Henry's Law: Why Soda Fizzes When You Open the Bottle

When you open a bottle of cold drink, you hear a hiss and see bubbles rushing up. Divers are warned to rise slowly from deep water. People living at high altitude can feel breathless. All these everyday facts are explained by Henry's law, proposed in 1803 by the English chemist William Henry. It states that the amount of a gas dissolved in a liquid at constant temperature is directly proportional to the partial pressure of that gas above the liquid. The law is simple, but it is important in chemistry, biology, medicine and industry. This blog explains the law, its limits and its use in exams.

The Scientific Problem Before the Discovery

By the end of the 18th century, Joseph Priestley had shown how to dissolve carbon dioxide in water under pressure to make soda water (1767). Chemists like Dalton and Henry were studying gases and trying to understand what controls the amount of gas in a liquid. It was not known how the solubility changes with pressure and how mixtures of gases behave. These questions were important because Dalton was developing his atomic theory and his law of partial pressures, and he needed to understand how gas mixtures dissolve in water.

The Discovery: Henry's Experiments

William Henry, working in Manchester, measured how much of various gases (CO₂, N₂, O₂, H₂, CH₄ and others) dissolved in water at different pressures. He placed the gas and water in a closed vessel, raised the pressure in steps, and measured the volume of gas absorbed. His results, published in the Philosophical Transactions of the Royal Society in 1803, showed that the mass of gas dissolved increases in direct proportion to the pressure. He also showed that for a mixture of gases, each dissolves independently, according to its own partial pressure. This second finding is linked with Dalton's law of partial pressures. Dalton, a friend and colleague in Manchester, used Henry's results in his own work.

The law in modern form:

p = K_H · x          (p = partial pressure of gas; x = mole fraction of dissolved gas)
or   S = k · p       (solubility proportional to pressure)

K_H is the Henry's law constant, which depends on the gas, solvent and temperature. In the first form, a higher K_H means lower solubility. For example, at 298 K for water: K_H of N₂ is about 87 kbar, of O₂ about 46 kbar, and of CO₂ about 1.67 kbar. So CO₂ is far more soluble than N₂ or O₂. (CO₂ also reacts with water to form carbonic acid, which adds to its solubility.)

Example: If the partial pressure of CO₂ over a drink is raised from 1 bar to 4 bar at constant temperature, the dissolved CO₂ becomes 4 times greater.

The Chemistry Behind It

At equilibrium, the rate at which gas molecules enter the liquid equals the rate at which they leave: gas ⇌ dissolved gas. A higher partial pressure means more collisions with the liquid surface, so more gas enters until a new equilibrium is reached. By Le Chatelier's principle, increasing pressure shifts the equilibrium towards the dissolved state, which reduces the number of gas molecules.

• Effect of temperature: Dissolving a gas in a liquid is usually exothermic. So the solubility decreases when temperature increases, and K_H rises. Warm soda goes flat faster, and aquatic animals face lack of oxygen in warm water.
• Limitations: The law holds at low pressure and for gases that do not react with the solvent. It fails for gases such as NH₃ and HCl, which react with water, and at very high pressures.
• Thermodynamic view: The law is the behaviour of a dilute solute (Henry's region) while Raoult's law describes the solvent. For a binary solution, when the solvent follows Raoult's law, the solute follows Henry's law at low concentration.
• Graph: A plot of the partial pressure of a gas against its mole fraction in solution is a straight line with slope K_H.

Applications Today

• Carbonated drinks: Soda is bottled under CO₂ at high pressure (about 2-4 bar) to dissolve a lot of gas. On opening, pressure falls to 1 bar and CO₂ escapes as bubbles. The sealed cap keeps CO₂ in.
• Scuba diving and "the bends": Under pressure at depth, more nitrogen dissolves in blood. If a diver rises too fast, nitrogen bubbles out in tissues and joints, causing decompression sickness. To prevent this, divers breathe a helium-oxygen mixture (helium is less soluble) and ascend slowly.
• Altitude sickness: At high altitude (such as Ladakh or Leh) the partial pressure of oxygen is lower, so less oxygen dissolves in the blood, giving low oxygen concentration (hypoxia) in climbers and residents who are not used to it. This causes weakness and confusion.
• Anaesthesia and medicine: The uptake of anaesthetic gases in blood follows Henry's law.
• Environmental science: Dissolution of CO₂ in oceans, which affects ocean acidification, and the oxygen content of rivers and lakes, important for fish in warm weather.

Exam Corner

• Henry's law: p = K_H · x; higher K_H means lower solubility at a given pressure.
• Solubility of gases decreases with increase in temperature; K_H increases with temperature.
• Applications: soda bottle, scuba diving (bends), altitude sickness.
• Law not valid for gases reacting with the solvent (NH₃, HCl in water) or at high pressure.
• In a mixture, each gas dissolves according to its own partial pressure.

Common Misconceptions

• "Higher K_H means higher solubility." In the form p = K_H x, it means lower solubility.
• "Gas solubility rises with temperature, as for solids." It usually falls.
• "Henry's law is only about carbon dioxide." It holds for many gases when they do not react with the solvent.

FAQ

What does Henry's law say?

The mass of a gas dissolved in a given volume of liquid at constant temperature is directly proportional to its partial pressure above the liquid.

Why do divers get the bends?

At depth, extra nitrogen dissolves in blood. If the diver comes up quickly, pressure drops suddenly and nitrogen forms bubbles in tissues.

Why does a soft drink go flat when warm?

Gas solubility decreases with temperature, so CO₂ escapes faster.

Is Henry's law the same as Dalton's law?

No. Dalton's law gives the total pressure of a gas mixture as the sum of the partial pressures; Henry's law links partial pressure to dissolved gas.

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