Hydrogen gas can be substituted for helium in a balloon. In fact, hydrogen was commonly used in the past for filling balloons and airships due to its lighter weight compared to helium. Hydrogen has a lower density than helium, which makes it buoyant in air and allows the balloon to float.
However, it is important to note that hydrogen is highly flammable and can be hazardous if not handled properly. The flammability of hydrogen makes it more dangerous compared to helium, which is inert and non-flammable. For safety reasons, helium is generally preferred for filling balloons, particularly for recreational and commercial purposes.