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Usually, the word 'real gas' refers to a gas that does not function as an ideal gas. The interactions between gaseous molecules can explain their behaviour. Such intermolecular interactions between gas particles are the explanation of why the ideal gas law does not adhere to real gases. A real gas can therefore be characterized as non-ideal gases whose molecules occupy a given amount of space and are capable of interacting with one another. In this article, we will study the real gas definition, real gas equation, and ideal and real gases in detail.

Real Gas Definition

A real gas is defined as a gas that at all standard pressure and temperature conditions does not obey gas laws. It deviates from its ideal behaviour as the gas becomes huge and voluminous. True gases have velocity, mass, and volume. They liquefy when cooled to their boiling point. The space filled by gas is not small when compared to the total volume of gas.

Ideal and Real Gas Equation

An ideal gas is defined as a gas that obeys gas laws at all pressure and temperature conditions. Ideal gases have velocity as well as mass. They have no volume. The volume taken up by the gas is small as compared to the overall volume of the gas. It does not condense and triple-point does not exist.

The ideal gas law is the equation of the state of a hypothetical ideal gas, also called the general gas equation. Under many conditions, it is a reasonable approximation of the behaviour of several gases, but it has many limitations. In 1834, Benoît Paul Émile Clapeyron first described it as a variation of the empirical law of Boyle, the law of Charles, the law of Avogadro, and the law of Gay-Lussac. In an empirical form, the ideal gas law is also written:


Real Gas Law

By explicitly including the effects of molecular size and intermolecular forces, the Dutch physicist Johannes van der Waals modified the ideal gas law to explain the behaviour of real gases. The Van der Waal real gas equation is given below-

Real gas law equation,

=(P+an2/V2) (V-nb)=nRT

Where a and b represent the empirical constant which is unique for each gas.

n2/V2 represents the concentration of gas.

P represents pressure

R represents a universal gas constant and T is the temperature 

Ideal and Real Gases

The difference below shows the properties of real gas and ideal gas, and also the ideal and real gas behaviour.

Ideal Gas

Real Gas

No definite volume

Definite volume

Elastic Collision of particles

Non-elastic collisions between particles

No intermolecular attraction force

Intermolecular attraction force

Does not really exists in the environment and is a hypothetical gas

It really exists in the environment

High pressure

The pressure is less when compared to Ideal gas


Interacts with others

Obeys PV = nRT

Obeys p + ((n2a )/V2) (V – nb ) = nRT

Did You Know?

A factor known as compressibility factor Z is determined by the deviation of real gas from ideal gas and is defined as the ratio of the actual volume to the volume predicted by the ideal gas law at the same temperature and pressure Z = Actual volume/volume predicted by the ideal gas = v/RT/P

But the ideal gas rate, Videal, is RT/P. The compressibility factor can therefore also be defined as the ratio of specific real gas volume to specific ideal gas volume, i.e.

Compressibility factor Z= vreal gas/videal gas

As we all know, at very low pressures and high temperatures, all gases act as ideal gases. So when the pressures are reduced, as the gas behaves as ideal, the value of Z tends to unite. 

It is to be remembered that, depending on the pressure and temperature, the value of Z can be less than unity or greater than unity. The compressibility factor chart shows the Z values corresponding to the pressure.

FAQ (Frequently Asked Questions)

Question: What is an Example of a Real Gas?

Ans: Any gas that exists is a real gas. Oxygen, hydrogen, carbon dioxide, helium, carbon monoxide, etc. Real gases between particles have small attractive and repulsive forces and ideal gases do not. There is a volume of true gas particles and ideal gas particles do not.

Question: What are the Assumptions of an Ideal Gas?

Ans: The ideal gas law assumes that gases behave ideally, meaning that they conform to the following characteristics: (1) the collisions between molecules are elastic and their motion is frictionless, meaning that the molecules do not lose energy; (2) smaller magnitude is the total volume of the individual molecules.

Question: What is Charle's Gas Law?

Ans: The physical concept known as Charle's law states that, as determined on the Kelvin scale, the volume of gas equals a constant value multiplied by its temperature (zero Kelvin corresponds to -273.15 degrees Celsius)