The state of the substance, most likely, would be a gas. Given the same amount of the substance being able to fill different volumes of containers, means that the substance is compressible. From the three phases of matter, gas is surely the compressible one while liquids are sometimes compressible but only up to certain extent. Also, the substance cannot be a a solid since a solid would have a definite shape and volume. Nor it cannot be a liquid since a liquid cannot fill the whole container and it does not change its volume no matter what is the container.
