## Experiment 3-6

Density of Gases

### Background Information

Presumably you found in experiment 3-5 that if you measured liquids precisely enough, it is possible to distinguish different liquid substances by differences in their densities.

Solids often (but not always) have densities greater than liquids.  In this experiment you should notice that gases generally have densities significantly smaller than liquids.  The result of these differences in density result in what the ancient Greeks called natural place.  They had noted that here on earth solids placed in liquids or air fall closer to the center of the spherical earth.  And air bubbled into water floats up out of the water.  The heat from a fire seems to cause the hot air and smoke to rise up through the air.  The Greeks presumed that these observations occurred because each element had its natural place with earth closest to the center of the earth, followed by water, air, and fire the furthest from center.  An element out of place would fall or rise seeking its natural place.

The concept of natural place is no longer in common use, having been replaced by the concept of buoyancy.  The competitive pull of gravity on different substances seems to judge using density.  When two objects of equal volume are pulled by gravity, the more massive experiences a greater gravitational pull, thus winning the lower position.  The less dense substance is buoyed up by the more dense substance displacing it from the lower position.  When energy is calculated, the situation with the more dense substance occupying the lower position results in less stored gravitational energy that the losing placement.  We note there seems to be a natural preference for less potential energy.  So when the hot smoke rises, we now believe it is due to the more dense cooler air displacing the hot smoke from the lower position, resulting in a decrease of stored gravitational energy.

Even steel ships and concrete pontoons can float if they are so constructed that the density of the entire ship or pontoon (including cargo and air) is less than that of the water.  In fact a launched ship sinks into the water until the mass of displaced water EXACTLY equals the mass of the ship and its contents.  Submarines change their density slightly by pumping water into or out of internal tanks to encourage submerging or emergency ascents.

Recall that density is defined by the equation

Density = mass / volume

### Experiment

In this experiment measure and calculate the density of a gaseous substance.

A couple tricks will be necessary to measure the mass and volume of gas:
• The problem with measuring the mass of a sample of gas is that because of the low density, the buoyance of the surrounding air would greatly reduce and perhaps give an apparent negative mass for the gas.  This difficulty can be overcome by using more dense solids and liquids to generate the gas then recalling our finding that any apparent small loss in their mass during the generation will be EXACTLY due to the mass of the escaping gas.

• Three problems with measuring the volume of the sample of gas are that the volume may be larger than your graduate, that most gases are invisible, and that the gas won't normally remain in a graduated container with the usual open top.  The trick is to displace water from a large inverted container filled totally with water and immersed in water in a still larger container.  The gas can be generated so that it bubbles up into the inverted container, the meniscus of the remaining water noted, and the container refilled to that level measuring the water needed in batches.
Gas called Carbon Dioxide:  This gas can be generated by many reactions.  But one of the simplest is to purchase a couple tablets of Alka Seltzer, a large commercial pill designed to provide a convenient way to deliver pain relief using the inexpensive drug called Aspirin.  When Alka Seltzer is placed in water, a combination of dry powders, citric acid and sodium hydrogen carbonate, react with the water generating carbon dioxide (fizzing bubbles) and a pleasantly sour tasting drink containing the aspirin.
1. Weigh a small container of water along side of one Alka Seltzer tablet.
2. Place the tablet in the water then reweigh after the fizzing is complete.
3. Subtract the mass after from the mass before reaction to determine the mass of escaped gas.
4. Obtain a relatively large transparent container such as a pitcher to collect the gas.  The tapered top could be cut off a large plastic soda bottle to serve as such a container. (Use caution with sharp knife.)
5. Partially fill with water a larger container.  (This could be gallon pail, bath tub, or any container with size in between!)
6. Fill completely full the collection container in the pail/tub then invert it so it's open mouth remains under water and the container remains filled with water.
7. Place perhaps half a Alka Seltzer tablet at a time under the collection container so that the carbon dioxide bubbles are collected in the inverted container.
8. When the bubbling is complete, mark the meniscus of the water still in the inverted container.
9. Remove the inverted container, allowing any remaining water to escape.
10. With this container now upright, refill to the meniscus mark with water, measuring how much water is needed.  This measured water has the same volume as the carbon dioxide had.

Calculate the density of the gas using scientific notation for the small number.

As always, record your investigation into your journal and, if credit is needed, construct a written report.

### Alternates

There are other ways to produce carbon dioxide.  For example baking soda (sodium bicarbonate) and nearly any sour substance (acid) will generate carbon dioxide.  Dry ice is frozen carbon dioxide and produces gas upon warming. Caution: Dry ice can freeze damage live tissue.

Other gases can also be generated and compared.
• For example butane is available compressed in cans for refilling lighters and other devices.  The butane container could be weighed, some butane released into the collection apparatus using water displacement, the butane container carefully dried and reweighed.  Caution: Butane is flammable.
• There are directions elsewhere for generating other gases.  Generally these will require caution for more dangers than carbon dioxide presents.  Generally other gases will require a capped apparatus for generating the gas and tubing to transport the generated gas into the collection apparatus.

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