Forces of Attraction
February 3, 2015
Valentine’s Day is just around the corner and attractive forces are on everyone’s mind. In science, a general rule is “opposites attract.” In solution chemistry, there is another saying, “like dissolves like.”
Although “like dissolves like” sounds as if it contradicts “opposites attract,” it is actually an extension of the same physical phenomenon. For example, polar molecules will be attracted to other polar molecules through the attraction of the opposite partial charges on the atoms. Therefore, charged (or partially charged) solutes will dissolve in charged (or partially charged) solvents. So “like does dissolve like.”
Hydrogen bonding will occur between the polar -OH group on the ethanol molecule and the polar water molecule.
A quick demonstration highlighting "like dissolves like" can be performed with some canola oil, water, and a colored ionic compound such as copper(II) chloride.
- Mix the water and the oil together.
There is a phase separation because they are not “like” enough. (The oil is nonpolar while the water is polar.)
- Add copper(II) chloride.
The ionic copper(II) chloride passes through the oil layer and into the water layer, where it dissolves and the water layer turns a blue-green.
Copper chloride dissolves in the water layer but not in the oil layer.
To demonstrate nonpolar solubility, you can use hexane, water, and iodine. In this case, the nonpolar iodine will dissolve in and color the nonpolar hexane, but it will not affect the polar water. London dispersion forces can be used to explain the nonpolar–nonpolar interaction.
Finally, you can create an inquiry experiment for your students by having them determine if unknown compounds are more polar or nonpolar, based on their relative solubility in water. If you are testing unknown compounds that are not colored, you can measure another property of the mixture, such as pH or conductivity, using our Wireless Sensors, to determine if the solute will dissolve in the polar solvent.
With these quick demonstrations and activities, you can use the students' established ideas about forces of attraction to introduce the important concepts of molecular structure and "like dissolves like."