A balloon filled up with gas would only go up in air if it is filled up with

A balloon ascends in the atmosphere based on the principle of buoyancy, which requires the gas inside the envelope to be less dense than the surrounding air. According to Archimedes' principle, the upward buoyant force is equal to the weight of the displaced fluid; thus, if the total weight of the balloon and its gas is less than the weight of the air it displaces, it will rise. Common lifting gases include helium and hydrogen because they have significantly lower densities than the nitrogen-oxygen mixture of the atmosphere [4]. Alternatively, hot air balloons achieve lift by heating the air inside, which increases its volume and decreases its density relative to the cooler ambient air. While water vapour is theoretically lighter than dry air, it is not typically used for ballooning due to condensation issues [2]. Therefore, the fundamental requirement for ascent is that the internal gas density is lower than the external air density.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 9: The Amazing World of Solutes, Solvents, and Solutions > 9.5.2 Effect of temperature on density > p. 147
2. [2] https://www.sciencedirect.com/topics/engineering/air-balloon
3. [4] https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/Balloon_Flying_Handbook_FAA-H-8083-11B/bfh_chapter_11.pdf