Classification of Materials

To start thinking like a chemist, one of the first steps is to begin asking "what is this thing made of?" Put another way, "what is its composition." You should already have some intuitions around questions of composition from your day-to-day life. For example, you surely know that you can take sugar and water and mix them to make sugar water or simple syrup. As a slightly more complicated example, you can mix oil, vinegar, and lemon juice (which are, themselves, already mixtures of things) to make salad dressing, maybe with some salt dissolved in there to boot. So the process of mixing is one you probably understand.

As the previous section on separations may have made clear, though, chemists are more often interested in going in the reverse direction: taking something that is made of multiple components and separating them out from each other. For example, you could take sugar water and distill it, evaporating and collecting the water to leave the sugar behind. At that point, you could legitimately claim to have fully separated the mixture: you have pure water in your collection flask and pure sugar in your beaker.

However, as you may know, even pure water can be further broken down into components. This process looks very different, however, from the distillation of sugar water. By running an electric current through water, you can decompose it into hydrogen gas and oxygen gas. If these gases are combined and ignited, they will react (violently), producing water once again.

So in one sense, water itself is made up of simpler components, just like sugar water is. However, it should be clear to you that these processes of separation are of distinctly different types. It's true that water is made up of hydrogen and oxygen, the way sugar water is made up of sugar and water ... but it's not true in the same way.

Let's dig further into these ideas and learn some terms that will be useful in understanding this difference.

Elements

In the example above, my hypothetical chemist started with sugar water and asked "what is this made of?" She answered the question by distilling it into two components: sugar and water. Then she took the water, a single uniform substance, and again asked "what is this made of?" Again she achieved a separation, this time using electric current to give two components: hydrogen and oxygen gas.

Our chemist might well then ask about the hydrogen (or oxygen) gas, "what is this made of?" She might embark on an effort to separate it into its components. She could try filtration, distillation, electrical current, heat, or any number of other techniques chemists use for separation. However, in this case, no matter what she tries, she will never be able to split her hydrogen gas into simpler components (at least not without moving from the realm of chemistry to the realm of high-energy physics). This is because hydrogen is an element: a substance that cannot be broken down into simpler substances.

Humans have been classifying things as elements for many millennia. Plato and other ancient Greek philosophers believed there were four elements - air, earth, wind, and fire - and that all the forms of matter in the universe were combinations of these four things. While the broad idea (matter is made of elements, which can be found alone or in combination with each other) is correct, none of these four things are actually elements.

Beginning in the middle ages, alchemists began identifying different sets of elements. Some of them were accurate (gold, silver, copper, mercury) while some were incorrect (water, which is not an element, and heat, which is not a form of matter at all). With the dawn of chemistry in the late 1700's, the list of known elements grew rapidly, as did our understanding of how elements combine.

We will learn more about what makes an element in future lessons, especially about elements at the microscopic scale. At the scale humans can observe, though, the only way to determine something is an element is to exhaustively test methods for separating it, until you feel sure it can't be broken down further.

Compounds

Hydrogen and oxygen are elements, as laid out above. If they are combined and given a spark, they will join together to form water. In this sense, water is "made of" hydrogen and oxygen. Other substances you may be familiar with are also made up of elements. The butane in a lighter is made of the elements carbon and hydrogen. The salt on your kitchen table is made of the elements sodium and chlorine. And the sugar in your sugar bowl is made up of three elements: carbon, hydrogen and oxygen.

Furthermore, these substances are not only made of these elements, but they always contain those elements in the same proportion. This is the Law of Constant Composition we discussed earlier in the lesson. If you weigh out the elements that make up water, you always find it to contain 7.94 times as many grams of oxygen as hydrogen. If you do the same process with salt, you always find 1.54 times as many grams of chlorine as sodium.

Substances like these are called compounds: combinations of elements with well-defined properties, which always have those elements combined in the same ratio (again, this is the Law of Constant Composition).

It is impossible to take a compound and pack a little "extra" of an element into it. If I take salt and try to add more chlorine, I don't get "Extra-chlorine-tastic Salt" ... the chlorine just fails to incorporate into it.

On the other hand, atmospheric air is a mixture of the elements nitrogen and oxygen (mostly). Since it is made of elements, you might be tempted to say air is a compound. However, the composition of air can be easily adjusted - indeed air at high elevations has less oxygen in it than air at sea level, but we still call both of them "air." Thus, air does not obey the Law of Constant Composition, and is not a compound.

The flowchart below should help you better understand these ways of classifying matter. In future sections and lessons, we will learn more about the microscopic-scale differences that allow us to classify matter in these ways. We will start with atomic theory in the next section.