In common terms, however, we commonly say water boils at 100 C/212 F, and freezes at 0 C/ 32 F. These phase change temperatures are properties of water - but they by themselves do not define the state. They are based on an assumption about another property of the system at that time. What assumption is built in to those statements? How does changing that assumption affect those properties? Do you know of any cases in everyday life where that assumption is broken?