Lighter than air

Some gases are buoyant in air because they have a density that is less than the density of air (about 1.2 kg/m3, 1.2 g/L). Lighter than air gases are used to fill craft called aerostats which include free balloons, moored balloons, and airship to make the whole aircraft, on average, lighter than air. (Heavier than air aircraft include aeroplanes, gliders and helicopters.)

Hot air
The density of a gas can be reduced by raising its temperature while leaving the pressure unchanged (Charles' Law).

Heated air is widely used as a lifting gas in hot-air balloons. (The gas in a hot-air balloon is not only heated air, but also includes the products of combustion from the balloon's burner.)

The altitude of a hot air balloon is controlled by regulating lift. To increase lift, more heat is applied. To decrease lift slowly, the hot air is allowed to cool. To decrease lift quickly, hot air is vented. Unlike balloons using low molecular mass gases (see below), hot air balloons require continual burning of fuel in order to remain aloft.

Low molecular mass gases
Because any given volume of any gas at a given temperature and pressure contains the same number of molecules (Avogadro's law), any gas with a lower molecular mass than that of air will be lighter than air (at the same temperature and pressure).

A sealed balloon expands as it rises because air pressure decreases with increasing altitude. As buoyancy depends on the mass of the displaced gas (Archimedes' principle) and because air is less dense at higher altitudes, balloons which rise to high altitudes (such as weather balloons) have to be allowed to expand as they climb so as to support the same weight. Weather balloons are made with strong elastic "envelopes" so that they do not burst as they expand.

Determining which gases are lighter than air is relatively straightforward. These gases must have a molecular mass less than 28.97 (the average molecular mass of air) and exist as a gas at atmospheric temperatures and atmospheric pressures.

Assuming one atom per molecule of gas, the heaviest possible atom that could meet these criteria is silicon, which has an atomic mass of 28.1. However, silicon does not become a gas until it reaches a very high temperature. The same applies to the metals aluminum, magnesium, sodium, beryllium and lithium and the hydrides of these. Carbon and boron have high boiling points, but methane and borane (the hydrides of carbon and boron) are lighter than air.

The following is a list of all stable materials with a molecular mass under 28.8 and a boiling point under 100°C. Although isotopes are not considered here, it should be remembered that for the replacement of hydrogen with deuterium (or even tritium) the large relative mass difference can alter some of the properties of that specific gas (eg rates of reaction). Therefore ND3 can be considered a different gas to NH3 in the extreme.

HAHAMICE
The acronym HAHAMICE was used to help emergency responders remember the gasses which are lighter than air. It stood for:
 * H - Hydrogen
 * A - Ammonia
 * H - Helium
 * A - Acetylene
 * M - Methane
 * I - Illuminating Gases (old term for natural gas)
 * C - Carbon Monoxide
 * E - Ethylene

This acronym left out several gases, and was later changed to 4H MEDIC ANNA:
 * H - Hydrogen
 * H - Helium
 * H - Hydrogen Cyanide
 * H - Hydrogen Fluoride


 * M - Methane
 * E - Ethylene
 * D - Diborane
 * I - Illuminating Gases
 * C - Carbon Monoxide


 * A - Acetylene
 * N - Neon
 * N - Nitrogen
 * A - Ammonia

As is noticeable, the acronyms do not include all lighter than air gases.

Many of these gases are not practical for use in balloons. The following combine poor lift with objectionable properties: carbon monoxide, hydrogen cyanide, hydrogen fluoride, diborane, ethylene and acetylene. Nitrogen has negligible lift. Neon is harmless and offers a modest degree of lift; however it costs roughly the same as helium, another noble gas with far superior lift. The four remaining gases (ammonia, methane, helium, and hydrogen) have been used as balloon gases.

Ammonia has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).

Methane (the chief 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 small-moleculed hydrogen and helium. (Most lighter than air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)

Hydrogen and helium
Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. (Both provide about 1 kilogram of lift per cubic meter of gas at room temperature and sea level pressure.) Helium is preferred because it is not combustible.

The relative lifting power of hydrogen and helium can be calculated using the theory of buoyancy as follows:

The density at sea-level and 0 °C for air and each of the gases is:
 * Air (ρair) = 1.292 grams per liter (g/l).
 * Hydrogen (ρH2) = 0.08988 g/l
 * Helium (ρHe) = 0.1786 g/l

Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the difference of the densities (ρgas) - (ρair) rather than upon their ratios. Thus the difference in buoyancies is about 8%, as seen from the buoyancy equation: The negative signs indicate that these gases tend to rise in air.
 * Buoyant mass (or effective mass) = mass × (1 - ρair/ρgas)
 * Therefore the buoyant mass for one liter of hydrogen in air as:
 * 0.08988 grams * (1 - (1.292 / 0.08988) ) = -1.202 grams
 * And the buoyant mass for one liter of helium in air as:
 * 0.1786 grams * (1 - (1.292 / 0.1786) ) = -1.113 grams

Thus hydrogen's additional buoyancy compared to helium is:
 * 1.202 / 1.113 = 1.080, or approximately 8.0%

Many countries have banned the use of hydrogen as a lift gas for manned vehicles. The Hindenburg disaster is frequently cited as an example of the risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lift gas: with good engineering and good handling practices, the risks can be significantly reduced. It has been suggested that policy might allow hydrogen for cargo airships (both those unmanned and those manned only by pilots) and require helium for passenger airships.

Water vapour
Although pure water is not a gas at room temperature and sea level pressure, water in the vapor phase mixes readily with dry air, as do any two gases, until the partial pressure of the water vapor reaches the saturation water vapor pressure at the current temperature. Such moist air is lighter than dry air at the same temperature, because the molecular mass of water is lower than the average molecular mass of dry air. Most hot air balloons burn propane (or some other hydrocarbon) to provide heat; the combustion products have an average molecular mass of 29.1; the "light" water vapor more or less compensates for the "heavy" carbon dioxide. Pure water vapor (steam) can be used to lift balloons; however, the question of condensation must be addressed somehow. One route would be simply to tolerate the condensation - this supposes a rather large balloon. Alternatively, the balloon could provide insulation (for example have a double-walled structure) or the water vapour (if pure) could be maintained at least at the boiling point of water at the altitude of use (100 degrees C at sea level; less higher) by a heating device. [There are intriguing possibilities in using a mixture of air and water vapor, at a temperature high enough for the water component to remain as vapor.] Two research efforts are currently underway to build steam-filled balloons (see external links below), taking rather different approaches; both of them have succeeded in practical demonstration of steam as a lift gas.

Vacuum balloon
First proposed by Italian monk Franceso de Lana in 1670, the vacuum balloon would be the ultimate expression of displacement lift power. A frequent topic of blue sky thinking, the basic principle has remained the same: A container strong enough to preserve a vacuum that displaces sufficient air to lift the container and an additional load. However to avoid crushing by atmospheric pressure would require materials far stronger than any currently available (see unobtainium)