( So ( ( when ( ( ( water molecules ) ( -LRB- ( vapor -RRB- ) ) ) ( are ( introduced ( into ( ( that volume ) ( of ( dry air ) ) ) ) ) ) ) ) ( , ( ( ( the number ) ( of ( ( air molecules ) ( in ( the volume ) ) ) ) ) ( ( must ( ( ( decrease ( by ( the ( same number ) ) ) ) , ) ( if ( ( the ( ( temperature and ) pressure ) ) ( remain constant ) ) ) ) ) . ) ) ) ) )
(ROOT (S (IN So) (SBAR (WHADVP (WRB when)) (S (NP (NP (NN water) (NNS molecules)) (PRN (-LRB- -LRB-) (NP (NN vapor)) (-RRB- -RRB-))) (VP (VBP are) (VP (VBN introduced) (PP (IN into) (NP (NP (DT that) (NN volume)) (PP (IN of) (NP (JJ dry) (NN air))))))))) (, ,) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NN air) (NNS molecules)) (PP (IN in) (NP (DT the) (NN volume)))))) (VP (MD must) (VP (VB decrease) (PP (IN by) (NP (DT the) (JJ same) (NN number))) (, ,) (SBAR (IN if) (S (NP (DT the) (NN temperature) (CC and) (NN pressure)) (VP (VBP remain) (ADJP (JJ constant))))))) (. .)))
So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant.
(ROOT (S (S (VP (VBG Reducing) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS molecules)) (PP (IN in) (NP (DT a) (JJ rigid) (NN container)))))))) (VP (VBZ causes) (NP (NN pressure)) (S (VP (TO to) (VP (VB decrease))))) (. .)))