---
title: "Henry's Law in Thermodynamics II"
description: "Henry's Law says a gas's dissolved concentration is proportional to its partial pressure above a liquid, a core relation in Thermodynamics II phase equilibrium."
canonical: "https://fiveable.me/thermodynamics-ii/key-terms/henrys-law"
type: "key-term"
subject: "Thermodynamics II"
unit: "Unit 10"
---

# Henry's Law in Thermodynamics II

## Definition

Henry's Law says that, at constant temperature, the amount of a gas dissolved in a liquid is proportional to the gas's partial pressure above the liquid. In Thermodynamics II, it shows up in phase equilibrium and gas solubility calculations.

## What It Is

Henry's Law is the thermodynamics relation you use when a gas and a liquid are in equilibrium and the gas is only slightly soluble in the liquid. It says the dissolved concentration of the gas rises as the partial pressure of that gas above the liquid rises, as long as temperature stays constant.

The usual form is C = k_H P, where C is the gas concentration in the liquid, P is the gas's partial pressure, and k_H is Henry's constant for that gas-solvent pair. Some courses write the law in terms of mole fraction or fugacity instead of concentration, but the idea is the same: more pressure from the gas phase usually means more gas stays dissolved.

In Thermodynamics II, this law is not just about a random solubility fact. It is a phase equilibrium model tied to chemical potential. At equilibrium, the chemical potential of the gas in the vapor phase matches the chemical potential of the dissolved gas in the liquid phase, and Henry's Law gives you a simple way to express that balance for dilute solutions.

The constant k_H depends on the gas, the solvent, and temperature. That is why carbon dioxide dissolves much more readily in a cold soda bottle than in an open warm drink. When temperature changes, the balance between the gas escaping and the gas dissolving changes too, so the same pressure does not give the same concentration.

A common trap is treating Henry's Law like a universal law for every gas-liquid mixture. It works best for dilute gases that do not react strongly with the solvent and at conditions where ideal or near-ideal behavior is reasonable. At higher pressures, higher concentrations, or when the gas reacts with the liquid, the straight-line relationship starts to break down and you may need a fugacity-based approach instead.

## Why It Matters

Henry's Law shows up whenever Thermodynamics II shifts from pure substances to phase equilibria and mixtures. It gives you a clean way to connect gas-phase pressure to liquid-phase composition, which is exactly the kind of link you need when solving equilibrium problems for vapors above liquids, dissolved gases in process streams, or gas exchange between phases.

It also gives physical meaning to chemical potential. Instead of treating chemical potential as an abstract symbol, you can see it as the quantity that decides whether a gas wants to stay in the vapor phase or enter the liquid. If pressure rises above the liquid, more gas dissolves until the phases reach equilibrium.

This comes up in engineering examples like carbonation, degassing, oxygen in water, and solvent stripping. If you are looking at a lake, a beverage container, or a unit operation with a gas-liquid contactor, Henry's Law helps predict which way mass transfer moves and how much gas can be held in the liquid at equilibrium.

It also sets the stage for more advanced equilibrium methods. Once you understand where Henry's Law works well, you can see why real systems sometimes need activity coefficients, fugacity corrections, or nonideal models. That makes it a bridge between simple solubility rules and the more detailed phase stability tools used later in the course.

## Connections

### Partial Pressure

Henry's Law uses the gas's partial pressure, not the total pressure of the whole mixture. If several gases are present, you only track the component you care about. That makes partial pressure the link between the gas mixture above the liquid and the amount of that one gas that dissolves.

### Solubility

Henry's Law is one of the cleanest ways to predict gas solubility in a liquid. It works best for dilute solutions, where the dissolved gas does not change the liquid's structure much. When solubility gets high or the gas reacts with the solvent, the simple proportionality starts to fail.

### Phase Equilibrium

This law is a phase equilibrium relation because it describes the balance between a gas phase and a liquid phase at constant temperature. In problems, it helps you find the composition at equilibrium or decide which phase a component prefers. It is often one of the first tools used before moving to more detailed fugacity calculations.

### [spontaneity of reactions](/thermodynamics-ii/key-terms/spontaneity-of-reactions)

Henry's Law is not a reaction law, but it connects to spontaneity through chemical potential. If the gas phase has a higher tendency to escape than to dissolve, mass transfer moves until equilibrium is reached. That same chemical-potential idea is also what you use when deciding whether a reaction or phase change is favored.

## On the AP Exam

A quiz question might give you a gas mixture above water and ask for the dissolved concentration or the effect of changing pressure. You use Henry's Law by identifying the correct gas partial pressure, checking the temperature, and applying the right constant for that gas-solvent pair. If the pressure doubles, the dissolved amount doubles too, as long as the system stays in the Henry's Law range.

Problem sets often test whether you can tell when the law applies. If the gas is highly soluble, reacts with the liquid, or the pressure is far from dilute conditions, you may need to say that the simple linear model is not reliable. In phase equilibrium problems, Henry's Law is often the first step before comparing it with vapor-liquid balance or chemical potential conditions.

## Henry's Law vs Raoult's Law

Henry's Law and Raoult's Law both connect pressure and liquid-phase composition, but they apply in different cases. Henry's Law is for a dilute solute gas dissolved in a liquid, while Raoult's Law is for a component that behaves like a solvent in an ideal solution. If you mix them up, you may choose the wrong constant or the wrong phase model.

## Key Takeaways

- Henry's Law says the dissolved amount of a gas is proportional to its partial pressure above the liquid at constant temperature.
- The law is a phase equilibrium tool, so it connects directly to chemical potential and the balance between vapor and liquid phases.
- Henry's constant depends on the gas, the solvent, and temperature, so the same gas does not behave the same way in every liquid.
- It works best for dilute, near-ideal gas-liquid systems and can break down at high pressure, high concentration, or when the gas reacts with the liquid.
- You will often use it for carbonation, dissolved oxygen, and other gas-liquid equilibrium problems in Thermodynamics II.

## FAQs

### What is Henry's Law in Thermodynamics II?

Henry's Law says that, at constant temperature, the concentration of a gas dissolved in a liquid is proportional to the gas's partial pressure above the liquid. In Thermodynamics II, it is used as a simple phase equilibrium relation for dilute gas-liquid systems. It shows up when you connect vapor composition to liquid solubility.

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

First identify the gas of interest and its partial pressure, then use the correct Henry's constant for that gas and solvent. If the equation is written as C = k_H P, you solve directly for concentration or pressure. The big check is whether the system is dilute and close enough to ideal behavior for the law to apply.

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

Henry's Law is usually used for a dilute solute, often a gas dissolved in a liquid. Raoult's Law is used for a solvent-like component in an ideal solution. They look similar mathematically, but they describe different physical situations and use different constants.

### Does Henry's Law always work for gases in liquids?

No. It works best at low concentrations and moderate pressures, especially when the gas does not react with the solvent. At higher pressures or in nonideal systems, the straight-line relationship can drift, so Thermodynamics II may push you toward fugacity or more detailed equilibrium models.

## Related Study Guides

- [10.3 Chemical Potential and Phase Stability Criteria](/thermodynamics-ii/unit-10/chemical-potential-phase-stability-criteria/study-guide/BPB7OWL4LxHJWfj9)

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