( ( Chemical Reactions ) ( ( looks ( at ( how ( ( ( atoms and ) molecules ) interact ) ) ) ) . ) )
(ROOT (S (NP (NNP Chemical) (NNP Reactions)) (VP (VBZ looks) (PP (IN at) (SBAR (WHADVP (WRB how)) (S (NP (NNS atoms) (CC and) (NNS molecules)) (VP (VBP interact)))))) (. .)))
Chemical Reactions looks at how atoms and molecules interact.
(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.