Gases 

Did you know that a spoonful of water will just sit in a cup, but the same amount of gas can fill an entire balloon? That’s the wonder of the gaseous state—tiny, invisible particles moving endlessly, spreading to every corner. From the air we breathe to the fuel that powers rockets, gas can surprise you with its characteristics. So let's explore and uncover everything about this important state of matter.

1.0Introduction

Gas is the most dynamic form of the fundamental three states of matter. It has neither a fixed shape nor a definitive volume, unlike its counterparts (solids or Liquids). The gaseous state of matter can occupy the whole space they are in, and is available to it. This is because the molecules of gases move freely, and, hence, can be compressed, expanded, and mixed easily. Some of the most common examples of gases are oxygen, nitrogen, carbon dioxide, and the mixture of gases, “the air” we breathe. 

2.0Characteristics of Gas

The characteristics of gases are very unique from the other three states of matter, these include: 

• Shape and Volume: Gases, unlike solids or liquids, do not have a fixed shape or volume. They expand to fill the complete space of the container they are kept in.
• Compressibility: Due to the large intermolecular spacing, gases are highly compressible, meaning they can be compressed easily under pressure. 
• Density: Very low density compared to solids and liquids due to the wide space between their molecules. 
• Diffusion: Diffusion is a fundamental property of gases, meaning they can mix and spread evenly with other gases. 
• Pressure and Temperature: Gases exhibit a strong response to changes in pressure and temperature. 

3.0Molecular Nature of Gases 

The properties of gases can be understood by studying their molecular nature. These properties include: 

1. Intermolecular Forces: The attractive forces between molecules are very weak in gases, allowing molecules to move freely in space. This explains high compressibility and indefinite shape and volume of gases. 
2. Motion: Gas molecules move at a very high speed, and that too without any fixed line of movement. 
3. Energy: The high-speed molecules give off their kinetic energy, which is directly proportional to temperature. 
4. Pressure: Though very small, the molecules of gases also produce pressure on the walls of the container they are kept in. 

4.0Gas Laws

It is mentioned earlier that the behaviour of gases largely depends upon the pressure and temperature of the surroundings. Gas laws describe the relation between these units, along with the volume and number of moles of these gases: 

Boyle’s Law: 

Boyle’s law states that at constant temperature and number of moles, the internal pressure of the gas is inversely proportional to its volume. Mathematically, it can be expressed as: 

$P\propto \frac{1}{v}$

Now, take the pressure of gas before the external pressure is applied, as P₁ and the volume be V₁. After an external pressure is applied, let the pressure of the gas be P_2, and the volume will be V₂. 

Hence, Boyle’s equation will be:  $P_1{V}_1=P_2{V}_2$

Charles’s Law: 

As per Charles’s law, at constant pressure, the volume of a gas is directly proportional to the absolute(kelvin) temperature. Mathematically, it is: $V\propto T$

To understand this, let the volume of a gas at a certain temperature, say T_1, be V₁. 

After increasing the temperature of the gas to T_2, the new volume of the gas will be V₂. 

Hence, Charles’s law will be: $V_1{T}_1=V_2{T}_2$

Gay-Lussac’s Law:

According to Gay-Lussac’s Law, the pressure of a gas on the container wall will increase/decrease with an increase/decrease in the absolute temperature, if the volume of the gas remains constant. Mathematically, it is: $P\propto T$

To understand this, let the volume of a gas in a container be constant and the pressure and temperature be P₁ and T₁, respectively. 

Now, increase the temperature of the container to T₂, and the pressure on the walls of the container will become P₂. 

And, Gay-Lussac’s Equation will be: $P_1{T}_1=P_2{T}_2$

Avogadro’s Law

Avogadro’s law states that at equal volumes for different gases, when kept at the same temperature and pressure, they contain the same number of molecules. 

Meaning, if two gases, say hydrogen and oxygen, have the same volumes, and are at the same temperature and pressure, they possess the same number of molecules. 

Mathematically, it can be said that the volume is directly proportional to the number of moles, under these conditions: $V\propto n$

5.0Ideal Gas Equation

An ideal gas is a theoretical concept which assumes that an individual gas molecule is negligible in comparison to the container, moves randomly, and does not exert any force on other molecules. It also assumes that the collision between any gas molecules does not result in a loss of energy. 

An ideal gas follows all the laws mentioned above, and hence, gives rise to an equation known as the Ideal Gas equation. That is: $V\propto \frac{nT}{P}$

By introducing in the equation, here, R, the universal gas constant with the value 8.314J/mol/K), we have the final equation as: PV=nRT

Note that real gases do not perfectly follow the ideal gas equation. In fact, at high pressure or low temperature, the factor affecting this equation becomes significant. So, to correct these derivations, another equation, the van der Waals equation, was introduced to modify the ideal gas equation as: 

$(P+\frac{a{n}^2}{V^2})(V-nb)=nRT$

Here, 

• a = correction factor for intermolecular attraction (greater attraction → higher a)
• b = correction factor for finite molecular volume (represents the excluded volume occupied by molecules)