( Students ( ( learn ( about ( atoms ( , ( molecules ( , ( and ( chemical reactions ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Students)) (VP (VBP learn) (PP (IN about) (NP (NNS atoms) (, ,) (NNS molecules) (, ,) (CC and) (NN chemical) (NNS reactions)))) (. .)))
Students learn about atoms, molecules, and chemical reactions.
(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.