( ( When ( ( two ( simple substances ) ) ( combine ( to ( ( make ( a ( ( more complex ) substance ) ) ) ( during ( a ( chemical reaction ) ) ) ) ) ) ) ) ( it ( ( is ( called synthesis ) ) . ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (CD two) (JJ simple) (NNS substances)) (VP (VBP combine) (S (VP (TO to) (VP (VB make) (NP (DT a) (ADJP (RBR more) (JJ complex)) (NN substance)) (PP (IN during) (NP (DT a) (NN chemical) (NN reaction))))))))) (NP (PRP it)) (VP (VBZ is) (VP (VBN called) (NP (NN synthesis)))) (. .)))
When two simple substances combine to make a more complex substance during a chemical reaction it is called synthesis.
(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)))))))))) (. .)))