( ( ( reactivity -- ) ( ( a description ) ( of ( ( the tendency ) ( of ( a ( substance ( to ( ( ( undergo ( chemical reaction ) ) ( either ( ( ( by itself ) or ) ( with ( other materials ) ) ) ) ) ( with ( ( the release ) ( of energy ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (NP (NP (NN reactivity)) (: --) (NP (NP (DT a) (NN description)) (PP (IN of) (NP (NP (DT the) (NN tendency)) (PP (IN of) (NP (DT a) (NN substance) (S (VP (TO to) (VP (VB undergo) (NP (NN chemical) (NN reaction)) (PP (CC either) (PP (IN by) (NP (PRP itself))) (CC or) (PP (IN with) (NP (JJ other) (NNS materials)))) (PP (IN with) (NP (NP (DT the) (NN release)) (PP (IN of) (NP (NN energy))))))))))))) (. .)))
reactivity -- a description of the tendency of a substance to undergo chemical reaction either by itself or with other materials with the release of energy.
(ROOT (S (NP (NP (DT The) (NN ability)) (PP (IN of) (NP (NN matter) (S (VP (TO to) (VP (VB combine) (ADVP (RB chemically)) (PP (IN with) (NP (JJ other) (NNS substances))))))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (NN reactivity))))) (. .)))