( ( a product ) ( ( is ( substance ( formed ( by reaction ) ) ) ) . ) )
(ROOT (S (NP (DT a) (NN product)) (VP (VBZ is) (NP (NP (NN substance)) (VP (VBN formed) (PP (IN by) (NP (NN reaction)))))) (. .)))
a product is substance formed by reaction.
(ROOT (S (NP (NP (NNS Substances)) (VP (VBN produced) (PP (IN by) (NP (DT a) (NN reaction))))) (VP (VBP are) (VP (VBN known) (PP (IN as) (NP (NNS products))))) (. .)))