MadSci Network: Chemistry

Re: Does the amount of salt in a water solution affect evaporation rates?

Date: Mon Mar 23 13:58:11 1998
Posted By: John Letourneau, Lab Technician, Canadian Forestry Service
Area of science: Chemistry
ID: 890286474.Ch

Carissa, You are correct in noticing that the amount of salt in solution can affect the evaporation rates of water. When salt is dissolved in water, each salt molecule breaks up into one sodium ion (Na+) and one Chloride ion (Cl-). Just like magnets, charged particles are attracted to other particles of opposite charge that might also be floating around in solution. What you may not be aware of is that water molecules can also act like weak magnets and have both positively and negatively charged ends. When water molecules evaporate they have to absorb enough energy from their surroundings to break free of the attractive forces of the other molecules around them. <<...>> When salt is dissolved in the water however, much stronger magnetic forces are present to hold the water molecules together. To break free (or evaporate) the molecules will require more energy than before the salt was added. <<...>> For your experiment, you are adding the same amount of energy to each container but now the molecules in the salt water need more energy per molecule to break free. This means that fewer molecules will have evaporated from the salted solution. Another experiment that demonstrates this phenomenon is the temperature at which water boils when it is salted as opposed to when it is not salted. Good Luck!!

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