1. 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 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.
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It is because of the density difference.

In the ballon, the density is less than that of the air, it float.
(hydrogen : air ~= 1 : 28.8)
The higher the difference on the density, the faster in will float.

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Balloon
From Wikipedia, the free encyclopedia

Balloons, like greeting cards or flowers, are given for special occasions.A balloon is a flexible bag normally filled with a gas, such as helium, hydrogen, nitrous oxide or air. Some balloons are purely decorative, others are used for specific purposes. Early balloons were made of dried animal bladders. Modern balloons can be made from materials such as rubber, latex, chloroprene or a nylon fabric. The modern balloon was invented by Michael Faraday in the 1800's, but mass production did not occur until the 1930's.
http://en.wikipedia.org/wiki/Balloon
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It is because the desity of H2(hydrogen) is lighter than the main air(99% of air)(N2 Nitrogen, O2 Oxygen).
The molar mass of H2 is 2, N2 is 28, O2 is 32. We can know that hydrogen is much ligher than N2 and O2 from the molar mass. Hence, the bollon go up in air.
When you are form 3 and studying chemistry, you will know more about that.

因為hydorgen比其他氣體輕