hey, anyone do know if is possible to make a PET bottle float with hydrogen gas?

The bottle is just too heavy.

 

You can calculate this yourself:

1. Measure the weight of the bottle.

2. Look up the density of air (hint: about 1.2 kg/m3)

3. Calculate the weight of the air inside the bottle.

 

If the weight of the bottle is more than the weight of the air inside it, then replacing all the air by hydrogen won't make it float.

Making an aerostat is nothing easy and demands seriously light construction. No single chance with a bottle. Space blanket is a good start. Build big, use glue rather than adhesive tape. Avoid hydrogen that leaks too easily. Do experiments with flammable gas outdoors.

So.. that wasn't a good idea to think about build something like a "zeppelin airship"

 

very well. thanks by the answers.

Or move to a planet with a much denser atmosphere.

Or you can create your own environment using gas that is denser than our air. Fill up a tub with Sulfur hexafluoride and then throw in your plastic bottle. I don't have any equations handy on me to see if it would work, but it is worth the experimemt if you want to have fun with it. Mythbusters did a similar experiment except it was with foil boats, not plastic bottles. I would try with and without helium in the bottle as a start.

Saw on Youtube an experiment in a university class. A paperboat floating over an invisible "sea" of SF₆. Fun.

Also, at a German source of sparkling water, where dioxide accumulates in a part of a cave: soap bubbles floating on the dioxide.

 

Zeppelin airship: I made some toys. A 3m long one with glued space blanket filled with helium. Just thin shopping bags filled with natural gas float also (do this outside please! Preferably with an anchor and without wind). But carrying any significant weight demands a huge volume, and Zeppelins are slow as well, explaining why aeroplanes replaced them.

 

For local uses, for instance sightseeing tours or as cranes, you may consider my regenerative buoyancy control:

http://www.scienceforums.net/topic/70114-aerostat-buoyancy-control/