Henry's LawJEE Main

Concept

The amount of gas dissolving in a liquid is proportional to its partial pressure above it: $p = K_Hx$. Big $K_H$ means the gas would rather stay in the gas phase (poorly soluble). Soda fizz, diver's bends, and mountain hypoxia are all Henry's law wearing different costumes.

Key formula

Derivation

At equilibrium the rate of gas molecules entering the liquid (∝ p) equals the rate leaving (∝ x). Proportionality constant = K_H.

Temperature: dissolution of gases is exothermic, so heating pushes gas OUT (Le Chatelier) — K_H grows with T. That's why warm soda goes flat and warm lakes suffocate fish.

Scenarios to explore

• Henry's Law — Gas solubility ∝ partial pressure — soda fizz & the bends.

Real-world applications

• Carbonated drinks bottled at ~3–4 atm CO₂.
• Scuba: N₂ dissolves at depth; fast ascent → bubbles → the bends (He mixes reduce it: huge K_H).
• Anoxia at altitude: lower pO₂ → less O₂ dissolved in blood.

JEE exam tips

• Helium in diving mixes: highest K_H = least dissolves = least bends risk.
• The p in the law is the PARTIAL pressure of that gas, not total.
• Cold water streams hold more O₂ — why trout like cold rivers.

Common mistakes

• Reading high K_H as high solubility — it's the INVERSE.
• Applying Henry to reacting gases (HCl, NH₃ in water — they ionise, law fails).
• Forgetting T-dependence direction (warm = less dissolved gas).

Exam traps to avoid

• Units of K_H are pressure units (atm/bar) when written p = K_H·x.
• Raoult's law is Henry's law with K_H = P° — the solvent's limiting case.