Lifting gas
A lifting gas or lighter than air gas is a gas that has a lower density than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include free balloons, moored balloons, and airships. Only certain lighter than air gases are suitable as lifting gases. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol, and so lighter than air gases have a  density lower than this. Gases used for lifting. Hot air. Heated atmospheric air is frequently used in recreational ballooning. According to the Ideal gas law, an amount of gas (and also a mixture of gases such as air) expands as it is heated. As a result, a certain volume of gas has a lower density as the temperature is higher. The average temperature of air in a hot air balloon is about . Hydrogen. Hydrogen, being the lightest existing gas (7% the density of air), seems to be the most appropriate gas for lifting. It can be easily produced in large  quantities, for example with the water-gas shift reaction or electrolysis, but hydrogen has several disadvantages: Helium. Helium is the second lightest gas. For that reason, it is an attractive gas for lifting as well. A major advantage is that this gas is noncombustible. But the use of helium has some disadvantages, too: Coal gas. In the past, coal gas, a mixture of hydrogen, carbon monoxide and other gases, was also used in balloons. It was widely available and cheap; the down side was a higher density (reducing lift) and the high toxicity of the carbon monoxide. Ammonia. Ammonia has  been used as a lifting gas in balloons, but while inexpensive, it is relatively heavy (density 0.769 g/L at STP, average molecular mass 17.03 g/mol), poisonous, an irritant, and can damage some metals and plastics. Methane. Methane (density 0.716 g/L at STP, average molecular mass 16.04 g/mol), the main component of natural gas, is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the smaller molecules of hydrogen and helium. Many lighter-than-air balloons are made of aluminized  plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons. However, methane is highly flammable and like hydrogen is not appropriate for use in passenger-carrying airships. It is also relatively dense and a potent greenhouse gas. Combinations. It is also possible to combine some of the above solutions. A well-known example is the Rozière balloon which combines a core of helium with an outer shell of hot air. Gases theoretically suitable for lifting. Water vapour. The gaseous state of water is lighter than air (density 0.804 g/L at STP, average molecular mass 18.015 g /mol) due to water's low molar mass when compared with typical atmospheric gases such as nitrogen gas (N). It is non-flammable and much cheaper than helium. The concept of using steam for lifting is therefore already 200 years old. The biggest challenge has always been to make a material that can resist it. In 2003, a university team in Berlin, Germany, has successfully made a 150 °C steam lifted balloon. However, such a design is generally impractical due to high boiling point and condensation. Hydrogen fluoride. Hydrogen fluoride is lighter than air and could theoretically be used  as a lifting gas. However, it is extremely corrosive, highly toxic, expensive, is heavier than other lifting gases, and has a high boiling point of 19.5 °C. Its use would therefore be impractical. Acetylene. Acetylene is 10% lighter than air and could be used as a lifting gas. Its extreme flammability and low lifting power make it an unattractive choice. Hydrogen cyanide. Hydrogen cyanide, which is 7% lighter than air, is technically capable of being used as a lifting gas at temperatures above its boiling point of 25.6 °C. Its extreme toxicity, low buoyancy, and high boiling point  have precluded such a use. Neon. Neon is lighter than air (density 0.900 g/L at STP, average atomic mass 20.17 g/mol) and could lift a balloon. Like helium, it is non-flammable. However, it is rare on Earth and expensive, and is among the heavier lifting gases. Nitrogen. Pure nitrogen has the advantage that it is inert and abundantly available, because it is the major component of air. However, because nitrogen is only 3% lighter than air, it is not an obvious choice for a lifting gas. Diborane. Diborane is slightly lighter than molecular nitrogen with a molecular  mass of 27.7. Being pyrophoric it is however a major safety hazard, on a scale even greater than that of hydrogen. Vacuum. Theoretically, an aerostatic vehicle could be made to use a vacuum or partial vacuum. As early as 1670, over a century before the first manned hot-air balloon flight, the Italian monk Francesco Lana de Terzi envisioned a ship with four vacuum spheres. In a theoretically perfect situation with weightless spheres, a 'vacuum balloon' would have 7% more net lifting force than a hydrogen-filled balloon, and 16% more net lifting force than a helium-filled one. However , because the walls of the balloon must be able to remain rigid without imploding, the balloon is impractical to construct with any known material. Despite that, sometimes there is discussion on the topic. Hydrogen versus helium. Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both significantly lighter than air, making this difference negligible. The lifting power in air of hydrogen and helium can be calculated using the theory of buoyancy as follows: Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the "difference