( ( Each group ) ( ( will ( ( ( combine ( their ( substances one ) ) ) ( at ( a time ) ) ) ( until ( ( a ( chemical reaction ) ) occurs ) ) ) ) . ) )
(ROOT (S (NP (DT Each) (NN group)) (VP (MD will) (VP (VB combine) (NP (PRP$ their) (NNS substances) (NN one)) (PP (IN at) (NP (DT a) (NN time))) (SBAR (IN until) (S (NP (DT a) (NN chemical) (NN reaction)) (VP (VBZ occurs)))))) (. .)))
Each group will combine their substances one at a time until a chemical reaction occurs.
(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)))))))))) (. .)))