The focus of "Chapter 15: Water" was water's role in the formation of aqueous solutions. We examined the primary characteristics of a solution and how water is able to dissolve solid solutes; we differentiated between a solution, a suspension, and a colloid. There are many examples of solutions that do not involve water at all, or that involve solutes that are not solids. The table below summarizes the possible combinations of solute-solvent states, along with examples of each.
|Solute State||Solvent State||Example|
|liquid||gas||water in air|
|gas||gas||oxygen in nitrogen (gas mixture)|
|solid||liquid||salt in water|
|liquid||liquid||alcohol in water|
|gas||liquid||carbon dioxide in water|
|solid||solid||zinc in copper (brass alloy)|
|liquid||solid||mercury in silver and tin (dental amalgam)|
Our air is a homogenous mixture of many different gases and therefore qualifies as a solution. Approximately \(78\%\) of the atmosphere is nitrogen, making it the solvent for this solution. The next major constituent is oxygen (about \(21\%\)), followed by the inert gas argon \(\left( 0.9\% \right)\), carbon dioxide \(\left( 0.03\% \right)\), and trace amounts of neon, methane, helium, and other gases.
Solid-solid solutions such as brass, bronze, and sterling silver are called alloys. Bronze (composed mainly of copper with added tin) was widely used in making weapons in times past, dating back to at least 2400 B.C. This metal alloy was hard and tough, but was eventually replaced by iron.
Perhaps the most familiar liquid-solid solution is dental amalgam, used to fill teeth when there is a cavity. Approximately \(50\%\) of the amalgam material is liquid mercury to which a powdered alloy of silver, tin, and copper is added. Mercury is used because it binds well with the solid metal alloy. However, the use of mercury-based dental amalgam has gone under question in recent years, because of concerns regarding the toxicity of mercury.