Gases are classified as real gases and ideal gases.

**GASES**

Gases are classified as
real gases and ideal gases.

If the molecules or
atoms of a gases interact with each other with a definite amount of
intermolecular or inter atomic force of attraction, then the gases are said to
be **real** **gases. **At very high temperature or low pressure,** **a
real gases behaves as an ideal gases because in this condition there is no
interatomic or intermolecular force of attraction.

If the atoms or
molecules of a gas do not interact with each other, then the gas is said to be
an **ideal gas** or a **perfect gas.**

Actually, in practice,
no gas is ideal. The molecules of any gas will have a certain amount of
interaction among them. But, these interactions are weaker when the pressure is
low or the temperature is high because the interatomic or intermolecular forces
of attraction are weak in ideal gas. Hence, a real gas at low pressure or high
temperature can be termed as a perfect gas.

Ideal gases obey Boyle’s
law, Charles’s law and Avogadro’s law. All these laws state the relationship
between various properties of a gas such as pressure (P), volume (V),
temperature (T) and number of atoms (n). In a given state of the gas, all
these parameters will have a definite set of values. When there is a change in
the state of the gas, any one or more of these parameters change its value. The
above said laws relate these changes.

The ideal gas equation
is an equation, which relates all the properties of an ideal gas. An ideal gas
obeys Boyle’s law and Charles’ law and Avogadro’s law. According to Boyle’s
law,

PV = *constant* (3.1)

According to Charles’s
law,

V/T = constant (3.2)

According to Avogadro’s
law,

V/n = constant (3.3)

After combining
equations (3.1), (3.2) and (3.3), you can get the following equation.

PV/nT = constant (3.4)

The above relation is
called the combined law of gases. If you consider a gas, which contains µ moles
of the gas, the number of atoms contained will be equal to µ times the Avogadro
number, N_{A}.

i.e. n = µN_{A}.
(3.5)

Using equation (3.5),
equation (3.4) can be written as

PV/ µN_{A}T =
constant

The value of the
constant in the above equation is taken to be k_{B}, which is called as
**Boltzmann constant (1.38 × 10 ^{–23} JK^{–1}). **Hence,

*PV/ µN _{A}T = k_{B}*

*PV = µN _{A} k_{B}
T*

Here, *µN _{A}k_{B}*

*8.31 J mol ^{−1} K^{−1}.*

PV = RT (3.6)

Ideal gas equation is
also called as *equation* *of state *because it gives the relation
between* *the state variables and it is used to describe the state of any
gas.

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10th Science : Chapter 3 : Thermal Physics : Gases |

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