( ( ( ( ( ( Pure substance ) , ) elements ) , ) compounds ) ( , ( mixtures ( ( are ( subjects ( of ( this unit ) ) ) ) . ) ) ) )
(ROOT (S (S (NP (NP (JJ Pure) (NN substance)) (, ,) (NP (NNS elements)) (, ,)) (VP (VBZ compounds))) (, ,) (NP (NNS mixtures)) (VP (VBP are) (NP (NP (NNS subjects)) (PP (IN of) (NP (DT this) (NN unit))))) (. .)))
Pure substance, elements, compounds, mixtures are subjects of this unit.
(ROOT (S (NP (PRP You)) (VP (VBP call) (NP (NP (DT a) (JJ pure) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (RB not) (VP (VB be) (VP (VBN broken) (ADVP (RB down) (PP (IN into) (NP (NP (JJ different) (NNS types)) (PP (IN of) (NP (NNS substances)))))) (NP (DT a) (PRN (-LRB- -LRB-) (NP (NN n)) (-RRB- -RRB-)) (NN element))))))))) (. .)))