Hydrogen

The discovery of hydrogen (H) is usually credited to Henry Cavendish, who, in 1766, named the gas produced in the reaction between a metal and an acid 'flammable air'. In 1781, Cavendish also observed that this gas burned in oxygen to form water. Antoine Lavoisier reproduced this finding in 1783, and named the gas 'hydrogen', from the Greek hydro meaning water and genes meaning creator.
 
Hydrogen is the most abundant chemical element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. Throughout the universe, it is usually found in its atomic or plasma forms, unlike on Earth, where it is always observed as a diatomic gas with formula H₂.

Hydrogen's ability to burn in air provided us with the famous 'squeaky pop' test, a favourite of Chemistry lessons everywhere. If hydrogen is produced in a reaction and a burning splint is placed near the gas, a loud 'pop!' will be heard. This sound is caused by the gas igniting.
 
Due to its low density, hydrogen was once widely used as a lifting gas for balloons and airships. This practice stopped after the 1937 Hindenburg disaster, in which the German airship LZ 129 Hindenburg caught fire and was destroyed during a landing attempt. 36 people were killed. Helium has now taken over as the lifting gas of choice, as, although it is heavier than hydrogen, it has the advantage of being non-flammable.
 
The most common modern uses of hydrogen are in the processing of fossil fuels and in the production of ammonia, an important ingredient of fertilisers. It is also used as a coolant for electrical generators at power stations, as it has the highest thermal conductivity of any gas. Liquid hydrogen is used as a rocket fuel in the space programme. Another use of hydrogen is as an energy carrier, as, unlike fossil fuels, it does not release CO₂ on burning. However, due to costs and difficulties with hydrogen storage, currently it isn't widely used.