Short Answer

Dalton’s law of partial pressures states that when two or more gases are mixed in a container, the total pressure is equal to the sum of the partial pressures of each gas. Each gas behaves independently and contributes pressure as if it were alone.

This law applies only to gases that do not react with each other. Because the gases move freely and randomly, their individual pressures simply add up. This makes it easier to calculate total pressure in mixtures such as air, which contains nitrogen, oxygen, carbon dioxide, and other gases.

Detailed Explanation

Dalton’s Law of Partial Pressures

Dalton’s law of partial pressures is an important principle used in studying gas mixtures. It states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the individual pressures (partial pressures) of all gases present in the mixture. Each gas in the mixture behaves as if it is alone in the container and contributes its own pressure. This pressure is called the partial pressure of that gas.

The law can be written as:

Pₜₒₜₐₗ = P₁ + P₂ + P₃ + …

Where P₁, P₂, P₃, etc., are the partial pressures of individual gases.

Dalton’s law works because gases in a mixture do not affect each other’s motion. They move freely, collide randomly, and occupy the entire volume independently. Their pressures simply add up to form the total pressure.

Meaning of Partial Pressure

Partial pressure is the pressure that a gas would exert if it were alone in the container at the same temperature and volume. For example:

• If air contains nitrogen, oxygen, carbon dioxide, and other gases, each gas has its own partial pressure.
• Nitrogen contributes the highest partial pressure because it makes up almost 78 percent of air.

Thus, partial pressures allow us to understand how much each gas contributes to the total pressure.

Why Dalton’s Law Works

Dalton’s law works because:

1. Gas molecules are far apart and rarely attract each other.
2. Gases mix completely and evenly throughout the container.
3. Each gas behaves independently of other gases.
4. Collisions between molecules of different gases do not change their individual pressures.

As long as gases do not react chemically, they follow Dalton’s law.

Conditions Required for Dalton’s Law

Dalton’s law applies best under certain conditions:

• Gases must be non-reacting:
  If gases react, the composition changes, and partial pressures cannot be added directly.
• Temperature must stay constant:
  Changing temperature changes gas pressure.
• Ideal or near-ideal behaviour:
  At low pressure and high temperature, gases behave ideally, making Dalton’s law more accurate.

Real gases at high pressure or low temperature may show slight deviations, but the law still works well for many practical situations.

Applications of Dalton’s Law

Dalton’s law is widely used in different fields:

1. Atmospheric Science

Air is a mixture of gases. The total pressure of air is the sum of partial pressures of nitrogen, oxygen, carbon dioxide, and other gases.

2. Breathing and Physiology

Human breathing depends on partial pressures. Oxygen moves into the blood because its partial pressure in the lungs is higher than in the blood.

3. Gas Collection Over Water

When gases are collected over water, water vapour also contributes pressure.
Dalton’s law helps subtract vapour pressure to find the pressure of the dry gas.

4. Industrial Gas Mixtures

Industries that use gas cylinders depend on Dalton’s law to measure and prepare mixtures of gases.

5. Scuba Diving

Divers breathe compressed air. Dalton’s law helps calculate how much oxygen and nitrogen they inhale at different depths.

Examples of Dalton’s Law

Example 1:
If nitrogen exerts 600 mmHg and oxygen exerts 160 mmHg in air,
Total pressure = 600 + 160 = 760 mmHg

Example 2:
If three gases exert 2 atm, 1 atm, and 0.5 atm,
Total pressure = 3.5 atm

These examples show that total pressure is simply the sum of all partial pressures.

Importance of Dalton’s Law

Dalton’s law is important because it:

• Helps understand gas mixtures scientifically
• Makes pressure calculations simple
• Explains how gases behave independently
• Helps in medical, industrial, and environmental applications
• Forms the basis for studying advanced gas laws

It remains a fundamental concept in chemistry and physics.

Conclusion

Dalton’s law of partial pressures states that the total pressure of a gas mixture is the sum of the partial pressures of each gas. Each gas contributes its own pressure independently, provided the gases do not react chemically. This law is essential for understanding atmospheric gases, gas collection methods, breathing processes, and many industrial applications. Its simplicity makes it one of the most useful gas laws in chemistry.