[Puchini 101]

A lab assistant asked me the following question today: we always assume that if the melting range of a compound is sharp, it is considered more pure. If that is the case, suppose you have two compounds A and B that form an eutectic mixture when you have 50% A and 50% B. This eutectic mixture melts at let's say 100 °C. Now, looking at the Temperature vs. Composition diagram we see that if we have a composition other tan the eutectic composition, the melting range will be GREATER if the solid is enriched in A or B (i.e. more pure).He suggested there is a contradiction or something that I'm not thinking about but I can't figure out what's wrong.  I  would appreciate any help or opinion on this matter!

http://www.foothill.edu/psme/armstrong/images/mpphasediagram.gif

[mjc123]

if the melting range of a compound is sharp, it is considered more pure.

That applies to a compound with small levels of impurity. It is pretty meaningless to say that e.g. a 70:30 mixture is "more pure" than 50:50. Both are mixtures, not pure compounds.
Have you come across the phase rule? How many degrees of freedom are there (ignoring pressure, which we assume fixed) for pure solid A in equilibrium with pure liquid A? How many for solid eutectic in equilibrium with liquid eutectic? How many for solid A in equilibrium with liquid A + B?