Morphosyntactic Change

Particle verbs (combinations of two words but lexical units) are a notorious problem in linguistics. Is a particle verb like look up one word or two? It has its own entry in dictionaries, as if it is one word, but look and up can be split up in a sentence: we can say He looked the information up and He looked up the information. But why can''t we say He looked up it? In English look and up can onl...