Definition
A physical law stating that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each individual gas in the mixture. Each gas behaves as if it alone occupied the entire volume, and its contribution to the total pressure is independent of the other gases present.
Plain English
When several gases are mixed together, each one pushes with its own pressure, and the total pressure is just all those individual pressures added up.
Context Anchor
Seen in high-altitude physiology, oxygen-system discussions, and explanations of why less oxygen reaches the body as altitude increases.
Derivation
Named after John Dalton, an English chemist and physicist (1766-1844), who first described the behavior of gas mixtures in the early 1800s. The 'law' label simply means a consistently observed physical principle.
Why Pilots Care
It explains why the partial pressure of oxygen drops at altitude, directly affecting breathing and the risk of hypoxia.
Analogy
Think of several people pushing on the same door. The total push on the door is the sum of each person's push; in the same way, the total pressure of air is the sum of each gas's pressure share.
Grounding Statement
At sea level, the air pushes on you with about 14.7 pounds per square inch of pressure. About 21% of that push comes from oxygen alone -- that's oxygen's partial pressure. At 18,000 feet, total pressure is roughly half, so oxygen's partial pressure is also roughly half, even though oxygen is still 21% of the air.
Intuition Check
Dalton's Law does not mean the oxygen percentage in normal air changes sharply with altitude. It means each gas's pressure share changes when the total air pressure changes.
Example Sentence 1
Dalton's Law explains why a pilot can become hypoxic at high altitude even though the air around them is still 21% oxygen.
Example Sentence 2
Using Dalton's Law, a pilot can understand why supplemental oxygen becomes necessary even before total pressure reaches critical levels.