hydrogen

[nickywild]

is hydrogen lighter or heavier than air and why

[umm]

surely it is lighter than air as the old german airships proved..they were filled with hydrogen but did not use them too long as hydrogen is very explosive..think the famous airship that blew up was the hindenberg,,no doubt someone will correct me if i am wrong..

[theprof]

Hydrogen is lighter than air. You may have seen pictures of airships that were used by the Germans during the war years. Hydrogen was used to inflate the airships for many years until it was found that another gas called Helium was less flammable.

The reason that hydrogen is lighter than air is because the density of hydrogen is less than that of air.

Air has a molecular mass of around 29. Any gas with a density below that figure at the same temperature and pressure as air will be lighter than air.

Typical gases with a density below 29 include Helium, Ammonia, Methane, Acetylene and even water (in the form of steam). In all there are fourteen gases that are lighter than air. Lower density gases such as Helium can be substituted for hydrogen in balloons but those gases such as Nitrogen (molecular mass 28) are next to useless to provide lift for balloons.

The density of a gas can also be reduced by increasing its temperature. This is why hot air is used to provide lift for hot-air balloons although combustion products from the burner also assist the process.

[theprof]

You're absolutely right umm.

[WaldoMcFroog]

Yes, it is. You can prove this by filling a balloon full of air and another full of helium and seeing what happens to them. Helium is less dense than air, thus rises.

[theprof]

Many of the fourteen gases lighter than air are unsuitable for use in balloons and for other practical purposes for an assortment of reasons.

Some such as Acetylene, Ethylene and Methane are explosive. Ammonia is poisonous and little used. Hydrogen Fluoride is both poisonous and corrosive.

The gases with molecular masses closest to that of hydrogen provide the best buoyancy. As we ascend towards that 29 figure, the lift provided by these gases becomes less and less.

[mibn2cweus]

Air is made of a combination of various elements. Hydrogen is the lightest of about 100 elements that seperately and in combination make up everything thing that can be seen (and most everything else that can not be seen) accept ironically for light which is energy.

Each element is made of a specific type of atom. Atoms are the single units of an element that determine and retain the charateristics and properties essential to a specific element.

Atoms of the particular type that make up each element vary somewhat in size (the space they occupy under certain conditions) but vary much more in weight, depending largely on the particular element they comprise.

Atoms of the element Hydrogen occupy the greatest volume of space in relation to their weight compared to all other elements. Because of the weight to volume ratio of Hydrogen, individual atoms of Hydrogen are displaced by other heavier elements present in the air and so 'float to the top'.

[shammydodger]

I haven't read all the posts on here, but one point I would pick up on is the Prof's statement that air has a molecular mass of around 29. I think I know what he means, but there are no such things as molecules of air, so the term molecular mass in relation to air is nonsensical. And he is interchanging the terms "molecular mass" and "density" when comparing gases, when they are different properties, although I would concede that the density of a gas is a function of its molar mass.

[theprof]

You are right shammydodger, I could have made the replies clearer.

Nevertheless, I'm pleased that you knew what I meant.

I hadn't long come out of a meeting yesterday when I wrote the posts. The alcohol had been flowing freely.

[theprof]

To clarify, I should add that the 29 figure I quoted is the summation of the individual constituent molecular masses of Oxygen, Nitrogen, Carbon Dioxide, Hydrogen, Argon, Neon, Krypton, Helium and Xenon. This is derived from the volume ratio compared to dry air of the individual gas multiplied by the molecular mass (in kg/kmol) of each of the individual gases.

As an example oxygen has a volume ratio of 0.2095, which is multiplied by a molecular mass of 32.0, to arrive at a molecular mass in air of 6.704. The total of the gases calculated in this manner provides us with a figure of 28.97, which is the total �molecular mass� of air.

Alternatively, we may say that the mean molar mass of air is 28.97 g/mol

I do concede that this is a theoretical figure and that air molecules do not exist. Nevertheless I felt it necessary to illustrate the reply to this question in this manner for the sake of simplicity based on premises that may not be unfamiliar to the original questioner or others with passable knowledge of chemistry.

Whichever way you look at it, in no sense could this 28.97 calculation be termed nonsensical.