The definition of a molecule as "the smallest indivisible portion of a substance that retains chemical and physical properties" is widespread among encyclopaedias and vocabularies. However,it looks *very puzzling* to me. Most chemical and physical properties of a substance have sense only when we talk of a *set* of molecules together. For example,I see as plain nonsense asking something like "at what temperature/pressure will that single isolated water molecule boil/ice/melt?". I think no one will deny phase transitions of a substance are among the most basic physico-chemical properties of a substance,but at what number of molecules does this make sense?
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