Re: how does a hot air balloon rise to do with the particles in the air

Hi Ruth,
A hot air baloon rises because the density of air in the balloon is less 
than the density of air outside it (Density is essentially the number of 
particles of air in a given volume).

When that happens the balloon floats upwards.

If your question was how the density of the air in the balloon gets 
lowered when it heats up read on...

When air is heated, the particles of air gain energy. When that happens 
they start bumping into each other and trnasferring their energies between 
themselves. So the particles in the centre will start to push the 
surrounding particles outwards while those outside will continue moving 
outwards since they have no other particles to collide with. So the same 
number of particles will now occupy a larger space and hence have a lower 
density. 

I hope you have understood what I meant to convey. If not please do write 
back to us and we'd be glad to help you out.

Cheers,
Nauzad