( ( An acid ) ( ( is ( ( any chemical ) ( that ( ( releases ( hydrogen ions ) ) ( when ( dissolved ( in water ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (NN acid)) (VP (VBZ is) (NP (NP (DT any) (NN chemical)) (SBAR (WHNP (WDT that)) (S (VP (VBZ releases) (NP (NN hydrogen) (NNS ions)) (SBAR (WHADVP (WRB when)) (S (VP (VBN dissolved) (PP (IN in) (NP (NN water))))))))))) (. .)))
An acid is any chemical that releases hydrogen ions when dissolved in water.
(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.