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Real Gas| Behaviour and Deviations

Real gases are different from ideal gases because they have actual molecules that take up space and interact with each other. These interactions, affected by forces between molecules and their size, make real gases behave differently than ideal gases. Here we will learn behavior and deviations of real gas.

It’s important to understand how real gases behave because it helps scientists and engineers make accurate predictions about how they will act in different situations like temperature and pressure changes.

This knowledge is especially useful in fields like chemistry, physics, and engineering where precise calculations and predictions are needed for practical purposes.

Differences between Real and Ideal Gases

Real gases and ideal gases may seem similar, but they have some key differences. Let’s explore these differences in more detail.

Occupying Space and Molecular Forces

Unlike ideal gases, real gases actually occupy space. This means that the molecules of a real gas take up physical space as they move around.

On top of that, real gases experience attractive or repulsive forces between their molecules. These forces can cause the gas particles to interact with each other and affect their behavior.

Deviation from Ideal Gas Law

Real gases do not always follow the ideal gas law equation (PV = nRT). While the ideal gas law assumes that gas particles have no volume and do not interact with one another, these assumptions are not valid for real gases.

Under certain conditions, such as high pressures or low temperatures, real gases can deviate significantly from the predictions of the ideal gas law.

Factors affecting the behavior of Real Gases

The behavior of real gases is influenced by several factors.


Pressure plays a crucial role in determining how closely a real gas behaves like an ideal gas. Higher pressures tend to bring out the deviations from ideality in real gases.


Temperature also affects the behavior of real gases; at low temperatures, intermolecular forces become more significant and can cause deviations from ideality.

Molecular interactions

Molecular interactions also play a part in how real gases behave. Depending on whether these interactions are attractive or repulsive, they can affect properties such as compressibility and thermal expansion.

Deviations from Ideal Behavior in Real Gases

Real gases don’t always behave like ideal gases, especially under certain conditions. Let’s explore the deviations from ideal behavior in real gases.

High pressures and low temperatures

Real gases exhibit non-ideal behavior at high pressures and low temperatures. As the pressure increases or the temperature decreases, the behavior of a gas becomes less ideal.

Compressibility factor (Z)

To understand these deviations, scientists use a concept called the compressibility factor (Z). The compressibility factor takes into account intermolecular interactions between gas particles.

Factors contributing to non-ideal behavior

Several factors contribute to the non-ideal behavior of real gases:

  1. Molecular size: When gas molecules are large, they occupy more space and experience stronger intermolecular forces, leading to deviations from ideal behavior.

  2. Molecular shape: The shape of gas molecules can affect their ability to pack closely together, influencing intermolecular interactions and causing deviations.

  3. Polarity: Polar molecules have an uneven distribution of charge, resulting in stronger intermolecular attractions and non-ideal behavior.

Applications of Ideal Gases

These deviations from ideal behavior have practical implications. For example:

  • In industrial processes that involve high pressures or low temperatures, understanding how real gases deviate from ideality is crucial for accurate calculations and predictions.

  • In research settings, studying these deviations helps scientists develop more accurate models and equations to describe real gas behavior.

By considering factors such as molecular size, shape, and polarity, scientists can better explain why real gases don’t always behave ideally under certain conditions. This understanding is essential for various applications in industries and scientific research alike.

Van der Waals Equation

The Van der Waals equation is an important tool in understanding the behavior of real gases. It provides a more accurate description compared to the ideal gas law by incorporating corrections for molecular volume and intermolecular forces.

Corrections for Molecular Volume and Intermolecular Forces

The Van der Waals equation takes into account two key factors that affect real gas behavior: molecular volume (b) and intermolecular forces (a).

  • Molecular Volume (b): Unlike ideal gases, real gases occupy space due to their molecular size. The Van der Waals equation includes a correction term (b) to account for this volume.

    This correction helps us understand how the actual volume of a gas affects its behavior under different conditions.

  • Intermolecular Forces (a): Real gases experience attractive or repulsive forces between molecules, known as intermolecular forces. The Van der Waals equation incorporates another correction term (a) to consider these forces.

    By including this factor, we can better understand how interactions between gas molecules impact their behavior.

Accounting for Deviations in Real Gases

One of the main advantages of using the Van der Waals equation is its ability to account for deviations observed in real gases under various conditions.

These deviations can occur when a gas is subjected to high pressures or low temperatures.

  • High Pressure: At high pressures, real gases tend to deviate from ideal behavior by occupying more space than predicted by the ideal gas law. The Van der Waals equation helps us accurately calculate the pressure-volume relationship under such conditions.

  • Low Temperature: Real gases also deviate from ideality at low temperatures due to increased intermolecular attractions. The Van der Waals equation considers these intermolecular forces, providing a more precise estimation of gas properties at lower temperatures.

Understanding and applying the Van der Waals equation allows scientists and engineers to more accurately predict the behavior of real gases in a wide range of conditions.

