Definition
A physical law stating that the amount of a gas that will dissolve in a liquid at a given temperature is directly proportional to the partial pressure of that gas above the liquid. As the pressure of the gas increases, more of it dissolves into the liquid; as the pressure decreases, the dissolved gas comes back out of solution.
Plain English
The more pressure pushing a gas onto a liquid, the more of that gas gets absorbed into the liquid. Reduce the pressure, and the gas escapes back out.
Context Anchor
Encountered in high-altitude physiology, cabin pressure, decompression sickness, and flying after scuba diving.
Derivation
Named after William Henry, an English chemist who described the relationship in 1803. The law itself is just his observation about how gases behave in liquids under pressure.
Why Pilots Care
Explains why rapid drops in cabin or ambient pressure can release nitrogen bubbles in the blood, causing decompression sickness.
Analogy
Think of opening a bottle of soda. While the cap is on, pressure keeps the carbon dioxide dissolved in the liquid. Pop the cap, the pressure drops, and the gas fizzes out of solution. Your bloodstream behaves the same way with nitrogen when cabin pressure drops.
Grounding Statement
Picture a pilot climbing to a high altitude: the outside pressure drops, so gas that was safely dissolved in body fluids may begin to come out of solution.
Intuition Check
Henry'S Law is not an aviation regulation. It is a physics rule about how pressure affects gas dissolved in liquid.
Example Sentence 1
Henry's Law explains why a rapid loss of cabin pressurization at high altitude can cause nitrogen bubbles to form in a pilot's blood.
Example Sentence 2
At 18,000 feet the reduced pressure allowed dissolved nitrogen to form bubbles in the bloodstream according to Henry's Law.