Evaporation

I

Introduction

Evaporation, gradual change of state from liquid to gas that occurs at a liquid’s surface. Examples of evaporation include rainwater evaporating from warm pavement after a thunderstorm and wet paint drying as solvents in the paint evaporate. Fingernail polish also hardens as acetone (CH₃COCH₃) evaporates from the liquid polish.

A liquid is made up of atoms or molecules (bound groups of atoms) that are in constant motion, traveling at different speeds. The average speed of these particles depends only on the liquid’s temperature. If the particles have enough energy, fast-moving particles striking other particles near the liquid’s surface may impart enough speed, and therefore enough kinetic energy (energy of motion) to cause the surface particle to leave the liquid and become gas atoms or molecules. The particle’s kinetic energy is directly related to its speed.

As particles with the most kinetic energy evaporate, the average kinetic energy of the remaining liquid decreases. In a similar way, if a basketball team loses its tallest players, the average height of the team is diminished. Because a liquid’s temperature is directly related to the average kinetic energy of its molecules, the liquid cools as it evaporates (see Heat Transfer).

II

Rate of Evaporation

A liquid’s surface area and temperature affect its rate of evaporation. Evaporation rates also depend on the type of liquid, since liquids made up of different molecules differ in the amount of attraction that exists between the molecules (see Intermolecular Attractions).

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A

Surface Area and Temperature

Because molecules or atoms evaporate from a liquid’s surface, a larger surface area allows more molecules or atoms to leave the liquid, and evaporation occurs more quickly. For example, the same amount of water will evaporate faster if spilled on a table than if it is left in a cup.

Higher temperatures also increase the rate of evaporation. At higher temperatures, molecules or atoms have a higher average speed, and more particles are able to break free of the liquid’s surface. For example, a wet street will dry faster in the hot sun than in the shade.

B

Intermolecular Forces

Most liquids are made up of molecules, and the levels of mutual attraction among different molecules help explain why some liquids evaporate faster than others. Attractions between molecules arise because molecules typically have regions that carry a slight negative charge, and other regions that carry a slight positive charge. These regions of electric charge are created because some atoms in the molecule are often more electronegative (electron-attracting) than others. The oxygen atom in a water (H₂O) molecule is more electronegative than the hydrogen atoms, for example, enabling the oxygen atom to pull electrons away from both hydrogen atoms. As a result, the oxygen atom in the water molecule carries a partial negative charge, while the hydrogen atoms carry a partial positive charge. Water molecules share a mutual attraction—positively charged hydrogen atoms in one water molecule attract negatively charged oxygen atoms in nearby water molecules.

Intermolecular attractions affect the rate of evaporation of a liquid because strong intermolecular attractions hold the molecules in a liquid together more tightly. As a result, liquids with strong intermolecular attractions evaporate more slowly than liquids with weak intermolecular attractions. For example, because water molecules have stronger mutual attractions than gasoline molecules (the electric charges are more evenly distributed in gasoline molecules), gasoline evaporates more quickly than water.