Its density has decreased. When the balloon is warmed it becomes less dense, not lighter. When it cools, it becomes more dense, not heavier. It has the same mass, but at different volumes. Density is a mathematical relationship between mass and volume.
Air is over seven times more dense than helium.
Take the temperature in the room at the floor, at the ceiling, and two places in between. Based on the previous activity, explain why the temperature varies from the floor to the ceiling. To assess, look for the correct use of the density of hot and cold air in the explanations.