What is a real gas?

Short Answer

A real gas is a gas that does not follow all the assumptions of the ideal gas equation perfectly. Real gases have intermolecular forces and their molecules occupy some actual volume. Because of these factors, real gases behave differently from ideal gases, especially at high pressure and low temperature.

In everyday conditions, many gases behave almost like ideal gases, but under extreme conditions, real gases show noticeable deviations. Examples include carbon dioxide, ammonia, hydrogen, oxygen, and nitrogen, all of which are considered real gases.

Detailed Explanation

Real Gas

A real gas is a gas that shows actual behaviour found in nature, where gas particles attract each other slightly and occupy a definite volume. Real gases do not completely follow the assumptions of the ideal gas theory. Ideal gases are imaginary gases used to make understanding easier, while real gases are the gases we find in the real world. In an ideal gas, particles do not attract or repel each other and have no volume, but real gases do not follow these conditions. Because of these differences, real gases behave differently under certain conditions, especially at high pressures and low temperatures.

In practical life, all gases such as oxygen, nitrogen, hydrogen, carbon dioxide, helium, and ammonia are real gases. They behave nearly ideally under normal temperature and pressure but show deviation under extreme conditions. Understanding real gases is important because gas behaviour in industries, laboratories, and nature depends on these real conditions.

Characteristics of Real Gases

Real gases differ from ideal gases due to two main reasons:

  1. They have intermolecular forces.
  2. They have finite volume.

These characteristics affect the behaviour of gases and make them deviate from the ideal gas equation.

  1. Real gases have intermolecular forces
    In real gases, gas particles attract each other weakly. These attractive forces help particles stick closer when pressure increases or temperature decreases. Because ideal gases assume no attraction, real gases behave differently in real environments.
  2. Real gases have measurable volume
    Gas particles take up some space. Ideal gases assume particles have no volume, but in reality, gas molecules occupy a small but measurable amount of space. When gases are compressed at high pressures, the size of molecules affects the behaviour of the gas.
  3. Behaviour changes under extreme conditions
    Real gases show more deviation when:
  • Pressure is very high
  • Temperature is very low
    Under these conditions, particles come closer and the ideal gas assumptions fail.

Why Real Gases Deviate from Ideal Behaviour

The deviations arise mainly because of two factors:

Intermolecular Attractions

Real gases have attractive forces between molecules. At low temperatures, molecules move slowly and come closer. These attractions reduce the pressure exerted by the gas compared to ideal gases. As a result, real gas pressure becomes lower than the ideal gas pressure.

Finite Molecular Volume

Ideal gases assume zero volume of gas particles. But real gas molecules occupy space. When pressure is high, the volume taken up by particles becomes significant. Therefore, the actual volume of a real gas is larger than the volume calculated using the ideal gas equation.

Because of these factors, real gases do not follow the ideal gas equation PV = nRT perfectly.

Van der Waals Equation for Real Gases

To correct these deviations, scientists use the van der Waals equation, which adjusts the ideal gas equation for real gas behaviour. It accounts for:

  • ‘a’ → correction for intermolecular forces
  • ‘b’ → correction for molecular volume

This equation gives more accurate results for real gases under all conditions.

Conditions When Real Gases Behave Like Ideal Gases

Real gases behave almost like ideal gases when:

  • Temperature is high (particles move fast and attraction becomes negligible)
  • Pressure is low (particles are far apart and volume of particles becomes negligible)

Under these conditions, real gases follow the ideal gas equation closely.

Examples of Real Gases

Every natural gas is a real gas. Some examples include:

  • Oxygen (O₂)
  • Nitrogen (N₂)
  • Carbon dioxide (CO₂)
  • Helium (He)
  • Hydrogen (H₂)
  • Ammonia (NH₃)

All these gases show deviations from ideal behaviour at certain conditions.

Importance of Studying Real Gases

Studying real gases is important because:

  • Industrial processes involving gases require accurate predictions.
  • Gas cylinders, LPG tanks, and oxygen tanks behave as real gases under compression.
  • Weather patterns and atmospheric behaviour depend on real gas behaviour.
  • Chemical reactions involving gases, especially under high pressure, require real gas equations.
  • Understanding deviations helps design safer and more efficient equipment (engines, refrigeration systems, compressors).
Conclusion

A real gas is a gas that does not follow the assumptions of the ideal gas theory due to intermolecular forces and the finite volume of molecules. Real gases behave nearly ideally at normal conditions but deviate significantly at high pressure and low temperature. Understanding real gases allows accurate predictions and safer use of gases in science, industry, and daily life.