Will a hot air balloon still rise if the surrounding air is the same temp ?  

Interesting, using bacis physics, i would say it would rise to a certain point, then just maintain itself suspended in the air.

air when heated becomes lighter. When we heat the air inside the balloon it becomes lighter and the total weight of the air and balloon compared to the same volume of air outside becomes lesser. and that is the point at which the balloon starts to rise.

Therefore,

Weight of hot air + weight of Balloon < Same Volume of Cold air outside = Balloon Rise.

The weight of Balloon alone is always more than the air so it cannot rise on its own.

When the air surrounding the Balloon is at the same temperature as the air inside the balloon then the weight of Balloon and air inside will always be more than the same volume of air outside. Hence the balloon will start to drop down.


Therefore the balloon will not rise if the air inside is at the same temperature as the air outside. Else why wouldthe air inside be heated at all....?

I sense a conflict in the question.
How can the surrounding air can be the same temperature inside? They heat the air trapped in the baloon till to 100 degree celcius. Well no living creature can stand so much heat :) If air is cooled down to the surrounding temperature; before it gets even lets say 70 degree closer to surrounding temp, just begin whispering every pray you know and try to wear your parachute immediately.

The answer to your question is simply and definitely NO it can not rise under the circumstance you've cited. In fact it dives through the earth! As a mylotter mentioned up above clearly, the reason for heated air is to create a bouyant force similiar to Archimet's water rules.

And a baloon can not rise endlessly. Low pressure up in the very high altitude tries to squeze down the volume of the baloon, hence lower volume means lower capability to carry, even though it would be much hotter than the air outside.