DENSITY Applications with Gases

 DENSITY is a physical property of matter, as each element and compound has a unique density associated with it. Density defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume. For gases the density may vary with the number of gas molecules in a constant volume.
Density Comparison to Air: The density of several gases is compared to the density of air. Does an object float in air or sink in the air? If an object containing a gas floats on air, it is less dense than air vs. if it sinks, it is more dense than air.

Gas Density examples based upon differences in molecular weight.

Density of a gas with a constant number of molecules in a constant volume, varies according to the molecular weight. The higher the molecular weight the higher the density. Graphic.

Helium balloon: Compare the behavior of a helium filled balloon with that of an air filled balloon. Even taking into account the weight of the rubber balloon, the helium balloon floats on the air and is less dense than the air. The air filled balloon sinks because the weight of the rubber balloon makes is slightly heavier and thus more dense. Table of Densities

Applications: Mylar balloons, Goodyear blimp. Exit Vchembook site

Hydrogen balloon: Compare the densities of hydrogen, helium, and air to predict what will happen to a hydrogen balloon. In the hydrogen balloon demonstration, usually not enough hydrogen fills the balloon to overcome the weight of the balloon. Sometimes you can almost see the effect of the hydrogen filled balloon attempting to float. Table of Densities

Application: Hindenburg Exit Vchembook site

Carbon Dioxide: Did you ever see the effect of having dry ice (which is solid carbon dioxide) in a punch bowl at a party? Dry ice graphic. The dry ice is vaporizing to the gaseous state, as well as condensing some water vapor in the air to produce a white fog that sinks in the air. This shows that the carbon dioxide and cold water vapor are more dense than air. Table of Densities

Application: Fire extinguisher demonstration
In this case, the carbon dioxide produced by the vinegar and baking soda reaction sinks to the bottom of the beaker. In the process, the air which is less dense is pushed up and out of the beaker. As the oxygen in the air is pushed out of the beaker by the denser carbon dioxide, the candles go out one by one.

Application: Floating Flame. In this demonstration graphic the first picture shows that the container is filled with carbon dioxide gas. The white cloud that you see is condensed water vapor, but carbon dioxide gas is mixed in as well. In the second picture, the red glow on the bottom is caused by by red light sticks placed at the level of the gas delivery jet. The more dense carbon dioxide gas has filled the container. Natural gas, methane, is less dense than the carbon dioxide, so it floats to the top of the more dense carbon dioxide. At the level just above the layer of carbon dioxide, there is sufficient air and methane to support combustion - so the flame appears to float on top of the white cloud.

Gas Density examples based upon differences in temperature.

The density of gases depends upon the temperature. The higher the temperature, the more the molecules are spread out and the lower the density. Graphic. The result is that warm gases rise and cool gases sink. The same concept helps to explain the weather resulting in high and low pressures. High pressure means high density, cooler, sinking air. Low pressure means low density, warmer, rising air.

Hot Air Balloon: What makes a hot air balloon rise? As air is heated, it becomes less dense than the surrounding cooler air. The less dense hot air has enough lifting power to cause the balloon to float and rise into the air.

Hot Air Balloon Demonstration Exit Vchembook site

 Cold Water Vapor: What causes a cloud to sink to the earth surface and we call it fog. Tiny droplets of water are present in clouds and fog. If the surrounding warm air is cooled it sinks since it is more dense, and at the same time the water as a gas is condensed into tiny droplets of water.

Application: Liquid Nitrogen Demonstration
The white fog coming from the liquid nitrogen container is having the same effect as described above.

 Demonstrations with Density

Mysterious Ice
Layers of Liquids
Egg Densities - sugar water/oil
Smart Eggs - salt water and acid
Floating Eggs - sugar and water
Floating Spheres
Lava Lamp
Underwater Smoke Stack
Floating objects in water

 Mathematical Definition of Density

The formal definition of density is mass per unit volume. Usually the density is expressed in grams per mL or cc. Mathematically a "per" statement is translated as a division. cc is a cubic centimeter and is equal to a mL Therefore,

 Density =

 mass =

  g/mL
 

  volume

 

 Densities of Common Elements and Compounds

 Substance

 Density
grams per mL

 Hydrogen gas

0.000089

 Helium gas

0.00018

 Air

 0.00128

 Carbon Dioxide

 0.001977

Water

 1.00