I’ve always been taught that when a substance has the properties of an acid, it is considered acidic. However, recently I’ve noticed various things describing such substances as very acid as if the term were an adjective. Is this proper usage? Is it more common in Europe or other English-speaking places? It sounds quite strange to my American ears.
According to UK and US Oxford Dictionary online acid can be used as an adjective meaning the same thing as acidic in reference to PH (not only as in vulgar):
As a native speaker in Australia, I can vouch for "acidic" being the more common adjective, but not the one and only. One of our chief popular-science-celebrities, Karl Kruszelnicki (who is also Australian), almost always uses the term "acid" over "acidic", perhaps because he's trying to use simplified language to engage a larger audience. He'll often use phrases such as "less acid" or "more acid".
Here is an example of him talking about acidification in oceans - you'll get the idea. http://mpegmedia.abc.net.au/science/podcast/gmis/gmis20121211.mp3