Sentence:  The rate of the gas <WSD>phase</WSD> exchange reaction appears to be proportional to the first power of the absorbed light intensity indicating that the radical intermediates are removed at the walls or by reaction with an impurity rather than by bimolecular radical combination reactions . 
Meaning of the word marked with <WSD> tags:  (physical chemistry) a distinct state of matter in a system; matter that is identical in chemical composition and physical state and separated from other material by the phase boundary