Question

So I went through a lab experiment last week, basically, We have a mixture and we remove iron with a magnet, we than seperate naphthalene by sublimation (by heating a flask). Than we have urea which is the only component that can dissolve in water which is obtained by crystallization after seperation from the polypropylene which floats on the water and the sand which sinks.

1) Describe the prinicpal diffuclties you encountered in obtaining a clean seperation and in collecting the seperated components without loss.

I really forgot the diffuclties I ecountered, anyone got an idea?

Answer 1

Since Urea is highly soluble and non-volatile solute in water so becomes somewhat difficult to separate urea from water. Two methods are available for the separation of urea from water: We can evaporate the water or we can crystallize the urea.

Evaporation is the most common practice: the solution is heated and the water boils out of the solution. that results a urea melt, which still contains a little water (0.5 - 1 %). This method has one major drawback: impurities are not removed and the end product only contains 98 to 99 % urea.

The other method is crystallization. In that case we do not heat the solution, but we create a vacuum above the solution. The vacuum "pulls" out the water, and the solution is concentrated until there is more urea then can be dissolved in the water: The urea starts to crystallize. If the "slurry" (the mixture of crystals and solution) contains enough crystals, it is sent to centrifuges, where the solution is separated from the crystals. The crystals are dried and the result is a very pure product. In crystalline form the urea purity can be higher then 99.8 %.