Why does a balloon filled with HYDROGEN go up in air?

Because of the HYDROGEN's density is LOWER than air, so go up towards to sky

If so, why an iron container containing hydrogen gas does not go upwards.

You must consider the density of the whole filled balloon (rubber balloon and hydrogen inside), but not consider hydrogen only.

If you put an object in air, the object will goes up if its density is lower than that of air.

The density of hydrogen is as lower as 1/14.5 of air.

When filled up with hydrogen, the balloon (including the rubber sheet covering and the hydrogen inside) will have a density lower than that of air. As a result, the balloon will go up.

Because hydrogen is much lighter than air. (Density of hydrogen is smaller than air)

2006-11-13 21:57:17 補充：
More about density of gases and balloons...When we blow up a balloon, it falls on the ground because the breathed air inside is denser than the unbreathed air in the atmosphere. Notice that breathed air composes of a higher ratio of carbon dioxide, which is denser than air.