Can rocks and soil, or simply any other common form of matter with a mass, be vaporized in a nuclear thermal rocket at high temperatures to provide thrust for the rocket? Wouldn't that be convenient for interplanetary travel? What is so specific about liquid hydrogen that makes it commonly be used as the working liquid?
Any material at all can be theoretically used for thrust. If you throw material out the back, your rocket gets pushed forward. The big reason liquid hydrogen is used is because the specific impulse, which can be thought of as a proxy of fuel efficiency, is higher the faster your exhaust gas is moving as it's ejected. Because hydrogen is the lightest element, the average atom of hydrogen in a volume at a certain temperature will be traveling faster than an atom of any other element at that temperature. So this means hydrogen provides the best specific impulse, and thus is the most fuel efficient reaction mass. Due to the tyranny of the rocket equation, fuel efficiency is extremely important, so other reaction mass options are usually ignored. For a thermal rocket anyway; all bets are off if you're willing to include ion or photon drives.