1. Explain why a balloon filled with helium gas rises in air. Why does the balloon rise to a particular height above ground and does not rise further ?

The helium gas in the balloon is lighter and less dense than the air outside the balloon. This makes a helium balloon rise up.

The balloon will stop ascending if the balloon reaches a height where density of air is nearly equal to the density of helium gas within the balloon. So there is no upward force ( buoyant force by air) acting on the balloon and the balloon will remains at the same height.