15.2. Aqueous Solutions http://www.ck12.org
the density of the solution is different than the densities of the two starting materials.) Recall that salt is an ionic
substance comprised of cations and anions bonded tightly together in a crystal lattice. Because water molecules
are polar, they are capable of stabilizing isolated ions by surrounding each one with the appropriate partial charges.
The process of separating ions from one another is known asdissociation. After the sodium and chloride ions that
make up salt have separated from each other, the compound is said to have dissociated. Here is a video clip of what
dissociation looks like: http://www.youtube.com/watch?v=EBfGcTAJF4o.
Although they appear identical, a solution of sodium chloride has properties that are significantly different than those
of pure water. For example, pure water is a poor conductor. However, if we pass electricity through a solution of
sodium chloride, we find that it is quite conductive (seeFigure15.11).
FIGURE 15.11
Conductivity of a solution of sodium chlo-
ride.InFigure15.11, the bulb on the right is fully illuminated. Electricity is able to flow through the circuit because the
salt solution is a good conductor. In contrast, the bulb on the left is not lit, because the poorly conductive beaker
of water provides too much resistance to the complete electric circuit. Why does this difference occur? In order to
conduct electricity, a substance needs to have charge carriers (either ions or electrons) that are able to move freely. In
pure water, there are essentially no ions, and all electrons are tightly associated with their corresponding molecules.
However, in the salt solution, plenty of positively and negatively charged sodium and chloride ions are available to
conduct an electric current. Because this test tells us the relative number of free ions that are in solution, it can be
used to determine whether a given substance dissociates into separate ions when dissolved in water.
Scenario 2 –Sucrose Added to Water
In our second scenario, we are going to add cane sugar to water. Again, the solid will gradually disappear over time,
and assuming there is enough water to dissolve all of the sugar, we will eventually be left with a clear solution. What