( ( An antioxidant ) ( ( is ( ( a substance ) ( that ( ( ( reduces or ) prevents ) ( ( oxidation , ) ( ( a ( chemical reaction ) ) ( ( in which ) ( oxygen ( reacts ( with ( another substance ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (NN antioxidant)) (VP (VBZ is) (NP (NP (DT a) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (VBZ reduces) (CC or) (VBZ prevents) (NP (NP (NN oxidation)) (, ,) (NP (NP (DT a) (JJ chemical) (NN reaction)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (NN oxygen)) (VP (VBZ reacts) (PP (IN with) (NP (DT another) (NN substance))))))))))))) (. .)))
An antioxidant is a substance that reduces or prevents oxidation, a chemical reaction in which oxygen reacts with another substance.
(ROOT (S (NP (NNP Combustion) (NN reaction)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN substance)) (VP (VBZ reacts) (ADVP (RB quickly)) (PP (IN with) (NP (NN oxygen))))))) (. .)))