In practice, it is very rare for a solidifying system to be composed of a single, highly pure element or compound. The liquid normally contains other elements or compunds in solution, i.e. solutes, present either as deliberate additions or as inadvertent impurities. Redistribution of such solute affects the nature of the process and the microstructure of the product.

Attention centres on the partitioning of solute at an advancing solidification front. Even if the system as a whole is not in thermodynamic equilibrium, this can normally be assumed to apply at the interface itself (i.e. the kinetic undercooling is small).

Therefore, for an interfacial temperature of T*, the composition of solid and liquid at the interface, C_s* and C_L*, will be those given by the phase diagram. The partition coefficient, k, is given by the ratio of these compositions.

k = C_s* /C_L*

Since the solidus and liquidus lines are usually more or less straight, k is approximately constant for a given system. For many systems, k has a value less than unity, so that solute is rejected from the the solid as it forms, and accumulates in the liquid.    
Explore different solidifying systems by changing interfacial temperature, T*  and the composition of solid and liquid at the interface, C_s* and C_L*.