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1. Chapter 4
Elements ,Mixture and
compounds
Gases Pressure

2.  What is the pressure of a gas?
 Pressure is a force exerted by the substance per unit area on another
substance. The pressure of a gas is the force that the gas exerts on the walls
of its container. When you blow air into a balloon, the balloon expands
because the pressure of air molecules is greater on the inside of the balloon
than the outside.

3.  How is it measured?
 All gases that occupy a space have a pressure of some measurable
degree. Gas Pressure is measured in mmHg (millimeters of mercury) with a
barometer, which gives the barometric pressure. ... However, the SI unit
for pressure is the pascal (Pa)

4.  What Is a Gas?
 A gas is 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.

5.  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 below. The force of the particles against things they bump into
creates pressure. Pressure is defined in physics as the amount of force
pushing against a given area. How much pressure a gas exerts depends on the
number of gas particles in a given space and how fast they are moving. The
more gas particles there are and the faster they are moving, the greater the
pressure they create.

7.  Pressure in the Atmosphere
 We live in a “sea” of air called the atmosphere. Can you feel the air in the
atmosphere pressing against you? Not usually, but air actually exerts a lot of
pressure because there’s so much of it. The atmosphere rises high above
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.

8.  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.

9.  Vocabulary
 gas: State of matter that has neither a fixed volume nor a fixed shape.
 pressure: Result of force acting on a given area.