What is the X in X-mas?
the one mixture of a set of substances able to dissolve in one another as liquids that, of all such mixtures, liquefies at the lowest temperature. If an arbitrarily chosen liquid mixture of such substances is cooled, a temperature will be reached at which one component will begin to separate in its solid form and will continue to do so as the temperature is further decreased. As this component separates, the remaining liquid continuously becomes richer in the other component, until, eventually, the composition of the liquid reaches a value at which both substances begin to separate simultaneously as an intimate mixture of solids. This composition is the eutectic composition and the temperature at which it solidifies is the eutectic temperature; if the original liquid had the eutectic composition, no solid would separate until the eutectic temperature was reached; then both solids would separate in the same ratio as that in the liquid, while the composition of the remaining liquid, that of the deposited solid, and the temperature all remained unchanged throughout the solidification.