The Behaviour of Gases

Introduction

The behaviour of gas molecules is completely dependent on the laws and properties possessed by them. The scattering of gas molecules is very much different from the distribution in solids and liquids. To understand the behaviour of gas molecules, you need to understand the laws and properties that are related to gas molecules. There are 5 properties and 5 laws that you need to learn. 

The behaviour of gases contains various formulas, concepts, and real-life problems. Once you are good at basic concepts, you can solve various types of problems and logic. If you want to score good marks in examinations, then read these notes before appearing for the exam. 

What is gas?

Gas is interpreted as a homogeneous fluid that has low viscosity and low density. The volume of gas is also assumed to have equal volume that of the vessel. Gases are classified into the following categories- 

• Ideal gas
• Non-ideal gas( also known as real gas)

Properties of gas are below:

What is the kinetic theory of gases?

It is an important concept while studying the behaviour of gases. The behaviour of gases can be easily understood with the help of the kinetic theory of gases. Kinetic theory is nothing but the study of gases at the macroscopic level. There are 5 well-known postulates of the kinetic theory that are mentioned below- 

1. The volume of the gas molecules is nothing when compared the distance between these molecules is much greater than its size. It is taken as negligible.
2. Gas is a mixture of a large number of molecules or atoms that are constantly moving at a random speed which gives rise to its kinetic energy. 
3. The intermolecular interactions in gases are also considered to be negligible. 
4. The average kinetic energy of all the molecules is completely dependent on its temperature. As the temperature rises, the molecule’s speed increases due to which kinetic energy also increases and vice versa.
5. The crash of molecules with each other and with the wall of the vessel is always elastic in nature.

What are the factors that affect the behaviour of gases?

These are some factors that are responsible for the behaviour of gases at various levels- 

• Volume (V)
• Temperature (T)
• Quantity (n)
• Pressure (P)

The above-mentioned factors are related to each other and they can be better understood from the following points- 

• When the temperature decreases, as the gas molecules contract it also reduces the volume.
• When the temperature increases, the molecules expand which leads to an increase in the pressure of gas molecules. 
• When the temperature increases, the gas molecules expand and the volume also expands or increases.
• When the temperature decreases, as the molecules contract the pressure also reduces. 
• To convert gas into liquid or solid, we need to lower the temperature, and pressure should be very high.
• To reduce the pressure, the quantity of the gas should be less. 
• When the quantity of gas decreases, it also reduces the pressure, and vica versa is also true. 
• To increase pressure, the quantity or volume of gas should be more. 

Why do real gases behave as ideal gases?

An ideal gas is nothing but the gas which obeys all the laws. Generally, when the temperature rises to an extreme level and when it archives lower pressure, the real gases behave as ideal gases. At this level,  the potential energy due to intermolecular forces becomes less notable when compared to its kinetic energy. And the size of gas molecules becomes less notable when compared to the space between them. This leads to its ideal behaviour. 

What are gas laws?

As discussed above, there are 5 laws of gas that follow. These laws are explained below. 

• Ideal gas law
• Boyle’s law
• Avogadro’s law
• Gay-Lussac’s law
• Charles’s law

Let’s read about each of these laws one by one!!

1.Ideal gas law

As per the ideal gas law, when we multiply the pressure and volume of one gram molecule of an ideal gas, it is equal to the product of the universal gas constant, a number of moles of the gas, and the absolute temperature. The representation is shown below- 

Where,

• P is the pressure of the gas
• T is the temperature of the gas
• V is the volume of the gas
• N is Avogadro’s number, NA = 6.0221×1023 
• n is the number of moles
• R is the universal gas constant = 8.3145 J.mol-1.K-1
  
  

2.Boyle’s law

As per Boyle’s law of gas, the volume of gas is inversely proportional to its pressure when we fix the temperature. 

P/V = constant

P1/V1 = P2/V2 = P3/V3 = constant

Where

• V is the representation of the volume of gas
• P is the representation of pressure of a gas

3.Avogadro’s law

As per Avogadro’s law, the number of molecules is directly proportional to the volume of gas at a contact pressure and temperature. The representation is shown below- 

Where,

• V is the volume of the gas
• k is the proportionality constant
• n is the number of moles

4.Gay-Lussac’s law

As per Gay-Lussac’s law, the pressure of a specific mass of gas is directly proportional to the absolute temperature of the gas. The representation is shown below- 

Where

• T1 is the initial temperature
• T2 is the final temperature
• P1 is the initial pressure
• P2 is the final pressure

5.Charles’s law

As per Charles’s law, the volume of the gas of a specific mass is directly related to the temperature. The representation is shown below-

Where,

• V is the volume of gas
• T is the temperature of a gas

Conclusion

The behaviour of gases contains various formulas, concepts, and real-life problems. Once you are good at basic concepts, you can solve various types of problems and logic. It is an important concept while studying the behaviour of gases. The behaviour of gases can be easily understood with the help of the kinetic theory of gases. Kinetic theory is nothing but the study of gases at the macroscopic level. An ideal gas is nothing but the gas which obeys all the laws. Generally, when the temperature rises to an extreme level and when it archives lower pressure, the real gases behave as ideal gases.