In fact, the study of the properties of gases was the beginning of the development of modern chemistry from its alchemical roots. Of the three basic phases of matter-solids, liquids, and gases-only one of them has predictable physical properties: gases. Today all airships use helium, a legacy of the Hindenburg disaster. In addition, the best source of helium at the time was the United States, which banned helium exports to pre–World War II Germany. Why, then, was helium not used in the Hindenburg? In the 1930s, helium was much more expensive. Helium gas is also lighter than air and has 92% of the lifting power of hydrogen. So although hydrogen is an obvious choice, it is also a dangerous choice.
However, hydrogen also has one obvious drawback: it burns in air according to the well-known chemical equation 2H 2(g) + O 2(g) → 2H 2O(ℓ) This makes hydrogen an obvious choice for flying machines based on balloons-airships, dirigibles, and blimps. Any balloon filled with hydrogen gas will float in air if its mass is not too great. Thirty-six people, including one on the ground, were killed. The actual cause of the explosion is still unknown, but the entire volume of hydrogen gas used to float the airship, about 200,000 m 3, burned in less than a minute. Perhaps one of the most spectacular chemical reactions involving a gas occurred on May 6, 1937, when the German airship Hindenburg exploded on approach to the Naval Air Station in Lakehurst, New Jersey.