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Chapter: 10th Science : Chapter 3 : Thermal Physics


Gases are classified as real gases and ideal gases.


Gases are classified as real gases and ideal gases.


1. Real 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.


2. Ideal Gases

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.


3. Ideal Gas Equation

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, NA.

i.e. n = µNA. (3.5)

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

PV/ µNAT = constant

The value of the constant in the above equation is taken to be kB, which is called as Boltzmann constant (1.38 × 10–23 JK–1). Hence, we have the following equation:

PV/ µNAT = kB

PV = µNA kB T

Here, µNAkB = R, which is termed as universal gas constant whose value is

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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