Henry's Law

Henry's Law says the concentration of a gas dissolved in a liquid is proportional to that gas's partial pressure above the liquid at constant temperature. In Heat and Mass Transfer, it shows up in absorption, stripping, and solubility calculations.

What is Henry's Law?

Henry's Law is the rule that links how much gas dissolves in a liquid to the gas's partial pressure above that liquid, as long as temperature stays constant. If the gas pressure goes up, more gas is driven into the liquid. If the pressure drops, less gas stays dissolved.

In Heat and Mass Transfer, you use this law whenever a gas and a liquid are in equilibrium at an interface. The gas phase does not dissolve in random amounts, it settles at a concentration set by the gas's partial pressure and the particular gas-liquid pair. That is why different gases have different Henry's law constants. Oxygen, carbon dioxide, and ammonia do not all behave the same in water or in solvents used for separation.

The usual form is C = k_H P, where C is the dissolved-gas concentration, P is the partial pressure of that gas, and k_H is Henry's law constant. The exact units of k_H can vary by convention, so in problem solving you always check how the course or textbook defines it. What matters is the proportional relationship, not memorizing one universal number.

A common way to read the law is as a solubility rule. Higher partial pressure means a stronger driving effect for gas molecules to enter the liquid phase. That is why pressurized carbonated drinks hold more CO2, and why gas absorption columns use contact with a liquid solvent to remove a component from a gas stream.

Temperature also changes the picture. For many gases in liquids, solubility drops as temperature rises, so the same gas pressure does not dissolve the same amount at a higher temperature. In separation problems, that means you cannot treat Henry's Law as a pressure-only shortcut. You need to know the temperature and whether the process is absorption, stripping, or equilibrium analysis.

The big thing to keep straight is that Henry's Law describes equilibrium at the interface, not how fast the gas gets there. Mass transfer rate depends on the driving force, fluid flow, interfacial area, and transport resistance. Henry's Law gives you the equilibrium endpoint that those transfer calculations work around.

Why Henry's Law matters in Heat and Mass Transfer

Henry's Law is one of the main links between phase equilibrium and separation design in Heat and Mass Transfer. It tells you what concentration a dissolved gas tends toward at a given gas pressure, which is the starting point for figuring out whether absorption or stripping will work well.

That matters in gas-liquid absorption columns, where you want a solute in the gas stream to move into a solvent. If the gas has a low Henry's law constant in the chosen liquid, it dissolves more easily, so absorption is more favorable. If the constant is high, the gas stays in the vapor phase more readily and absorption becomes harder.

The same idea shows up in stripping, where the goal is to remove a volatile component from a liquid by contacting it with another gas. Here, Henry's Law helps you predict how much of that component wants to stay dissolved versus leave the liquid.

It also gives you a quick way to interpret equilibrium data and check if a design choice makes physical sense. If pressure changes, the equilibrium concentration changes. If temperature changes, the solubility usually changes too. That means Henry's Law often appears right next to process conditions in homework, design problems, and lab analysis.

How Henry's Law connects across the course

Partial Pressure

Henry's Law is built on partial pressure, not total pressure. You only use the pressure of the gas species you care about, since that is what drives how much of that gas dissolves at the interface. In mixed-gas problems, separating the partial pressure from the total pressure is often the first step before you can apply the law correctly.

Solubility

Henry's Law gives a direct way to think about gas solubility in a liquid. A higher partial pressure usually means higher dissolved concentration, while temperature shifts can raise or lower solubility depending on the system. In separation problems, solubility is what tells you whether a gas prefers the liquid phase or the gas phase.

Gas-Liquid Absorption

Absorption problems often start with Henry's Law because you need the equilibrium concentration at the gas-liquid interface. That equilibrium value helps you compare the incoming gas composition with the liquid's capacity to take up the solute. If the actual concentration difference is large, the driving force for absorption is larger too.

Driving Force

Henry's Law gives the equilibrium endpoint, and the driving force comes from how far the current state is from that endpoint. In a mass transfer column, the difference between actual gas composition and equilibrium composition tells you whether mass moves into the liquid or back out of it. No equilibrium difference means no net transfer.

Is Henry's Law on the Heat and Mass Transfer exam?

A quiz or problem-set question usually gives you a gas, a liquid, a temperature, and a partial pressure, then asks for the dissolved concentration or whether absorption or stripping is favored. Your job is to pick the correct form of Henry's Law, track units for the constant, and use the partial pressure of the target gas, not the total pressure.

In design-style questions, you may also compare equilibrium concentrations at two temperatures or two pressures and explain which condition makes gas removal easier. If the problem includes a column or interface, Henry's Law often gives the equilibrium line before you move on to a driving-force or stage-count calculation.

Henry's Law vs Raoult's Law

Henry's Law and Raoult's Law both connect phase equilibrium to pressure, but they apply to different situations. Henry's Law is usually used for a dilute gas dissolved in a liquid, while Raoult's Law is used for the vapor pressure of a component in an ideal liquid mixture. If you see a solute gas above a solvent, think Henry's Law first.

Key things to remember about Henry's Law

• Henry's Law says dissolved gas concentration is proportional to the gas's partial pressure above the liquid at constant temperature.

• In Heat and Mass Transfer, the law is most useful in absorption, stripping, and other gas-liquid equilibrium problems.

• The Henry's law constant depends on the gas and solvent, so you have to use the value that matches the system in the problem.

• Higher temperature usually lowers gas solubility in liquids, so temperature can change the equilibrium concentration a lot.

• Henry's Law tells you the equilibrium condition, but it does not by itself give the transfer rate.

Frequently asked questions about Henry's Law

What is Henry's Law in Heat and Mass Transfer?

Henry's Law says the amount of a gas dissolved in a liquid is proportional to that gas's partial pressure above the liquid, as long as temperature stays constant. In this course, that relationship is used to model gas-liquid equilibrium in absorption and stripping problems. It tells you the concentration the liquid is trying to reach at the interface.

How do you use Henry's Law in absorption problems?

You use it to find the equilibrium dissolved concentration for the gas you want to remove. That equilibrium value becomes the reference point for the driving force between the gas stream and the liquid solvent. If the solvent can hold more of the gas at the given pressure, absorption is easier.

What is the difference between Henry's Law and Raoult's Law?

Henry's Law is usually used for a dilute gas dissolved in a liquid, while Raoult's Law describes the vapor pressure contribution of a component in an ideal liquid mixture. They both deal with equilibrium, but they are applied in different phase and composition ranges. If the problem is about gas dissolving into a solvent, Henry's Law is the better match.

Does temperature affect Henry's Law?

Yes. For many gas-liquid systems, raising temperature lowers gas solubility, so less gas stays dissolved at the same pressure. That is why temperature changes can strongly affect absorption and stripping design calculations.