( acidity ( ( is ( caused ( by ( ( high concentrations ) ( of ( hydrogen ions ) ) ) ) ) ) . ) )
(ROOT (S (NP (NN acidity)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (JJ high) (NNS concentrations)) (PP (IN of) (NP (NN hydrogen) (NNS ions))))))) (. .)))
acidity is caused by high concentrations of hydrogen ions.
(ROOT (S (NP (NP (DT The) (NN concentration)) (PP (IN of) (NP (NP (NN hydrogen) (NNS ions)) (PP (IN in) (NP (DT a) (NN solution)))))) (VP (VBZ is) (VP (VBN called) (ADVP (RB acidity)))) (. .)))