couldn't grow idiomorphic
  • Crash out - Either your product is very insoluble, or you were way too impatient to let it cool slowly. Your product will form as a a fine powder, a clump of microcrystals or something along those lines.
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Crystallisation is formed from a super saturated solution while precipitation is formed due to reaction of 2 substances to form sparingly soluble product

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    $\begingroup$ Well, I guess this could count as a difference but who says precipitation can only occur because of reactions? $\endgroup$ – M.A.R. Jun 25 '16 at 7:55
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    $\begingroup$ No, this is incorrect. The terms are not defined like this. $\endgroup$ – Jan Jun 25 '16 at 17:06

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