Difference between Ideal Gas and Real Gas

Introduction

An ideal gas is nothing but a theoretical(exists only in theory) gas because, in reality, it is not possible to have an ideal kind of gas. An ideal gas is formed of large numbers of randomly(in any direction) rotating point-like particles that are not subjected to interaction with one another. This ideal gas (theoretical gas)concept or idea is useful or important because it completely follows the ideal gas law(or ideal gas equation). A real gas is a non-ideal gas and here the molecules occupy the desired space and also interact with one another and they do not obey the ideal gas law or ideal gas equation. Cool air at normal pressure and temperature behaves as an ideal gas but on increasing the pressure and temperature of the same gas it starts behaving like a real gas because of the interactions between the molecules of the gas.

What is an ideal gas?

An ideal gas is nothing but a theoretical gas that does not exist in reality or the real world. It is composed of a large number of randomly(in any direction) moving points or dot-like particles that are not allowed to interact with one another. But the concept or idea of an ideal gas is essential because it follows the ideal gas law that is P(pressure in atm or bar)×V(volume in l or mL)= n(moles given)×R(gas constant)×T(temperature). In some special cases like perfectly elastic collisions or point-like collisions, the basic requirement or necessity of zero interaction(collision with one another) can often be relaxed. Although under various circumstances like changes in temperature(T) and pressure(P), many real gases start behaving like an ideal gas where the gaseous molecules act as ideal particles. Generally, At higher temperatures and lower pressure, the gases start behaving like an ideal gas because the potential energy(U) due to the intermolecular forces(kind of molecular force) becomes very low or less significant as compared to the particles with the kinetic energy.

Василь Іванович Сидоров, Molecular power 42. Model of ideal gas, CC BY-SA 4.0

Properties of an ideal gas

Some of the properties or characteristics of an ideal gas are as follows-

• An ideal gas is compressible because the molecules contain so much energy that they force each other to move inward, which is what we call compression.

• In an ideal gas, most of the collisions with the container are elastic which means they lose no energy or zero energy after the collision and keep moving.

• An ideal gas is capable of taking the shape of the container in which they are kept because of its compressibility. So, the volume of the container can be considered as the overall volume of the gas present inside the container.

• An ideal gas behaves inert which means they are assumed to not react with one another.

• The particles or molecules of the ideal gas are point masses with no or zero volume.

What is real gas?

Real gases are not ideal or non-ideal gases whose molecules occupy space or fill the space and interact with each other. Real gas does not follow the ideal gas law or ideal gas equation. So, the real gases are further designed or modelled or represented by taking into consideration their molar or molecular weight and molar or molecular volume. That is $\mathrm{RT\:=\:(p\:+\:\frac{a}{v^{2}_{m}})(V_{m}\:-\:b)\:or\:,p\:=\:\frac{RT}{(V_{m}\:-\:b)}\:=\:\frac{a}{V_{m}^{2}}}$ where p=pressure(in atm or bar), T= temperature(mostly in kelvin), R= ideal gas constant or gas constant, and 𝑉𝑚= molar volume. a and b are the variables that vary and are determined empirically for individual gases. When we increase the pressure or temperature of the ideal gas then the interactions between the molecules become very fast or rapid which cannot be calculated or predicted using the gas law and hence considered a real gas. Here the gas molecules collide inelastically with each other. Here in the real gas either attractive or repulsive forces are found or present between the molecules or particles.

Pbsouthwood, Real gas compression graph (breathing gases for diving), CC0 1.0

Properties of real gas

Some of the properties or characteristics of a real gas are as follows-

• Real gas contains a definite amount of volume.

• In real gas, most of the collisions either with the container or with each other are non-elastic collisions.

• In the real gas either there will be intermolecular attraction or repulsion force between the molecules.

• Here the pressure applied by the molecules on the container in which they are placed is less as compared to the ideal gas.

• In real gas, the gaseous molecules can freely interact or collide with each other.

• Real gas does not obey the ideal gas equation but they obey van der Waals real gas equation which is $\mathrm{RT\:=\:(p\:+\:\frac{a}{v^{2}_{m}})(V_{m}\:-\:b)}$

Difference between Ideal gas and Real gas

Ideal gas	Real gas
An Ideal gas does not have any definite or fixed volume as it takes the shape of the container in which it is placed.	Real gas occupies a definite volume as it does not occupy the shape of the container.
Here there are no intermolecular forces (either attraction or repulsion) between the molecules.	Here there is either an intermolecular force of attraction or repulsion present.
Here the particles undergo elastic collision.	Here the particles undergo non-elastic collision.
An ideal gas is a theoretical concept that does not exist in nature.	A real gas is present or exists in reality.
Here the molecules move randomly with very high pressure.	Here the molecules move randomly with less pressure as compared to an ideal gas.
It follows the ideal gas law that is $\mathrm{PV\:=\:nRT}$.	It obeys the van der Waals real gas equation which is $\mathrm{RT\:=\:(p\:+\:\frac{a}{v^{2}_{m}})(V_{m}\:-\:b)}$

Conclusion

An ideal gas is nothing but a theoretical(exists in theory only) gas that does not exist in nature. This gas obeys the ideal gas law equation that is PV = nRT. In an ideal gas, molecules move with very high pressure in a random direction. Here the particles present undergo elastic collision and there are no intermolecular forces present there. A real gas is a gas that exists in nature and they are also called non-ideal gas because they do not obey the ideal gas law or ideal gas equation. In real gas, the molecules move in a random direction with less pressure as compared to the ideal gas, and here the particles can interact with each other and undergo non-elastic collision.

FAQs

1. What do you mean by ideal gas?

An ideal gas is a theoretical(not practical) gas that obeys or follows the ideal gas law that is given by - PV = nRT. But in reality, no such gas exists. It is composed of a large number of randomly(in any direction) moving dot or point-like particles that are not allowed to interact with one another.

2. What is non-ideal gas?

A real gas is also called non-ideal gas that does not obey the ideal gas law or ideal gas equation. This gas exists in nature and here the molecules undergo non-elastic collision and the molecules can interact with each other.

3. Write the van der waals equation?

Van der Waals gas equation is given by $\mathrm{RT\:=\:(p\:+\:\frac{a}{v^{2}_{m}})(V_{m}\:-\:b)}$ where, p or P= pressure applied, T= temperature required, R= ideal gas constant, and 𝑉𝑚= molar volume(occupied by the gas). a and b are the variables that vary from one gas to another.

4. What is elastic collision?

An elastic collision is a type of collision that takes place between two bodies where the kinetic energy(K.E.) before and after the collision remains the same or there is no net loss in the energy(kinetic).

5. Which gas undergoes non-elastic collision?

Real gas undergoes non-elastic collision(kinetic energy does not remain the same before or after the collision).