http://www.ck12.org Chapter 2. Matter
2.14 Gases
- Identify properties of gases.
- Explain why gases exert pressure.
- Describe how atmospheric pressure changes as altitude increases.
A hiker pauses to view the impressive peak of Mount Everest, the tallest mountain in the world. At the top of Mount
Everest, the air is very thin. Climbers may need oxygen tanks to get enough oxygen to breathe, even though oxygen
is the second most plentiful gas in the atmosphere.
What Is a Gas?
Agasis one of four well-known states of matter. (The other three are solid, liquid, and plasma). The particles of a
gas can pull apart from each other and spread out. As a result, a gas does not have a fixed shape or a fixed volume.
In fact, a gas always spreads out to take up whatever space is available to it. If a gas is enclosed in a container, it
spreads out until it has the same volume as the container.
Q:The sketches in theFigure2.29 represent two identical sealed boxes that contain only air particles (represented
by dots). There are more air particles in box B than box A. Which box contains a greater volume of air?
A:This is a trick question! The air inside each box expands to fill the available space, which is identical for both
boxes. There are more air particles in box B, but the volume of air is exactly the same in both boxes.
Pressure of Gases
Particles of gas are constantly moving in all directions at random. As a result, they are always bumping into each
other and other things. This is modeled in the Figure 2.30. The force of the particles against things they bump into