Sentence:  The rate of the <WSD>gas</WSD> phase 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:  the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container