# Dalton’s Law of Evaporation

### Author: Paul J. Laumakis

**Model Reality Check**

We present a differential equation that models the process of evaporation from a free-water surface, based on the principle of conservation of mass and Dalton’s evaporation law. We collected experimental data to verify the accuracy of the analytic solution to the differential equation model. This verification process required the computation of the two vapor-pressure terms appearing in Dalton’s equation:

• the saturation vapor pressure of the evaporating water, and

• the partial vapor pressure of the thin layer of moist air just above the evaporating water surface.

We explain these terms and others involved in the underlying physics. Finally, we calculate the proportionality constant in Dalton’s equation, based on the slope coefficient of the least-squares linear fit to the experimental data.

The result is excellent agreement between the theoretical model and the experimental results, and it and serves to illustrate the exciting relevance of proper mathematical modeling.

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