Raoult’s Law, Dew Point, Bubble Point, and Flash Calculations Reference Page
Dew Your Bubbles Have Flash?

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Intro to Dew Your Bubbles Have Flash?

Raoult’s Law describes vapor–liquid equilibrium for mixtures of volatile components that behave approximately as ideal solutions. It provides the basis for calculating bubble points, dew points, and flash equilibria for many binary mixtures.

The videos on this page show how Raoult’s Law is used to calculate bubble pressures, dew pressures, bubble temperatures, dew temperatures, and flash equilibria for mixtures of volatile liquids.

Part 1

This video introduces Raoult’s Law and demonstrates how it is used to calculate bubble pressures and dew pressures for binary mixtures.

Visuals

Part 2

This video continues with bubble temperature and dew temperature calculations using Raoult’s Law.

Visuals

Part 3

This video demonstrates flash calculations, where a mixture separates into vapor and liquid phases at a specified temperature and pressure.

Visuals

Examples and Definitions

Definitions

Raoult’s Law
For an ideal solution, the partial pressure of component \(i\) in the vapor phase is proportional to its mole fraction in the liquid phase and the vapor pressure of the pure component.

\[ p_i = y_i P = x_i p_i^* \]

where \(x_i\) is the liquid mole fraction, \(y_i\) is the vapor mole fraction, \(P\) is the system pressure, and \(p_i^*\) is the vapor pressure of the pure component.

Ideal Gas Mixture
A mixture of gases that obeys the ideal gas law and Dalton’s Law of partial pressures.

\[ P = \sum p_i \]

Each component behaves as if the other gases were not present.

Ideal Solution
A liquid mixture in which intermolecular interactions between unlike molecules are the same as those between like molecules. Such mixtures follow Raoult’s Law over the full composition range.
Dew Pressure
The pressure at which a vapor of a given composition and temperature begins to condense.
Bubble Pressure
The pressure at which a liquid of a given composition and temperature begins to vaporize.
Dew Temperature
The temperature at which a vapor of a given composition and pressure begins to condense.
Bubble Temperature
The temperature at which a liquid of a given composition and pressure begins to vaporize.
Flash
A process in which a mixture with known overall composition is brought to a temperature and pressure within the two-phase region. The mixture separates into vapor and liquid phases, and the compositions and amounts of the phases are calculated.