Difference Between Ideal Gas and Real Gas

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States of Matter

Gas, Solids and liquids are considered as the three states of matter. The chemical constitution of a substance remains the same in all three states. All these states are interconvertible. State change is also known as a phase change. Every phase change is accompanied by free energy change ∆G.

∆G = ∆H - T∆S

∆H is the enthalpy change associated with making/breaking of bonds and ∆S is the entropy change. Entropy is the degree of disorderliness. As compared to liquids, gases are more disordered and have more entropy.

By the process of Sublimation, solid can directly change into a gas. By condensation, the gaseous state changes into a liquid state. By solidification, liquid changes into a solid-state.

Characteristics of The Gaseous State of Matter


  • Gases have no definite shape. They fill the containers.

  • Gases are highly compressible.

  • Gases have low density.

  • Large empty spaces between the particles of gases are present.

Kinetic- Molecular Theory of Gases

This theory is based on certain assumptions. This theory deals with the microscopic model of a gas. The properties studied above is a macroscopic model.

  • Gases consist of tiny particles moving randomly.

  • The volume of gas molecules is negligible compared to the total volume of the container.

  • The gas particles act independently of one another. The attractive forces are absent between the particles.

  • Collisions of a gas particle with other particles or with the wall are elastic collisions. This leads to the total kinetic energy to be constant.

  • The average kinetic energy of the particles of the gas is directly proportional to the temperature of the gas.

  • The pressure exerted by a gas is a result of collisions of gas particles with walls of the container.

  • The speed of particles in gas at a particular time is different. Thus, the kinetic energy will also be different. On collision, the speed changes but the distribution of speed always remains constant. This distribution is Maxwell-Boltzmann distribution.

Gas Laws

Boyle's Law

  • The law states that "Gas volume is inversely proportional to gas pressure, at a constant temperature."

This is true for fixed amounts of gas.

  • According to this law, pressure increases when the volume decreases.

  • P is inversely proportional to V

  • PV is constant

  • => P1V1 = P2V2

Charles's Law

  • This law states that Volume is directly proportional to the temperature at constant pressure and for a fixed mass of a gas.

Gay - Lussac's Law

  • According to this law, at constant volume, Pressure is directly proportional to the temperature.

  • PT = constant.

  • P1/T1 = P2/T2

Avogadro's Law

  • Two gases that have equal volumes also have the same number of molecules. This is true at the same temperature and pressure.

  • The gas’ volume is proportional to the moles.

  • V1/n1  = V2 / n2

The Ideal Gas Equation

All the variables can be related according to the gas laws in a single equation known as the Ideal Gas Equation.

PV = nRT

This is the ideal gas equation and R is the ideal gas constant.

R is expressed in the unit of work or energy per mole per Kelvin.

Ideal Gas Real Gas Difference

An ideal gas is that gas that follows all the gas laws.

For an ideal gas,

PV/RT= Z = 1

Z is the compressibility factor. The graph of PV/RT against p at 0°C is a horizontal straight line.

However, at high and low temperatures, gases are not found to behave ideally.  There are various deviations seen from the ideal state.

The Ideal Gas and Real Gas

Real gases behave differently from ideal gas behaviour because of the assumptions taken in the kinetic theory of the gases.

  • The volume of molecules of a gas is not negligible. This volume correction is required.

  • It was assumed that there were no intermolecular forces between the gas molecules. Thus a pressure correction is also needed.

At low temperatures, the above effects were negligible but at high temperatures, these assumptions do not prove right.

[Image will be Uploaded Soon]

The figure shows the deviation from ideal gas behaviour.

Difference Between Ideal and Non- Ideal gas

The deviation of the compressibility factor from 1 is the measure of the non-ideality of a gas. For an ideal gas, Z is 1 but for real gas, Z is greater than or less than 1 but not equal to 1.

Van Der Waals Equation represents the behaviour of real gas-

[ p + n2a/V2 ] [ V - nb ] = nRT

a and b are called van der Waals constants. a is the correction of intermolecular attraction and b is the volume correction.

FAQ (Frequently Asked Questions)

Q1. What is Dalton's Law of Partial Pressure and Law of Partial Volumes?

Dalton's Law of Partial Pressure

If two gases which are non-reactive with each other are kept in the same container, then the total pressure exerted by the mixture of gases on the wall of the container is equal to the sum of the Partial Pressure of each constituent gas.

Thus P (total) = P1 + P2 + P3……….. ( V and T constant)

Where P1, P2 P3 are partial pressures of respective constituent gas. 

Law of Partial Volumes

According to this, the total volume of the mixture of gases is the sum of Partial Volumes of the constituent gases of the mixture.

V(total) = V1 + V2 + V3……….

V1, V2, V3 are partial volumes of the constituents.

Q2. Write the Characteristics of Solid and Liquid States of Matter.

Following are the characteristics of solid and liquid states of matter:


  • Solids have a definite shape, size and volume. They resist deformation.

  • They have an orderly arrangement of particles that are placed very close to each other.

  • Solids are denser than liquids.

  • They are not compressible.

  • They are not fluids as they cannot flow.

  • They cannot diffuse.


  • Liquids do not have a definite shape. They acquire the same shape as the container

  • Liquids are fluids.

  • Liquids are slightly compressible as compared to solids.

  • Liquids have a density lower than solids but greater than gases.

  • They have a definite volume.

  • They are diffusible in other liquids.