( ( ( ( ( ( The ( boiling point ) ) ( is ( ( a ( characteristic property ) ) ( of ( ( a ( pure liquid ) ) ( at ( constant pressure ) ) ) ) ) ) ) , ) but ) ( it ( varies ( with ( external pressure ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (JJ boiling) (NN point)) (VP (VBZ is) (NP (NP (DT a) (JJ characteristic) (NN property)) (PP (IN of) (NP (NP (DT a) (JJ pure) (NN liquid)) (PP (IN at) (NP (JJ constant) (NN pressure)))))))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ varies) (PP (IN with) (NP (JJ external) (NN pressure))))) (. .)))
The boiling point is a characteristic property of a pure liquid at constant pressure, but it varies with external pressure.
(ROOT (S (NP (NNP External) (NN pressure)) (VP (VBZ affects) (NP (NP (DT the) (JJ boiling) (NN point)) (PP (IN of) (NP (NN water))))) (. .)))