can someone describe crystallisation in words i can understand?

Question

it says here rather than completely evaporating the solvent, some solids can be separated from a solution by evaporating some of the solvent and then letting the solute slowly crystallise. This works because as the saturated solution cools, solute that can no longer stay dissolved crystallises. The solution is usually heated until crystals start to form and then left to cool. the crystals can be separated from the remaining solvent by filtration

Answer 1

Crystallisation is a simple method of evaporation.If we take a solution containing salt and water,and heat it, after some time the complete water is evaporated and salt is left back in the form of cryastals.

Answer 2

Most solids are more soluble in hot solvent than in cold solvent. That means that if you mix salt (ionic) or sugar (nonionic) in water more will dissolve on the stove than in the fridge. (This is NOT true of some of the solids in water - the calcium salts are less soluble in hot water so can plug up your hot water pipes! This is a very very rare exception)
So if almost all saturated solutions will contain more of the solid when hot than they will when cold the extra has to go somewhere and many chemicals will come out as crystals.
This is only true if the temperature has dropped low enough so that quite a bit of the solid which is dissolved is going to precipitate out. And of course some things will precipitate out as amorphous solids rather than crystalline solids.
But you have it wrong: The solution is allowed to saturate at some temperature and then is allowed to cool. One way to do this is to heat a solution together with some extra solids until all of the solids have dissolved. IF you started with a saturated solution, when the solution cools the extra solids will come back out. If you are careful to keep the water (solvent) clean and free of contamination (especially dust) the crystals can be grown to be large by seeding the cooled solution with just one tiny crystal of that solid. Thats because crystalization is easier when it happens on an already existing crystal surface - the atoms or molecules have an easier time falling into the ordered periodic arrangement which is what a crystal is.

Answer 3

Some materials are miscible - soluble within each other as a homogeoenous solution in all proportions (water and ethanol). Some materials have limited solubility over a composition range (water and table salt).

Increase the concentration of solute in solvent by evaporating solvent. When solute concentration exceeds its solubility limit it will precipitate out of solution (solid) or separate into two immiscible phases (liquids).

Solubility limits are generally increased by increasing the temperature of the solution. Cool the solution and solid precipitates (look up the Born-Haber cycle).

Anhydrous ethanol is miscible with gasoline in all proportions. Add a few percent water - humidity out of the air - and two liquid phases separate: a heavy ethanol plus water phase (corrosive) and a light gasoline phase. That is one reason why gasohol is an incredibly stupid thing to do. There are many others (the Second Law of Thermodynamics cannot be evaded, you can eat food or burn food but not both, etc.

Answer 4

Crystallisation. with salt and water as an example.

A solid(perhaps salt) which has dissolved in a liquid (called a solution)(perhaps water)
can be separated by crystallisation.
The dissolved substance(salt) is called the solute.
The liquid(water) used for dissolving is called the solvent.

The solution(salt and water) is warmed in an open container,
allowing the solvent(water) to evaporate, leaving a saturated solution.
A solution which has as much solid dissolved in it
as it can possibly contain, is called a saturated solution.

As the saturated solution is allowed to cool,
the solid(salt) will come out of the solution and crystals will start to grow.
The crystals can then be collected and allowed to dry. making salt crystals to sprinkle on your chips.

Answer 5

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, 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.

Many compounds have the ability to crystallize with different crystal structures, a phenomenon called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products.

Answer 6

When liquids turn to solids.