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.
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.