Rather than trying to define a mixture, it might be easier to define what a mixture is not. A mixture is not a pure substance. A pure substance, in chemical terms, is something with only one chemical present. That chemical might be either an element or a compound, but either way there is only one single chemical present. All other matter must be mixtures.
A mixture occurs whenever two or more chemicals are physically combined, in any proportion. It is important to remember the “physical” part. Mixtures do not involve chemical bonds or reactions. Chemical reactions may produce mixtures, and they may occur within mixtures, but the mixture exists only because two or more separate chemicals are present.
There are many types of mixtures, including many things that do not visibly appear to be mixtures. Generally, mixtures are classified as either homogeneous or heterogeneous. Homogeneous mixtures are evenly dispersed mixtures – the chemicals are arranged throughout the mixture in the same proportion, so that a sample from one place in the mixture will exactly match in composition a sample taken from anywhere else in the mixture. Heterogeneous mixtures are not uniform. The compositions of two samples from a heterogeneous mixture are not guaranteed to match.
Some examples of heterogeneous mixtures include: sand at the beach, Mom’s casserole, writing paper, Italian dressing, and even your blood. In each case, there’s definitely more than one chemical present, and they aren’t evenly mixed. Sand has a lot of silicon dioxide present, but there are plenty of other things mixed in, including broken up bits of calcium carbonate from seashells. Mom’s casserole needs no explanation. Writing paper is made of bits of cellulose mixed together with chemical binders and usually coated with various chemicals too. You can see individual fibers if you look closely, of course, and they clearly aren’t uniform. Italian dressing separates into multiple parts just while sitting in the bottle. That’s why you have to shake it up before pouring it, you want to make the mixture as even as possible so that the flavor will be more uniform. Your blood, of course, has various types of blood cells, plasma, platelets, gases, and more.
Some examples of homogeneous mixtures include the air in a gas cylinder, bronze, and the sugar-water my uncle feeds to the hummingbirds. There are multiple gases that make up air, mainly nitrogen and oxygen. Gases disperse to fill their container evenly, and since air is primarily empty space, the individual gases don’t interact too much with one another. (When compressed, they get in each others’ way more, but they are still able to move about.) Bronze is an alloy – a uniform mixture of two (or more) metals that is formed when they are melted together, blended, and allowed to cool. Sugar-water, of course, is sugar that is dissolved in water, and is an example of a solution.
A solution is a homogeneous mixture. Using that basic definition, air, alloys, and sugar-water are all solutions. One is a gas-phase solution, one is a solid-phase solution, and one is a liquid-phase solution. Typically, the word solution alone is used to refer to liquid-phase solutions. To talk more about liquid-phase solutions, a few vocabulary words are necessary.
The solvent is the main component of a solution. In a liquid-phase solution, it will be a liquid. Most commonly, people expect water to be the solvent, but any liquid may be a solvent – oil, octane, acetone – you name it. When water is the solvent, the solution is called an “aqueous solution”.
“Dissolve” means to enter into solution. When a chemical is first mixed with a solvent, it has to break down into individual molecules, and those molecules are surrounded by molecules of the solvent. (This process may sometimes be called dissolution or solvation.) Once this is accomplished, the dissolved molecules disperse throughout the solution, much like gases do.
The solute is any chemical that gets dissolved in the solvent. A solution may have more than one solute. A sports drink, for example, is a solution of salts, sugar (or other sweetener), and flavoring in water.
In a liquid-phase solution, the solute may be a solid, liquid, or gas. When the material is dissolved, the solution as a whole behaves as a liquid, but the solute is simply single molecules of whatever chemical it may be, surrounded by molecules of the solvent.
A solution may have properties that are distinct from those of the solvent or the solute alone. Consider the case when salt (sodium chloride) dissolves in water. Neither water nor salt crystals alone make a good conductor of electricity. When salt dissolves, it breaks up into individual ions, creating a more conductive environment. (The ions move, carrying charge through the water.)
Solutions are often described in terms of concentration. Concentration is a measure of how much solute there is relative to the amount of solvent, or relative to the total amount of solution. While beyond the scope of this article, you can read more about how to calculate concentration in the Helium article is the concentration of a chemical solution calculated?”
The chemistry of solutions is vast. Many reactions are performed in solution. Much of the chemistry of life relies on solutions, making it no surprise that so much of the body is made from water. The very fact that solutions are homogeneous mixtures is a part of why they are so important. Because they are uniform, they behave in predictable, reproducible ways. Imagine how disappointed you would be otherwise, when your favorite drink had splotches of flavorlessness, and other bits that were so intense it made your mouth sting. That’s what you might get with a heterogeneous beverage mixture.
