I am trying to replace the valve connecting the water line to the sink in my bathroom. The piping and sink were installed in 1975. I'm stuck trying to remove the valve under the sink (see picture below). When I try to unscrew the valve it will rotate but it's not actually unscrewing from the water line. I thought maybe I needed two wrenches (one on the nut closet to the pipe and one on the valve itself). But if they do rotate independently then I couldn't break the seal.
Is there some trick to getting this model of valve off? I tried searching online but I just cant find any info. Likely Im just butchering the terminology quite a bit which is hurting the search.
I'm quite a bit new at doing this type of project so its possible I'm looking right past the obvious.