When the Melting and Freezing Points are not the Same

Abstract

Almost every solid has a melting point, and almost every liquid has a freezing point. These two points are one and the same, just viewed from different perspectives: ice melts at zero degrees Celsius--the highest temperature at which it can be a stable solid--whereas water freezes at zero degrees C--the lowest temperature at which it can be a stable liquid. Hardly anything could appear simpler. But appearances can be deceiving. Small clusters of atoms or molecules are offering new clues about melting and freezing points--namely, that they are not so simple to pinpoint. These clusters--aggregates of atoms or molecules numbering from four or five to perhaps 100 or 200--can coexist as solids and liquids over a finite range of temperature and have distinctly different melting and freezing points. It is the unique character of clusters that has allowed investigators, including myself, to probe the secrets of their freezing and melting points. Clusters are bigger than individual molecules yet smaller than bulk matter--which consists of so many atoms that the number can be treated as infinite--and so they exhibit the properties of both. Because of their intermediate size, clusters can be studied in almost as precise detail as the atoms ormore » molecules constituting them, while simultaneously illustrating some features of bulk matter.« less