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Directions

Move the temperature slider to heat or cool a closed piston-cylinder containing a mixture of 20 mol% water and 80 mol% air at a constant total pressure of 1 atm. Blue particles represent air, and red particles represent water. Above the dew point (60.4°C), all of the water is vapor; below the dew point, liquid water (blue region at the bottom) is in equilibrium with a saturated vapor, and the piston moves down as gas-phase moles and temperature decrease. The system state box shows whether the vapor is superheated or saturated and displays the mole fraction of water in the vapor phase.

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Details

The saturation pressure of water is calculated from the Antoine equation (valid from 10°C to 110°C):

\[ \log_{10} P^{sat} = 7.07406 - \frac{1657.46}{227.02 + T} \]

where \(P^{sat}\) is in kPa and \(T\) is in °C.

The dew point is the temperature at which water first condenses, found by setting the partial pressure of water equal to its saturation pressure:

\[ y_{H_2O} \, P = P^{sat}(T_{dew}) \]

With \(y_{H_2O} = 0.20\) and \(P = 1\) atm, this yields \(T_{dew} = 60.4\,^{\circ}\mathrm{C}\).

Below the dew point, the vapor is saturated with water, so the vapor-phase water mole fraction is:

\[ y_{H_2O} = \frac{P^{sat}}{P} \]

Because air is non-condensable, the moles of water remaining in the vapor are:

\[ n_{H_2O}^{vap} = n_{air} \, \frac{y_{H_2O}}{1 - y_{H_2O}} \]

and the moles of liquid water are \(n_{H_2O}^{liq} = n_{H_2O}^{total} - n_{H_2O}^{vap}\).

The piston position is set by the ideal gas law at constant pressure, so the gas volume is proportional to the product of the total gas-phase moles and absolute temperature:

\[ V = \frac{n_{gas} R T}{P} \]

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About

This simulation was generated by Professor David L. Silverstein and Dr. Loyal Murphy of the University of Mississippi using Google Gemini. It was modified for LearnChemE using Claude AI.