( ( ( ( ( ( Reversible reactions ) ( are ( ( symbolized ( in ( chemical equations ) ) ) ( by ( a ( double-headed arrow ) ) ) ) ) ) , ) but ) ( ( the standard ) ( remains ( to ( ( call ( the materials ) ) ( on ( ( ( ( the left ) ( the reactants ) ) and ) ( ( ( the materials ) ( on ( the right ) ) ) ( the products ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (JJ Reversible) (NNS reactions)) (VP (VBP are) (VP (VBN symbolized) (PP (IN in) (NP (NN chemical) (NNS equations))) (PP (IN by) (NP (DT a) (JJ double-headed) (NN arrow)))))) (, ,) (CC but) (S (NP (DT the) (NN standard)) (VP (VBZ remains) (S (VP (TO to) (VP (VB call) (NP (DT the) (NNS materials)) (PP (IN on) (NP (NP (NP (DT the) (NN left)) (NP (DT the) (NNS reactants))) (CC and) (NP (NP (NP (DT the) (NNS materials)) (PP (IN on) (NP (DT the) (NN right)))) (NP (DT the) (NNS products)))))))))) (. .)))
Reversible reactions are symbolized in chemical equations by a double-headed arrow, but the standard remains to call the materials on the left the reactants and the materials on the right the products.
(ROOT (S (NP (JJ Reversible) (NNS reactions)) (VP (VBP are) (VP (VBN indicated) (PP (IN by) (NP (NP (JJ opposite-headed) (NNS kinds)) (PP (IN of) (NP (NNS arrows))))))) (. .)))
Reversible reactions are indicated by opposite-headed kinds of arrows.