( ( During ( a ( chemical reaction ) ) ) ( ( , ( ( atoms ( can ( form molecules ) ) ) , ) ) ( ( ( ( ( molecules ( can ( ( break apart ) ( to ( form atoms ) ) ) ) ) , ) or ) ( molecules ( can ( react ( with ( other molecules ) ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN During) (NP (DT a) (NN chemical) (NN reaction))) (PRN (, ,) (S (NP (NNS atoms)) (VP (MD can) (VP (VB form) (NP (NNS molecules))))) (, ,)) (S (NP (NNS molecules)) (VP (MD can) (VP (VB break) (PRT (RP apart)) (S (VP (TO to) (VP (VB form) (NP (NNS atoms)))))))) (, ,) (CC or) (S (NP (NNS molecules)) (VP (MD can) (VP (VB react) (PP (IN with) (NP (JJ other) (NNS molecules)))))) (. .)))
During a chemical reaction, atoms can form molecules, molecules can break apart to form atoms, or molecules can react with other molecules.
(ROOT (S (ADVP (RB Replace)) (NP (NP (DT another)) (PP (IN in) (NP (DT a) (NN molecule)))) (VP (VBZ happens) (PP (TO to) (NP (NNS atoms))) (PP (IN during) (NP (DT a) (NN substitution) (NN reaction)))) (. .)))
Replace another in a molecule happens to atoms during a substitution reaction.