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As a result, liquids with strong intermolecular attractions evaporate more slowly than liquids with weak intermolecular attractions. Intermolecular attractions affect the rate of evaporation of a liquid because strong intermolecular attractions hold the molecules in a liquid together more tightly. Water molecules share a mutual attraction-positively charged hydrogen atoms in one water molecule attract negatively charged oxygen atoms in nearby water molecules. As a result, the oxygen atom in the water molecule carries a partial negative charge, while the hydrogen atoms carry a partial positive charge. 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. These regions of electric charge are created because some atoms in the molecule are often more electronegative (electron-attracting) 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. 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. For example, a wet street will dry faster in the hot sun than in the shade.
#Chemlab 12 compare rates of evaporation free#
At higher temperatures, molecules or atoms have a higher average speed, and more particles are able to break free of the liquid’S surface. Higher temperatures also increase the rate of evaporation. For example, the same amount of water will evaporate faster if spilled on a table than if it is left in a cup.
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The stronger the forces keeping the molecules together in the liquid or solid state the more energy that must be input in order to evaporate them.īecause 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. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is in part related to the concentration points above. If the substance is hotter, then evaporation will be faster. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating. Concentration of other substances in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly. Concentration of the substance evaporating in the air. When the process of escape and return reaches equilibrium, the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur.įactors influencing rate of evaporation:. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state.