Short Answer

Partial pressures are affected by several factors such as the amount of gas (number of moles), temperature, volume of the container, and the total pressure of the gas mixture. Each gas in a mixture contributes its own pressure depending on how much of it is present and how fast its molecules move.

Changes in conditions like heating, cooling, compression, or expansion influence the motion of gas particles. As a result, the partial pressure of each gas increases or decreases depending on how these factors change inside the container.

Detailed Explanation

Factors Affecting Partial Pressures

Partial pressure is the pressure that each gas in a mixture exerts independently. It depends on how many molecules of that gas are present and how much space and energy they have. Even though gases are mixed together, each one behaves separately and contributes a part of the total pressure.

Several factors influence partial pressure, including the number of moles of gas, temperature, volume, and total pressure. Understanding these factors helps explain how gas mixtures behave in different environments such as the atmosphere, human lungs, industrial gas cylinders, and laboratory experiments.

Amount of Gas (Number of Moles)

The quantity of gas present is one of the main factors affecting partial pressure.
If the number of moles of a gas increases, its partial pressure increases because:

• More gas molecules collide with container walls.
• More collisions increase the force exerted, raising pressure.

For example, if oxygen is added to a container already containing nitrogen, the partial pressure of oxygen increases because more oxygen molecules are present. Similarly, if some gas is removed, its partial pressure decreases.

Thus, partial pressure is directly proportional to the number of moles of gas present.

Mole Fraction of Each Gas

Partial pressure can be calculated using the mole fraction:

Pᵢ = Xᵢ × Pₜₒₜₐₗ

Where Xᵢ is the mole fraction.
A higher mole fraction means a higher partial pressure.

For example, in dry air:

• Nitrogen has the largest mole fraction, so it has the highest partial pressure.
• Oxygen has a lower mole fraction, so its partial pressure is lower.

Temperature

Temperature strongly affects partial pressure because it changes the kinetic energy of gas molecules.

• When temperature increases, molecules move faster.
  They collide with walls more often and with greater force.
  This increases partial pressure.
• When temperature decreases, molecules move slowly.
  Collisions become weaker and less frequent.
  This lowers partial pressure.

This explains why air pressure in a closed tyre rises in summer and falls in winter.

Volume of the Container

Partial pressure is also influenced by the space available.
According to Boyle’s law:

• If volume decreases, the gas is compressed.
  Molecules collide more frequently, increasing partial pressure.
• If volume increases, molecules spread out and collisions reduce.
  This results in lower partial pressure.

For example, in the lungs, the volume of the chest cavity increases during inhalation, causing partial pressures of gases to change and allowing oxygen to move into the blood.

Total Pressure of the Gas Mixture

Total pressure affects partial pressure because each gas contributes a fraction of the total.
If total pressure increases due to compression or addition of gases:

• The partial pressures of individual gases also increase proportionally.

This relationship is used in scuba diving, where divers must control the mixture to avoid dangerous increases in nitrogen partial pressure.

Presence of Water Vapour

When gases are collected over water or when humidity is high, water vapour adds its own partial pressure. This reduces the partial pressures of the dry gases in the mixture.

Total pressure = partial pressure of dry gas + vapour pressure of water

Thus, water vapour affects the effective contribution of other gases.

Chemical Reactions in Gas Mixtures

Partial pressure can change when gases react:

• If a gas is consumed in a reaction, its partial pressure decreases.
• If a gas is produced, its partial pressure increases.

This is important in combustion, respiration, and industrial chemical processes.

External Conditions and Altitude

Atmospheric pressure changes with altitude:

• At high altitude, total pressure decreases.
  As a result, partial pressure of oxygen decreases, making breathing difficult.
• At sea level, total pressure is higher, so oxygen partial pressure is also higher.

This shows how external conditions influence partial pressures.

Importance of Understanding Partial Pressure Changes

Knowing what affects partial pressure is important because:

• It explains gas exchange in the lungs.
• It helps in preparing industrial gas mixtures.
• It ensures safety in diving and aviation.
• It aids in predicting chemical reaction behaviour.
• It helps understand atmospheric and environmental processes.

Partial pressure is a key concept in both science and daily life.

Conclusion

Partial pressures are affected by several factors including the number of moles of gas, temperature, volume of the container, total pressure, and external influences like water vapour and altitude. These factors change the collisions and distribution of gas molecules, which directly impact the pressure each gas contributes. Understanding these factors helps explain gas behaviour in physical, chemical, environmental, and biological systems.