|MadSci Network: Chemistry|
There are several approaches we could take to explain why pure water evaporates faster than saltwater, and some of them can become rather complicated. It would probably be easiest to explain what exactly is happening to the water and salt molecules themselves. Salt is what we call a nonvolatile substance. This means that it will not easily evaporate. Water is a slightly volatile substance, meaning that if left standing it will evaporate (go from the liquid to the gas phase). When we have pure water, there is nothing to prevent it from evaporating. That is, on the surface of the water there are only water molecules, and we know that evaporation takes place on the surface. When we have saltwater, the surface now contains both salt and water. Salt does not like to evaporate (because it is nonvolatile), so it will stay in the solution. The water will still evaporate, but not as quickly because now salt takes up part of the surface area at the top of the solution. Since the water molecules in salt water don't have as much surface area to evaporate from as the water molecules in pure water, the water in salt water will take longer to evaporate. We can also look at the forces between the salt and water in saltwater. We call these intermolecular forces and they result from the attraction of the positive and negative parts of a water molecule to the positive and negative ions in salt. Salt is made up of ions, which are just atoms with either a positive or negative charge. This charge comes about when an atom has more or less electrons than it does protons. Sodium and chloride ions make up salt, and when we put salt in water, these ions separate from each other (we call this dissociation) and the salt dissolves. The chloride ions, which are negatively charged, are attracted to the partial positive charge on the hydrogen atoms in a water molecule while the positively charged sodium ions are attracted to the partial negative charge on the oxygen atom in a water molecule. When the water in saltwater tries to evaporate it has a harder time because now it has sodium and chloride ions holding it back. Remember that salt doesn't like to evaporate so it tries to keep the water in the solution, too. In order for the water in saltwater to evaporate it needs more energy than pure water, so it will take a longer time to evaporate. Pure water, on the other hand, does not have to worry about intermolecular forces with ions. It does have something we call "hydrogen bonding," which is basically a weak force between the negative and positively charged parts of a water molecule, but hydrogen bonding is not as strong as the forces between the water and ions in saltwater. In hydrogen bonding, the hydrogen atoms (which have a partial positive charge) of one water molecule are attracted to the oxygen atom (which has a partial negative charge) of another water molecule. The electrons in the bond between hydrogen and oxygen in water are not shared evenly. They spend more time closer to the oxygen atom, which is what gives it the partial negative charge. To get a good picture of how hydrogen bonding works, you can look up the following website: http://www.mc.maricopa.edu/users/belford/scc130/pp130/ch14/sld023.htm
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