Final answer:
Surface tension is a physical property that cannot be determined for a material in the gas state.
Step-by-step explanation:
The physical property that would be impossible to determine for a material in the gas state is surface tension. Surface tension is a property that arises due to the cohesive forces between the molecules at the surface of a liquid. In the gas state, molecules are highly spaced apart and do not have the strong intermolecular forces necessary for surface tension to occur.