13-5: How Gases Work

Gases LIFT THINGS!!!! Some gases can be used to lift balloons or aircrafts. Other gases can’t be used to lift anything at all! But why is there a difference?

Why There Is A Difference… Gases with low densities can inflate things such as balloons or blimps. However, in order for the balloon to rise in the air it must weigh less than the air it is displacing. Also, if the air is more dense than the balloon it will slide under the balloon and lift it in the air. Think of a log submerged in water.

How Do You Find Low Density? But how do you know which gases have lower densities? By using the Ideal Gas law you can compare densities. Because the density of a gas depends on its: pressure, temperature, molar mass (all variables in the Ideal Gas Law), you can adjust these variables to give you a low density.

One Exception… However, if the pressure of the gas in the balloon is too small, the atmosphere would crush it. Leaving you with two ways to attain a low density gas: Choosing a gas with a low molar mass OR Heating the gas, inside of the balloon.

The Molar Mass Aspect There are two principal lifting gases: Helium AND Hydrogen Hydrogen was once the principal lifting gas for aircrafts, but if you recall the HINDENBURG TRAGEDY you would remember that Hydrogen burns rapidly and fiercely.

Boo Hydrogen! …YAY Helium! Due to the dangers of Hydrogen, Helium is now the only lifting gas used for aircrafts today. Helium has half the lifting power of Hydrogen (its molar mass is doubled) and it’s very rare and expensive. BUT…Helium has no tendency at all to burn, thus the overriding safety factor.

The Temperature Factor As we said before, another way of attaining low density is by heating the gas. Maybe that’s why hot-air balloons have been gracing our horizons for the past 200 years! Hot air balloons use regular atmospheric air to lift their balloons by heating the air by using a propane burner. This decreases the density of the gas.

So, Are Hot-Air Balloons Better? The lift of a hot-air balloon is relatively weak when compared to Hydrogen and Helium, but since light loads are always involved in recreational hot-air ballooning it really doesn’t matter all that much. Especially since the air would need to be heated to 2146K (a higher temp. than the melting point of most solids), which is just not feasible.

Gas Effusion Recall: Diffusion is the process by which the particles of one substance move through one another. Effusion is a process in which atoms or molecules of a gas move through a hole so tiny that they pass through one particle at a time.

Gas Effusion Also, lighter gases effuse and diffuse faster than heavier gases. Why? Because at a given temp., the particles of the lighter gases have greater speeds than the heavier gases. In what order would the following gases effuse? N2,O2, He, H2 H2 – He – N2 – O2