Henry's Law describes how much gas dissolves in a liquid when the gas is in contact with the liquid surface. It matters because gases like carbon dioxide, oxygen, and nitrogen are constantly moving between air and water in drinks, oceans, blood, and industrial tanks. The law says that increasing the partial pressure of a gas above a liquid increases the concentration of that gas dissolved in the liquid.
This is why a sealed soda stays fizzy under pressure but releases bubbles when opened.
At constant temperature, Henry's Law is usually written as C = kH P, where C is the dissolved gas concentration, P is the gas partial pressure, and kH is Henry's law constant for that gas and solvent. The constant depends on the gas, the liquid, and temperature, so it is not the same for every situation. In a pressurized container, more gas molecules hit the liquid surface each second, so more enter and remain dissolved until a new equilibrium is reached.
Applications include carbonated beverages, oxygen transfer in water, gas exchange in the lungs, and nitrogen absorption during scuba diving.
Understanding Chemistry: Henry's Law
At the particle level, dissolving is a two-way process. Gas molecules enter the liquid at its surface, while dissolved molecules escape back into the gas space. Equilibrium does not mean that motion stops.
It means the rate of entry matches the rate of escape. A greater partial pressure places more molecules of one gas near the surface. This raises the rate at which that gas enters the liquid.
The liquid eventually reaches a new balance with a larger amount of dissolved gas. Stirring can help the system reach this balance faster because it replaces liquid near the surface, but stirring does not change the final equilibrium amount when temperature and pressure remain fixed.
A gas does not need to react chemically in order to dissolve. Oxygen can occupy spaces between water molecules and remain as oxygen molecules. Carbon dioxide behaves differently because some dissolved carbon dioxide reacts with water to form weak acids and related ions.
This reaction removes some carbon dioxide molecules from the dissolved gas form, allowing more carbon dioxide to enter from the air. For this reason, real carbon dioxide systems can be more complicated than a simple Henry's law calculation.
Students should separate physical dissolving from chemical reaction. Henry's law best describes the gas that is physically dissolved and is most accurate when the solution is dilute and the gas does not react strongly with the liquid.
Partial pressure is especially important for mixtures such as air. Air contains mostly nitrogen and oxygen, with only a small amount of carbon dioxide. Each gas behaves as though the other gases are not present when considering its tendency to dissolve.
Adding a gas that does not dissolve much can raise the total pressure without greatly changing the amount of oxygen or carbon dioxide dissolved. This idea matters in medicine. In the lungs, oxygen moves from air in tiny sacs into blood because oxygen has a higher partial pressure in the lung air than in blood.
Carbon dioxide moves in the opposite direction because its partial pressure is higher in blood. Blood chemistry, circulation, and binding to hemoglobin make the full process more complex, yet partial pressure remains a central driving factor.
Scuba diving shows why changing pressure slowly can be important. At depth, the pressure of nitrogen in compressed air is higher than it is at the surface. More nitrogen can dissolve in a diver's blood and tissues over time.
A rapid ascent lowers the surrounding pressure quickly. Nitrogen may then leave solution as bubbles before it can be carried to the lungs and breathed out. Those bubbles can cause decompression sickness.
In laboratory work, pay attention to the conditions stated in a problem. Check the identity of the gas, the liquid, the temperature, and whether the pressure given is total pressure or the pressure of one gas. Remember that opening a container changes the conditions, so an equilibrium result from a sealed container may no longer apply.
Key Facts
- Henry's Law: C = kH P, where C is dissolved gas concentration and P is gas partial pressure.
- At constant temperature, gas solubility in a liquid is directly proportional to the gas's partial pressure above the liquid.
- If the partial pressure doubles, the dissolved concentration doubles, as long as kH and temperature stay constant.
- Only the partial pressure of the specific gas matters, not the total pressure by itself.
- Henry's law constant kH depends on the gas, the solvent, and temperature.
- For many gases in water, higher temperature lowers gas solubility, so warm liquids often hold less dissolved gas.
Vocabulary
- Henry's Law
- A law stating that the concentration of a dissolved gas is proportional to the gas's partial pressure above the liquid at constant temperature.
- Partial Pressure
- The pressure a single gas in a mixture would exert if it occupied the container by itself.
- Solubility
- The maximum amount of a substance that can dissolve in a given amount of solvent under specific conditions.
- Henry's Law Constant
- The proportionality constant that connects dissolved gas concentration to partial pressure for a specific gas, solvent, and temperature.
- Equilibrium
- A state in which gas molecules enter and leave the liquid at equal rates, so the dissolved concentration stays constant.
Common Mistakes to Avoid
- Using total pressure instead of partial pressure, which is wrong because Henry's Law depends on the pressure of the specific gas dissolving.
- Assuming kH is always the same, which is wrong because the constant changes with gas identity, solvent, and temperature.
- Forgetting the constant temperature condition, which is wrong because changing temperature can change gas solubility and the value of kH.
- Thinking bubbles form because gas disappears from the liquid, which is wrong because bubbles form when dissolved gas comes out of solution after pressure drops.
Practice Questions
- 1 A gas has kH = 0.030 mol/L·atm in water at a certain temperature. What concentration dissolves when its partial pressure is 2.0 atm?
- 2 At 25°C, a gas has a dissolved concentration of 0.12 mol/L when its partial pressure is 3.0 atm. What is kH in mol/L·atm?
- 3 A sealed soda bottle is opened, lowering the CO2 pressure above the liquid. Explain why bubbles form and why the soda eventually tastes flat.