pure gold vs Au

pure gold and Au both are nouns.

Word	Noun	Adjective	Verb	Adverb
pure gold	Yes	No	No	No
Au	Yes	No	No	No

As nouns, Au is a hypernym of pure gold; that is, Au is a word with a broader meaning than pure gold:

pure gold: 100 per cent gold
Au: a soft yellow malleable ductile (trivalent and univalent) metallic element; occurs mainly as nuggets in rocks and alluvial deposits; does not react with most chemicals but is attacked by chlorine and aqua regia

Other hypernyms of pure gold include atomic number 79, gold.

pure gold (noun)	Au (noun)
100 per cent gold	a soft yellow malleable ductile (trivalent and univalent) metallic element; occurs mainly as nuggets in rocks and alluvial deposits; does not react with most chemicals but is attacked by chlorine and aqua regia