is one of the three main states of matter. The other two
states are solid and liquid. These states differ from each
other in the way they fill space and change shape. A solid,
such as rock, always occupies a fixed volume (amount of
space) and has a fixed shape. A liquid, such as water, always
occupies a fixed amount of space. But it has no shape of
its own, so it takes on the shape of its container. A gas,
such as air, has neither a fixed shape nor a fixed volume.
It fills any container that holds it, and takes on the container's
shape. Like solids and liquids, gases have weight. But except
under great pressure, gases are thinner and lighter than
solids and liquids.
Many gases, including the nitrogen and oxygen in air, have
no colour or odour. They can be identified by their chemical
behaviour, weight, ability to absorb heat, and other properties.
But some gases have a colour, or an odour, or both. For
example, nitrogen dioxide is brown. Hydrogen sulphide smells
like rotten eggs.
Under special conditions, gases change into a fourth state
of matter called a plasma. Plasmas are formed by heating
a gas to an extremely high temperature or by passing an
electric current through it. Matter exists in a plasma state
in stars and the regions between stars.
How gases behave. The behaviour of gases is explained by
what scientists call the kinetic theory. According to this
theory, all matter is made of constantly moving particles--atoms
or molecules. An atom is one of the basic units of matter,
and a molecule is a combination of atoms. The number of
atoms or molecules of gas that could be held in a container
the size of a pin-head is many millions of times as large
as the number of people on the earth. But these gas particles
are so small that they occupy only about one-thousandth
of the space inside the container. The remaining space between
the particles is empty.
Gas particles fly around in all directions at about the
speed of sound. Their exact speed is determined by their
weight and by the temperature of the gas. Gas particles
move faster when the gas is hot than when it is cold. But
light particles move faster than heavy ones at all temperatures.
Each moving gas particle crashes into billions of other
particles each second. Gas particles crashing into the walls
of their container produce an effect called pressure.
A gas liquefies (changes to a liquid) when it is cooled
to a temperature called its boiling point. At this temperature,
the gas particles gather together to form a liquid. If the
pressure of the gas is increased, it liquefies at a higher
temperature. But pressure can raise the liquefying temperature
only to a limiting value called the critical temperature.
For example, oxygen under normal atmospheric pressure liquefies
at its boiling point, -183 °C. But under a pressure
of 5,171 kilopascals, oxygen liquefies at -119 °C, its
Gas laws. Three laws explain approximately how the pressure,
temperature, volume, and the number of particles in a container
of gas are related. These laws are Boyle's law, Charles's
law, and Avogadro's law.
Boyle's law says that pressure increases as the volume
of gas decreases. According to Boyle's law, the product
of the pressure (P) multiplied by the volume (V) remains
constant if there is no change in the temperature or in
the number of particles inside the container. This law is
PV = constant. Boyle's law says that the pressure doubles
when a gas is compressed to half its volume at constant
Boyle's law was first published by the Irish chemist Robert
Boyle in 1662. But other chemists had discovered the law
earlier. In 1660 and 1661, Richard Towneley and Henry Power
of England experimented with air below atmospheric pressure.
They found that the product of the air's pressure and volume
remained constant. At about the same time, Robert Hooke
of England experimented with air above atmospheric pressure.
Hooke's findings agreed with those of Towneley and Power.
Additional experiments by Boyle confirmed all these findings.
In 1679, Edme Mariotte of France published the results of
his own experiments with gases. Mariotte's writings became
well known in Europe. Thus, the law known today as Boyle's
law in North America and Great Britain is called Mariotte's
law in continental Europe.
Charles's law states that a gas expands by the same fraction
of its original volume with each degree that its temperature
rises. According to Charles's law, the ratio between the
volume (V) of a gas and its temperature (T) remains constant
if the pressure does not change. The law is written:
V/T = constant. In this equation, T is the absolute temperature
of the gas. It is usually measured in kelvins (Celsius degrees
above absolute zero). Kelvin is abbreviated K. For example,
when a gas is heated from 300 K (room temperature) to 600
K, its absolute temperature doubles. Doubling the temperature
doubles the gas's volume if the pressure does not change.
See ABSOLUTE ZERO.
Charles's law was discovered in 1787 by the French chemist
Jacques Alexandre Cesar Charles. He found that carbon dioxide,
hydrogen, oxygen, and nitrogen all expand at constant rates
as their temperatures rise. Charles did not publish his
findings, but explained his experiments to the French chemist
Joseph Gay-Lussac. Gay-Lussac performed similar experiments
and published his results in 1802. As a result, Charles's
law is sometimes called Gay-Lussac's law.
Avogadro's law was first proposed in 1811 by the Italian
scientist and philosopher Amedeo Avogadro. It says that
equal volumes of different gases all contain the same number
of particles if they all have the same pressure and temperature.
It was later discovered that a volume of 22.4 litres of
gas at 0 °C and atmospheric pressure contains about
602,000,000,000,000,000,000,000 (602 billion trillion) particles.
This number is usually written 6.02 X 10 to the power of
23 and is called the Avogadro constant. This number of particles
of any substance is called one mole of the substance.
The universal gas law combines Boyle's law, Charles's law,
and Avogadro's law into a single statement. This law is
PV = nRT.In this equation, P represents the pressure of
the gas, V represents its volume, n represents the number
of moles of gas, and T represents its absolute temperature.
R is a constant called the universal gas constant. It has
a value of 8.314 joules per kelvin per mole. According to
the universal gas law, the pressure of a gas can be doubled
in three ways: (1) the gas can be squeezed into one-half
its original volume, (2) twice as much gas can be forced
into he original volume, or (3) the absolute temperature
can be doubled.