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.

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    People often use acid "adjectivally. In some very common usages, such as an acid comment, the "morphologically correct" version is so uncommon it can't even be shown on an NGram chart. – FumbleFingers Jan 5 '15 at 20:00
  • Have you checked a dictionary? Mine lists it as both a noun and an adjective. – Barmar Jan 5 '15 at 22:24
  • @Barmar - Yes, I checked a dictionary. While it does list acid as an adjective, I have never heard this usage in speech and I didn't know if it was archaic or just more popular elsewhere. – Chase Sandmann Jan 5 '15 at 23:11
  • I think the adjective form tends to be used more in metaphoric senses, e.g. acid tongue. – Barmar Jan 5 '15 at 23:13
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    If you speak using basic English, you can counteract acid remarks. – Oldcat Jan 6 '15 at 0:12

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):

http://www.oxforddictionaries.com/definition/english/acid http://www.oxforddictionaries.com/definition/american_english/acid

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

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