Solutions and Basic Chemistry

As I prepared my lectures on solutions and then about acids and bases I decided a post about solutions would be in the same vein as a post about salts in basic chemistry. Both are everyday words that many people use in the kitchen and when preparing basic cold/flu remedies. In the kitchen, we refer to salt exclusively as table salt or NaCl. In chemistry, anything that makes ions in solution is a salt. It is such a basic term in science that the first classification system of the discipline was based on the term.
"Salts played a pivotal role in the historical development of chemistry as a discipline. The medieval alchemist Paracelsus named salt as a third principle of nature. However, he treated it more as a category of nature and his work became known throughout Europe in the middle of the 1700s. As the chemical behavior of salts became better understood, there was a significant increase in the number of salts known and this resulted in a need for a classification system. Classification as a method of understanding became a defining characteristic of science in general and of chemistry as a discipline."
Solutions are similar to salts in that the chemical meaning of the term spreads far wider than simply the kitchen or medical application. In the kitchen we prepare cornstarch solutions to thicken our gravy. We prepare saltwater solutions to cleanse our throats when we have a cold. A baking soda solution might be used as a basic household cleaner. Around the house, a solution seems to be primarily a small amount of a solid dissolved in at least enough liquid to make all of it go into solution.
In chemistry it is far, far more than that. A solution is basically anything that makes up a homogeneous mixture or a mixture of uniform composition. In class I clarify this to say that if you can't see the constituent components of the mixture, it can be classified as homogeneous. Then I joke that this would mean I would have to clarify what type of mixture I was referring to, for example, if I said "air." The choice would be Los Angeles air (heterogeous and not a solution) or desert island air (homogeneous mixture = solution). I'm just pointing out that if particles can be seen in the air, it cannot be considered a solution.
Up until the last paragraph of this entry, a beginning student might still assume that a homogeneous mixture would have to be a solid dissolved in a liquid. This is not true from a chemical standpoint. A homogeneous mixture can be a solid dissolved in a liquid (saltwater solution), liquid dissolved in a liquid (ethanol in water), a gas dissolved in a liquid (carbonated beverage), a liquid dissolved in a gas (water vapor), or a solid dissolved in another solid (alloy of metal like brass). Basically, any phase intermixed with any other phase of matter can qualify as a solution.
Sometimes you may not be able to tell a solution from a pure substance (element or compound). The best way to know if you have a solution as opposed to something pure would be to have it undergo some kind of physical change. In many cases, simply heating it up to melting point/boiling point will separate the two pure substances by the difference in their melting points/boiling points.
When I get into acid/base chemistry, suddenly the students must add the acid/base neutralization concept to their understanding of concentration and volume. Although they have seen this before in the chapter on chemical reactions, many of them do not remember that the neutralization of an acid/base gives a salt and water as products.  They must get comfortable interconverting between concentration, volume and moles. They must know the molar ratio of the acid to base that they are working with in their reaction. Then, they must be able to perform a titration to neutralize all of an acid and correctly determine the concentration of it based on a known concentration of base.
Solutions are a challenging part of introductory chemistry. And one that stretches people beyond their basic kitchen comprehension.