( ( ( One definition ) ( of ( an acid ) ) ) ( ( is ( ( a substance ) ( that ( produces ( ( hydrogen ions ) ( in ( aqueous solution ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (CD One) (NN definition)) (PP (IN of) (NP (DT an) (NN acid)))) (VP (VBZ is) (NP (NP (DT a) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (VBZ produces) (NP (NP (NN hydrogen) (NNS ions)) (PP (IN in) (NP (JJ aqueous) (NN solution))))))))) (. .)))
One definition of an acid is a substance that produces hydrogen ions in aqueous solution.
(ROOT (S (NP (NNS Acids)) (VP (VBP release) (NP (NN hydrogen) (NNS ions)) (, ,) (PP (IN by) (NP (NN definition)))) (. .)))
Acids release hydrogen ions, by definition.