Can you suggest an experiment to prove that a helium balloon floats because helium is lighter than oxygen? — CR
If you have a balance scale, you can do a series of comparisons. First compare a cup of water to a cup of salad oil, using the balance, to show that the salad oil is less dense than the water. Then show that the salad oil floats on water. Then compare an air-filled balloon to an identical helium balloon, using the balance, to show that the helium is less dense than air. Then show that the helium floats on air. It’s just like the salad oil on water, but now it’s the helium on air. You can’t simply pour the helium on the air to show that it floats, because they’ll mix. So you leave the helium wrapped up in a rubber balloon and then let it float on air. It floats just fine!