re: Does air have mass?

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From: Ronald Wong (ronwong@inreach.com)
Date: Fri Sep 28 2001 - 00:06:18 PDT 

Message-Id: <l03102801b7d87a8d767f@[209.209.18.104]>
Date: Fri, 28 Sep 2001 00:06:18 -0700
From: Ronald Wong <ronwong@inreach.com>
Subject: re:  Does air have mass?

David Fairman <thefairman@yahoo.com> asked:

>Does anyone have a good demo or activity that proves
>that air has mass?

To which Lori Lamberson replied:

>...
>>Here are two pretty easy things your students can do to show that air has
>>mass.
>>1) Mass an uninflated balloon and an inflated balloon. Compare the two
>>masses. Paul says that when you do this you are actually weighing the
>>compressed part of the air - but this still does show that compressed air
>>has mass.
>>2) Mass some water in a beaker and mass an unwrapped "alka seltzer" like
>>tablet (use generic stuff to keep the price down) and add those two masses
>>together. Then drop the tablet in the water, noticing that gas is given
>>off. Mass the water and beaker with the dissolved alka seltzer and compare
>>it to the original sum of the two masses before adding the tablet to the
>>water. Triple beam balances are sensitive enough to show a difference.

Dave,

If you look around your lab you may find a pair of Magdeburg hemispheres
stashed away in a cabinet somewhere. They usually are used to show, in a
dramatic way, that the atmosphere exerts pressure on objects here on the
surface of the earth.

Here's what you can do with them:

1. Stick the two hemispheres together and use a rubber band to hold
   them together.

2. With the valve open, place the hemispheres on a platform balance
   and bring it to balance. Remove the sphere from its platform and

3. Pump the air out of the sphere.
   (Ask the students:"What did the vacuum pump just do to the container?")

4. Place it back on it's platform.

5. If the balance is sensitive enough, you and your students will see
   that the sphere has become lighter (a liter of air has a mass of
   only about 1.2 grams) - the balance will become unbalanced in a manner
   consistent with the fact that the sphere is lighter.
   (Ask the students:"What does this tell you about air?". After they
   answer that question, ask them to predict what will happen when you
   open the valve.)

5. Open the valve and watch it get heavier as the air returns - the
   system becomes balanced once again.

The nice thing about this approach is that you don't have to measure the
mass of the sphere - with or without air in it. You could just as well have
put an identical sphere on the other platform or used the right amount of
sand/washers/coins/etc. to balance the system. The conclusions are simply
based on interpreting the meaning of the state of balance or the lack
thereof (Of course, if you did measure the masses, you could, if you also
had measured the inside diameter of the sphere, determine the density of
air).

You are also not put in a situation where you are magically producing a gas
which you must claim is air (the alka-selzer+water approach) or determining
the mass of COMPRESSED air instead of the air itself as pointed out by Paul.

The drawback of course is that you'll need the Magdeburg hemispheres (or a
similar container designed to withstand the effects of atmospheric pressure
when the air is pumped out of it).

Just a thought - ron

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