http://www.ck12.org Chapter 2. Matter
Earth’s surface, so it contains a huge number of gas particles. Most of them are concentrated close to Earth’s surface
because of gravity and the weight of all the air in the atmosphere above them. As a result, air pressure is greatest at
sea level and drops rapidly as you go higher in altitude. TheFigure2.31 shows how air pressure falls from sea level
to the top of the atmosphere. In the graph, air pressure is measured in a unit called the millibar (mb). The SI unit of
pressure is newton per square centimeter (N/cm^2 ).
FIGURE 2.31
Q:The top of Mount Everest is almost 9 km above sea level. What is the pressure of the atmosphere at this altitude?
A:Air pressure at the top of Mount Everest is about 260 mb. This is only about 25 percent of air pressure at sea level,
which is 1013.2 mb. No wonder it’s hard for climbers to breathe when they get close to Mount Everest’s summit!
Summary
- Gas is a state of matter in which particles of matter can pull apart from each other and spread out. As a result,
a gas does not have a fixed shape or a fixed volume. - Gas particles are constantly moving and bumping into things, and this creates force. The amount of force
pushing against a given area is called pressure. - The pressure of gases in the atmosphere is greatest at sea level and decreases rapidly as altitude increases.
Vocabulary
- gas: State of matter that has neither a fixed volume nor a fixed shape.
- pressure: Result of force acting on a given area.