Definition
A physical law stating that the total pressure of a mixture of gases is equal to the sum of the pressures that each gas would exert if it occupied the same space alone. Each gas in the mixture contributes its own share of the total pressure, called its partial pressure.
Plain English
When several gases are mixed together, each one pushes with its own pressure as if the others weren't there. Add all those individual pressures together and you get the total pressure of the mixture.
Context Anchor
Seen in aviation physiology, oxygen system discussions, cabin pressure discussions, and basic aircraft maintenance training involving gases and pressure.
Derivation
Named after John Dalton, the English chemist who described this behaviour of gas mixtures in the early 1800s. Knowing it's named after a person (not a technical concept) helps the term feel less abstract — it's simply Dalton's observation about how gases share pressure.
Why Pilots Care
Understanding how each gas contributes its own pressure supports correct diagnosis of pneumatic and oxygen system performance at varying altitudes.
Analogy
Think of several people pushing on the same door. The total push on the door is the combined push from each person. In air, each gas adds its own part to the total pressure.
Grounding Statement
At sea level, oxygen makes up about 21% of the air's total pressure. Climb to 18,000 feet and total pressure is roughly half — so oxygen's share is also halved, even though it's still 21% of the mix.
Intuition Check
Dalton’s Law is not an aviation regulation. Here, “law” means a physical rule that describes how gas pressures add together.
Example Sentence 1
Dalton's Law explains why a pilot flying at high altitude gets less oxygen with each breath, even though the air is still 21% oxygen.
Example Sentence 2
As altitude increases and total atmospheric pressure falls, Dalton’s Law shows that the partial pressure of each gas component drops proportionally.