Free software and open source software are synonyms. Even Stallman agrees on this. He cares about the use of the terms because of their other implications.
Ok, that's interesting. My impression from watching his talks is that he does not consider the two ideas as synonymous, in that a software can technically be closed-source but have licensing that is libre-free so that it can be used by whomever however. But perhaps I got it wrong.
It is the other way around: just because the source is available free of charge does not make the software free as in freedom. For example, there are licenses that prohibit you from running the software for commercial use even though you have access to the source. This would not fit the FSF's definition of free software because the user should have the right to use the software as they wish.
Those licenses also aren't open source. 'Open source' doesn't mean 'you can view the source', it means you can view, run, compile, modify and distribute the source, and distribute modified copies.