A student we, recrystallizing a compound As the hot solution cooled to room temp
ID: 964640 • Letter: A
Question
A student we, recrystallizing a compound As the hot solution cooled to room temperature no crystal, appeared The flask was then placed in an.ice water bath Suddenly a large amount of solid material appeared in the flask The student isolated a good yield of product; however, the product was contaminated with impurities Explain You and your lab partner take melting points of the same sample. You observe a melting point of 101-107degreeC, while your partner observes a value of 110-112degreeC. Explain how you can get two different values with exactly the same sample. What can you do if you do not know which layer is which in an extraction procedure? How could you test to determine which was the aqueous layer?Explanation / Answer
1.The condition for crystallization is that the solution must be super saturated or at least saturated with the solute. No solid can separate from an unsaturated solution.
If the solubility of a substance in a solvent increases with increase in temperature, an unsaturated solution at a higher temperature will become saturated when the temperature is lowered.
Initially the solution was not saturated at the room temperatue. Hence no precipitate appeared . When the temperature is lowered by placing it in an ice bath it gets saturated and crystallization takes place.
Slow crystallization from a hot solution always results in pure crystals. During sudden crystallization the impurities get trapped in the crystal lattice and hence the crystal is contaminated with impurities
2) The distribution of the impurities in the sample may not be uniform. The melting point is lowered by the presence of impurities.The greater the amount of impurity the greater the depression of freezing point.
Your sample contained more impurity than your friend's sample. So you observed a lower melting point.
3) Anhydrous Copper sulphate CuSO4 is a colorless solid whereas hydrated Copper sulphate CuSO4.5H2O has a bright blue color. So if a drop of water is added to anhydrous Copper sulphate it will turn blue. This is the test for water.
To test whether a layer is aqueous or organic add a drop of the layer to anhydrous Copper sulphate. If the color changes to blue the layer is aqueous layer. Otherwise it is organic.
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