How big is a drop of water? That’s not an unanswerable question, like “How long is a piece of string?” A single H2O molecule isn’t a drop. As more and more molecules cluster together, there must be some threshold at which they start displaying behaviors associated with the condensed phase.
Nor is the question a mere philosophical curiosity. How many water molecules make a drop is relevant to the chemistry of such environments as Earth’s atmosphere and interstellar space, where molecules are typically found in isolation or in small clusters. It’s also relevant to computational chemistry: Simulating molecules takes a lot of computing power, so modelers want to know how many molecules they really need to reproduce liquid water’s properties.
How does one even tell whether water is acting like a bulk liquid? One thing that condensed-phase water can do, but single molecules can’t, is dissolve other substances;...
