Cooling Curves differences between Solution and Pure Solvent
During the phase change, the composition of the solution phase is changing from that of the original solution as the solidifying component is removed.  This doesn't happen for a pure liquid.  In a simple system, the slant continues until both components begin to solidify simultaneously - the eutectic point.  The temperature then flattens out because the composition of the liquid phase doesn't change.  It stays flat until no liquid remains.  This can be complicated if there are other solid phases or formation of new chemical components.