Three Balloons 

I knew something was up. O2 density 1.41g/m3, Air density 1.3g/m3. Even if they were reversed it would sink due to the weight of the balloon. Adding He makes sense.

Little He can help? How many percent, I'm curious

Not a stupid question. The air we breathe is about 78% Nitrogen (N_3), 20% Oxygen(O_2), and 2% other gases. Both Hydrogen (H) and Oxygen are less dense than the surrounding air, so they float for the same reason a ball filled with air floats in water.

The answer is Archimedes' thrust Hydrogen is the lightest element in the Universe.

At first I thought like you, but I also did the experiment, and I got the same result as the man in the video The burn of a mylar balloon filled with pure hydrogen is very different from the burn of a latex balloon filled with pure hydrogen. In the first case: the mylar balloon, it catches fire and burns due to exposure to the outside ambient air. This is very soft. But in the 2nd case: the latex balloon : the burn is much more violent. I will soon do the demo on my RU-vid channel!

Because pure oxygen is lighter than air. Air is only ~20% oxygen.

@@captaincannabis3321 No, that's false. Oxygen is heavier than air. The reason the red balloon is floating in the air is because it also contains Helium (this is for the purposes of the video)

if it's pure oxygen, yes

@@roxmox5142 How come we don't use it instead of helium? Isn't helium in short supply and also it's dangerous when people inhale helium as a prank, this would eliminate the hazard.

@@ThePeterDislikeShow helium is a noble gas, it practically doesn't react with other elements, in fact pure oxygen can be toxic. Don't quote me on this but I think helium is preferred for balloons because it is even less dense than pure oxygen.

​@@roxmox5142 No, pure oxygen in a balloon will not float. Air is filled with mostly nitrogen(~78%), and much less oxygen (~21%). Since pure Oxygen has a density of ~1.4 kg/cubic meter, compared to ~1.2 for Nitrogen, pure oxygen would sink compared to all of the nitrogen, especially with the weight of the balloon material itself. A comment was added to the video stating they explained in the classroom demo that helium was added so they would float, and that helium didn't change the reaction. That explanation didn't make it into this short video.

that's really cool