[briteyellowness]

in the experiment, it mixed cyclohexanone and 2-furaldehyde in ethanol with semicarbazide hydrochloride.  then the samples were allowed to come to room temperature and the crystals from each sample were collected on a hirsch funnel and washed with two portions of cold water, and dried overnight.  

i was asked why i needed to wash with cold water.  i thought it would be harder to solubilize (?) in colder water so it would be used to remove insoluble impurities, but the answer was to remove soluble purities.  so would i precipitate to remove insoluble impurities?

also, does drying overnight help crystallize more?  i'm not sure why that part was written in the experiment either.  thanks again for the help.

[Donaldson Tan]

what happen if you wash the crystals with hot water?

why dry overnight? the longer period of time ensures all the water in the crystal has evaporated.

[briteyellowness]

and if i wash the crystals with hot water, i'd dissolve it and basically the experiment would have come to naught then?

so washing with any solvent is to remove an impurity that is soluble in that solvent?  

and precipitation to remove insoluble solvent?

[alphahydroxy]

i was asked why i needed to wash with cold water.  i thought it would be harder to solubilize (?) in colder water so it would be used to remove insoluble impurities, but the answer was to remove soluble purities.  so would i precipitate to remove insoluble impurities?

also, does drying overnight help crystallize more?  i'm not sure why that part was written in the experiment either.  thanks again for the help.

cold water presumably doesn't dissolve much (if any) of your desired product, but does dissolve some impurities present in the reaction mixture. This is the reason for washing with cold water. So using hot water to wash your product may well result in a lot more of your product dissolving, and being lost.

To remove insoluble impurities from a reaction mixture often involves filtering the mixture so that your product is dissolved and your impurities are caught in the funnel.

If both your product and the impurities are insoluble, this is much more tricky to purify. This may involve finding suitable conditions to recrystallise your product - i.e. finding a solvent system which dissolves everything when hot, but only the impurities when cold.

Failing this, a standard method of purification involves taking advantage of the different polarity of molecules. You can dissolve everything in as non-polar a solvent as you can then separate the different compounds by using chromatography, usually using a column of silica. The fundamental premise here is that different compounds 'stick' to the silica more or less strongly. you can then use solvents to remove the products at different times - a less polar solvent will remove a less polar compound but leave a more polar one 'stuck' to the silica.

[movies]

The usual way to remove insoluble impurities is to dissolve your sample in some hot solvent and then filter the mixture while it is still hot.  The filtrate will contain your desired compound while the insoluble impurities get caught in the filter.  This is a convenient method because you can then cool the filtrate solution to induce crystallization, filter again, and wash with cold solvent.  Then you remove the soluble impurities too.

Generally speaking, the soluble impurities you are worried about will be a lot more soluble than your desired product, but you still want to avoid dissolving any of your desired product.