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Gas Density

Calculations of conversions between molar mass and the mass per unit volume of gases

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Up, Up, and Away

Up, Up, and Away

Credit: finchlake2000
Source: http://www.flickr.com/photos/finchlake/6295784018/
License: CC BY-NC 3.0

Since ancient times, mankind has been fascinated with the idea of flight.  In the modern world, airplanes have made it possible for millions of people to take to the skies every single day.  Most of these people spend more time thinking about the lack of legroom than the astonishing engineering feats required to launch passengers into the air and have them arrive safely hundreds or thousands of miles away.  However, just a few hundred years ago, the ability of humans to travel long distances without touching the ground was something that could only be imagined.

Amazing But True!

  • Airplanes are an example of “heavier than air” flight, in which the flying object is denser than air.  If a commercial airplane were completely stationary, it would drop to the ground.  
  • Before this technology was developed, scientists and inventors developed machines based on the fact that substances that are less dense than air will float.  
  • Party balloons filled with helium provide a common example of this process.  Because the helium inside the balloon is less dense than the air outside, the entire balloon will float away if not tied down.
  • Credit: Unknown
    Source: http://en.wikipedia.org/wiki/File:Zeppelin_Postkarte_1936_a.jpg
    License: CC BY-NC 3.0

    Blimps such as the Hindenburg airship were popular methods of international travel. The Hindenburg could travel up to 85mph and can carry up to 72 passengers [Figure2]

  • Another substance that is less dense than air is… air, when heated.  Charles’s Law states that the volume occupied by a given amount of gas will increase if its temperature is raised.  
  • Recall that density is mass divided by volume.  If we increase the volume without increasing the mass, the density will decrease.  Learn more about how hot air balloons work here: http://www.youtube.com/watch?v=77Ej_Ayugxk

Show What You Know

With the links below, learn more about various gases that can be used to make lighter-than-air transportation possible. Then answer the following questions.

  1. Rank the following gases from most dense to least dense at a temperature of 25-27oC (298-300 K): air, carbon dioxide, helium, hydrogen, oxygen.
  2. If balloons were filled with each of the gases from the previous problem, which would float in air, and which would sink to the ground?
  3. Could we create a vacuum inside a balloon in order to make it float?  Why or why not?
  4. Hydrogen is much less dense than air and is cheaper than helium.  Why are helium balloons common, both at parties and for aerial travel, while hydrogen balloons are rarely used?

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Image Attributions

  1. [1]^ Credit: finchlake2000; Source: http://www.flickr.com/photos/finchlake/6295784018/; License: CC BY-NC 3.0
  2. [2]^ Credit: Unknown; Source: http://en.wikipedia.org/wiki/File:Zeppelin_Postkarte_1936_a.jpg; License: CC BY-NC 3.0

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