Short Answer

Partial pressure is the pressure that a single gas in a mixture would exert if it occupied the entire container alone at the same temperature. In a mixture of gases, each gas behaves independently and contributes its own pressure to the total pressure.

The total pressure of a gas mixture is the sum of the partial pressures of all the gases present. Partial pressure helps in understanding how much each gas contributes in mixtures such as air, where nitrogen, oxygen, and other gases each have their own partial pressures.

Detailed Explanation

Partial Pressure

Partial pressure is an important concept used in studying gas mixtures. It describes the pressure exerted by an individual gas in a mixture of gases. Even when several gases occupy the same container, each gas acts independently and exerts pressure as if it were alone. This independent pressure is known as the partial pressure of that gas.

For example, air is a mixture of nitrogen, oxygen, carbon dioxide, and other gases. Each one contributes a certain portion of the total air pressure. The partial pressure of nitrogen is the pressure nitrogen would exert by itself in the same space, and the same applies to other gases.

Dalton’s law of partial pressures states:

Total pressure = Sum of all partial pressures

This means the pressure inside a container is the combined effect of all gases present. Partial pressure is useful in chemistry, biology, environmental science, and many industrial processes.

Meaning of Partial Pressure

Partial pressure has a simple meaning:
It is the pressure that a gas would create if it were the only gas in the container but kept at the same temperature.

Even though gases are together in a mixture, they do not lose their independent behaviour. Gas particles move randomly and freely. They collide with container walls and exert pressure. Because gases mix uniformly and do not react (in most cases), each gas contributes its own share of pressure.

How Partial Pressure Is Calculated

Partial pressure can be calculated using:

Pᵢ = Xᵢ × Pₜₒₜₐₗ
Where:

• Pᵢ = partial pressure of gas i
• Xᵢ = mole fraction of the gas
• Pₜₒₜₐₗ = total pressure of the mixture

For example:
If oxygen makes up 21 percent of air and total pressure is 1 atm,
Partial pressure of oxygen = 0.21 × 1 atm = 0.21 atm

This shows what proportion of the total pressure a gas contributes.

Why Gases Have Partial Pressures

Gases have partial pressures because:

1. They behave independently in a mixture.
2. They move randomly in all directions.
3. They occupy the entire volume of the container equally.
4. Their individual pressures add up to create the total pressure.

If gases reacted with each other, partial pressures would not work the same way, but most gases in the air are non-reacting under normal conditions.

Factors Affecting Partial Pressure

Several factors can change the partial pressure of a gas:

1. Amount of Gas (Moles)

More moles of gas increase its partial pressure because more molecules collide with the container walls.

2. Temperature

Higher temperature increases the kinetic energy of gas molecules and makes partial pressure rise.

3. Volume of the Container

If volume decreases, partial pressure increases due to more frequent collisions.

4. Total Pressure

Partial pressure depends on the total pressure of the mixture because it is a fraction of the total.

Partial Pressure in Real-Life Examples

Partial pressure is very important in many natural and scientific processes.

1. Breathing and Respiration

Oxygen enters the bloodstream because the partial pressure of oxygen in the lungs is higher than in the blood.
Carbon dioxide moves out of the blood because its partial pressure is higher in the blood than in the lungs.

2. Scuba Diving

Divers breathe compressed air. At greater depths, the partial pressure of nitrogen increases and can affect the body. Dalton’s law helps divers manage safe breathing gas mixtures.

3. Atmospheric Studies

Air pressure at different altitudes changes. Partial pressures of gases like oxygen decrease as altitude increases, causing difficulty in breathing at high places.

4. Gas Collection Over Water

When gases are collected over water, water vapour adds its own partial pressure. The dry gas pressure is calculated by subtracting the vapour pressure of water.

5. Industrial Gas Mixtures

Companies that prepare gas cylinders use partial pressure to mix gases in the correct proportions.

Role of Partial Pressure in Gas Mixtures

Partial pressure plays a key role in:

• Gas reactions
• Diffusion of gases
• Gas solubility
• Behaviour of gases in closed systems
• Calculating moles and concentrations of gases

It helps scientists understand how gases behave both individually and as part of mixtures.

Importance of Partial Pressure

Partial pressure is important because it:

• Reveals how much each gas contributes to total pressure
• Helps apply Dalton’s law
• Helps explain gas exchange in organisms
• Helps predict gas behaviour in different environments
• Makes calculations involving gas mixtures easier

Without the concept of partial pressure, studying gas mixtures would be much more difficult.

Conclusion

Partial pressure is the pressure that each gas in a mixture exerts independently, as if it were alone in the container. The total pressure of a gas mixture is the sum of the partial pressures of all the gases present. This concept is essential for understanding gas behaviour in chemistry, physiology, environmental science, and many industrial applications.