Examples of Real Gas Phenomena

Real gases exhibit various interesting phenomena that can be observed in everyday life. Let’s explore some examples to better understand these phenomena.


When a gas cools down sufficiently, it undergoes a phase change into a liquid due to intermolecular attractions.

This process is known as condensation. You may have noticed this phenomenon when you see droplets forming on the outside of a cold beverage glass on a hot day.

The water vapor in the air comes into contact with the cold surface of the glass and condenses into liquid water droplets.


Certain gases can be converted into liquids by applying high pressure or reducing temperature. This process is called liquefaction.

For instance, when you use an aerosol spray, such as deodorant or hairspray, you are utilizing liquefied gases that turn back into gas upon release from the container. These gases are compressed and cooled to become liquids inside the canister.

Critical Point

At specific combinations of temperature and pressure, a substance reaches its critical point where distinct liquid-gas boundaries cease to exist. At this point, the substance exhibits unique properties.

For example, carbon dioxide (CO2) has a critical point at -56.6 degrees Celsius and 73 atmospheres of pressure. At temperatures and pressures above this critical point, CO2 behaves as a supercritical fluid with properties between those of a gas and a liquid.

Models for Real Gases

The behavior of real gases can be quite different from that of ideal gases. To better understand and describe this behavior, scientists have developed various models. Let’s take a look at three popular models used to study real gas phenomena.

The Virial Equation

Expanding on the Van der Waals equation, the Virial equation incorporates higher-order terms to provide a more accurate representation of real gas behavior.

Z = P V R T = 1 + B ( T ) V + C ( T ) V 2 +

where B(T) and C(T) are the temperature-dependent second and third virial coefficients, respectively, and Z is the compressibility factor.

This model takes into account factors such as molecular volume and intermolecular forces, which play significant roles in determining how gases behave under different conditions.

Redlich-Kwong Equation

The Redlich-Kwong equation is another model that considers both molecular volume and intermolecular forces. It provides accurate predictions for a wide range of gases by incorporating additional parameters in its calculations.

Redlich-Kwong Equation
Redlich-Kwong Equation

By accounting for these factors, the Redlich-Kwong equation offers improved accuracy when describing real gas properties.

Peng-Robinson Equation

Widely used in the petroleum industry, the Peng-Robinson equation is specifically designed to account for non-ideal behavior in hydrocarbon mixtures.

PCO2] = φCO2 PCO2 / P0

where φ is the fugacity coefficient. The gas pressure is divided by the standard state (P0 = 1 atm) to make the [activity], or [fugacity] dimensionless.

This model takes into consideration factors such as temperature, pressure, and molecular interactions to accurately predict the behavior of complex hydrocarbon systems.

These models help us understand how real gases deviate from ideal gas behavior by considering additional factors such as molecular size and intermolecular forces.

By using these equations, scientists can make more precise predictions about how gases will behave under various conditions.


We have learned about the differences between real and ideal gases, understanding the deviations from ideal behavior in real gases, and delving into the Van der Waals equation.

Furthermore, we have examined examples of real gas phenomena and explored various models for real gases.

Understanding the behavior of real gases is crucial in many scientific and industrial applications. By recognizing the deviations from ideal gas laws, scientists and engineers can develop more accurate models to predict and analyze real gas behavior.

Whether it’s in chemical reactions, environmental studies, or designing efficient processes, a detailed understanding of how real gases behave enables us to make informed decisions that lead to better outcomes.


What are some common examples of real gases?

Real gases can be found all around us. Some common examples include air (which consists mainly of nitrogen and oxygen), carbon dioxide (CO2), methane (CH4), hydrogen (H2), helium (He), and many other natural or synthetic compounds that exist as gaseous substances at normal temperatures and pressures.

How do real gases differ from ideal gases?

Unlike ideal gases which follow simple mathematical relationships described by the ideal gas law, real gases deviate from these laws due to intermolecular forces between their particles. Real gases occupy space and experience attractive or repulsive forces between molecules, leading to deviations from ideal gas behavior under certain conditions.

What is the Van der Waals equation used for?

The Van der Waals equation is an improvement over the ideal gas law as it takes into account the volume occupied by gas particles and the intermolecular forces between them. This equation allows for a more accurate description of real gas behavior, especially at high pressures or low temperatures.

How do real gases behave differently under extreme conditions?

Under extreme conditions of high pressure or low temperature, real gases can exhibit significant deviations from ideal behavior. They may liquefy or solidify when subjected to these conditions due to increased intermolecular attractions and decreased molecular motion.

Why is understanding real gas behavior important in industrial processes?

In various industrial processes such as chemical reactions, manufacturing, and energy production, gases play a crucial role. Understanding the behavior of real gases allows engineers and scientists to optimize process parameters, design efficient equipment, and ensure safe operation by considering factors like compressibility, phase changes, and reaction kinetics.