( ( Chemical reactions ) ( ( involve ( combining ( different substances ) ) ) . ) )
(ROOT (S (NP (NNP Chemical) (NNS reactions)) (VP (VBP involve) (S (VP (VBG combining) (NP (JJ different) (NNS substances))))) (. .)))
Chemical reactions involve combining different        substances.
(ROOT (S (NP (DT A) (NN combination) (NN reaction)) (VP (VBZ is) (NP (NP (DT a) (NN reaction)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (CD two) (CC or) (JJR more) (NNS substances)) (VP (VBP combine) (S (VP (TO to) (VP (VB form) (NP (DT a) (JJ single) (JJ new) (NN substance)))))))))) (. .)))