MCAT topics → Fluids in Circulation and Gas Exchange → Henry's Law (Gas Solubility)

Henry's Law (Gas Solubility)

MCAT trap: Confuses the direction of Henry's constant depending on which form of the law is used. The convention varies: in the form C = kP, a higher k means more soluble, but in the form P = kC (common in chemistry), a higher k means less soluble.

Henry's Law states that the concentration of a dissolved gas is proportional to its partial pressure above the solution: C = kP. The MCAT tests it from multiple angles, and the biggest trap is the temperature effect — gas solubility decreases with increasing temperature, the opposite of most solids, and students flip this constantly. You need to understand not just the equation but what it means mechanistically: why pressure drives gas into solution, why temperature works in the opposite direction compared to solids, and why nitrogen (not oxygen) causes decompression sickness — a distinction the exam will test in clinical passage scenarios.

The trickiest part is the Henry's constant itself. The exam can present passages using either C = kP or P = kC, and students who only memorized 'higher k means more soluble' will get burned if the passage flips the convention. Always check which form is being used before interpreting k. The other major trap is conflating temperature effects on gas solubility with temperature effects on solid solubility — these go in opposite directions, and the MCAT loves to test whether you know why.

On the clinical side, this topic connects directly to gas exchange physiology. CO2 dissolves in plasma following Henry's Law, oxygen delivery under hyperbaric conditions bypasses hemoglobin entirely, and decompression sickness is a direct consequence of Henry's Law in reverse — rapid pressure drops force dissolved gas out of solution faster than the body can handle. These aren't just interesting facts; they're the passage contexts the MCAT uses to test whether you actually understand the underlying principle.

Common misconceptions

Common mistake
Wrong: A higher Henry's law constant (k) always means the gas is more soluble in the liquid.
Right: The convention varies: in the form C = kP, a higher k means more soluble, but in the form P = kC (common in chemistry), a higher k means less soluble.
The sign of Henry's constant depends entirely on which form of the law you're using. In C = kP (more common in biology/physiology contexts), a larger k means more gas dissolves per unit pressure — so higher k does mean more soluble. But in P = kH·C (the chemist's form), k appears in the denominator effectively, so a higher kH means the gas resists dissolving — less soluble. Before you interpret any Henry's constant, check which equation the passage is using. The MCAT will not always tell you explicitly.
Common mistake
Wrong: Increasing temperature increases the solubility of gases in liquid, just as it does for most solids.
Right: Increasing temperature decreases gas solubility in liquids because dissolved gas molecules gain enough kinetic energy to escape the solution.
Gas solubility in liquids decreases with increasing temperature — the opposite of most solids. The reason: dissolved gas molecules gain kinetic energy as temperature rises and have enough energy to escape back into the gas phase, shifting equilibrium toward less dissolution. This is why warm soda goes flat faster and why cold water holds more dissolved oxygen. Don't let your intuition about solid solubility mislead you here; the physical basis is completely different.
Common mistake
Wrong: Decompression sickness is caused by dissolved oxygen coming out of solution when pressure drops rapidly.
Right: Decompression sickness is caused by dissolved nitrogen (not oxygen) rapidly coming out of solution and forming bubbles in tissues when ambient pressure drops too quickly.
Decompression sickness is caused by nitrogen, not oxygen. At depth, elevated pressure forces large amounts of N2 to dissolve in blood and tissues per Henry's Law. If a diver ascends too quickly, the ambient pressure drops faster than N2 can be transported to the lungs and exhaled — so it comes out of solution as bubbles in tissues and joints, causing the bends. Oxygen doesn't cause this problem at the same scale partly because it's metabolically consumed by tissues, while nitrogen is inert and accumulates.
Common mistake
Wrong: Hyperbaric oxygen therapy works by forcing oxygen into cells through osmosis.
Right: Hyperbaric oxygen therapy works by increasing the partial pressure of O2, which by Henry's law increases the amount of O2 dissolved directly in plasma, bypassing hemoglobin saturation limits.
Hyperbaric oxygen therapy has nothing to do with osmosis. Osmosis involves solvent movement across a semipermeable membrane driven by concentration gradients — a completely different mechanism. Hyperbaric chambers work by raising the partial pressure of O2 above atmospheric levels, which by Henry's Law directly increases the amount of O2 dissolved in plasma. This matters clinically because at normal pressures, hemoglobin is the limiting factor for oxygen delivery; under hyperbaric conditions, enough O2 dissolves in plasma alone to support tissue oxygenation, which is why it's useful for carbon monoxide poisoning and wound healing.
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What the exam tests

  1. Know the definition of Henry's Law and be able to state that dissolved gas concentration is directly proportional to the partial pressure of that gas above the solution (C = kP).
  2. Given a Henry's constant and a partial pressure, calculate the concentration of dissolved gas — and recognize how doubling pressure doubles dissolved concentration.
  3. Explain the clinical applications of Henry's Law including why decompression sickness occurs, why nitrogen (not oxygen) is the culprit, how hyperbaric oxygen therapy increases plasma O2 directly, and how CO2 dissolves into blood.

Can you avoid these mistakes?

A patient is placed in a hyperbaric chamber where the partial pressure of O2 is tripled compared to normal atmospheric levels. Using Henry's Law, predict what happens to the concentration of O2 dissolved in plasma — and explain why this matters when hemoglobin is already nearly 100% saturated.
A Henry's constant is listed in a passage as k = 3.4 × 10⁻² mol/L·atm. If the partial pressure of CO2 above a solution is 0.04 atm, what is the dissolved CO2 concentration? What would happen to that concentration if temperature were increased significantly?
A chemistry textbook gives Henry's Law as P = kH·C with kH(N2) = 86,000 atm·L/mol and kH(CO2) = 29 atm·L/mol. Which gas is more soluble in water at the same partial pressure, and how do you know from these constants?
Explain mechanistically why a scuba diver who surfaces too quickly can develop decompression sickness. Which gas is primarily responsible, and why doesn't the same problem occur with O2 at the same dissolved concentrations?

Related topics

Partial Pressures (Dalton's Law) Density and Specific Gravity Pressure in a Fluid (Pascal's Principle, Hydrostatic) Buoyancy and Archimedes' Principle Continuity Equation (Conservation of Flow)

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