Why are some balloons able to float while others are not able to?

Answer 1

In order to float, a given mass (or volume) must be lighter than the equivalent mass that surrounds it. In other words, it must lighter than air.

Filling a balloon with the same air that surrounds it will mean it cannot possibly be lighter than air (at best it will be the same weight). But the weight of the balloon alone will make it heavier. It therefore makes sense to construct the balloon from the lightest possible material. Rubber is quite heavy but it is also elastic -- thus the air inside is compressed. Thus even the air inside would be heavier than the equivalent mass of uncompressed air outside. Therefore it also makes sense to use a material that does not stretch.

In order to make the balloon float, we must fill it with a gas that is not as dense as the air that surrounds it. Helium is ideal because the molecules are spread out much more than the molecules in the surrounding air -- and is therefore lighter (fewer molecules means less weight for a given mass of gas). Moreover, the material used in helium-filled balloons ("party" balloons) is wafer-thin and extremely light. It also doesn't stretch as easily as rubber, so the helium is not compressed. Thus the balloon easily floats upwards with very little helium.

The same effect can be achieved simply by warming the air inside the balloon. This forces the gas to expand, increasing the balloon's mass but without increasing its weight. With sufficient mass, the balloon eventually becomes lighter than the equivalent mass of the surrounding air -- and begins to float upwards.

Filling a rubber balloon with helium is also possible, provided the balloon is small enough to begin with (making it lighter), and elastic enough to accommodate the amount of helium required to overcome the balloon's overall weight. The helium will be compressed, however, thus much more helium is required to increase the mass sufficiently to overcome the density of the surrounding air -- far more helium than is required in a normal helium-filled "party" balloon.

Answer 2

Balloons that float do so because they are filled with a gas that is lighter than the surrounding air. Balloons that float are typically filled with helium; previously balloons were filled with hydrogen, but this is rare now due to the high flammability of hydrogen. Other lighter than gases include neon, methane and hot air; hot/warm air is lighter than air of ambient temperature and vice versa.

Balloons that sink are filled with a gas equal or heavier than the surrounding air. Typically this includes air from a person's lungs or an oxygen tank. These balloons sink even though the air density is equal both inside and outside the balloon because of the weight of the balloon drags itself down.