Sentence:  In an essentially static system , an oil cannot be replaced by <WSD>water</WSD> on a surface unless the interfacial tensions of the water phase are reduced by a surface-active agent . 
Meaning of the word marked with <WSD> tags:  binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent