Classification of Matter

Matter is anything that has mass and occupies space. In other words, matter is anything. Chemistry is the scientific study of matter, which means it is a very broad branch of knowledge.

In order to make sense of the tremendous amount of information that we can learn about matter, chemists have developed categories of kinds of matter in which to organize the information. If we examine a thing, and decide which category it belongs in, we are classifying it. Keep in mind that there are several different systems of categories which are not mutually exclusive. When we classify organisms in biology, for instance, we can use two different category systems to decide that a particular animal is male and that it is a reptile. That does not imply that all reptiles are male or all males are reptiles.

For a chemistry student, the most useful categories for classifying matter are physical form (state), composition (degree of purity), and fundamental make-up.

States of Matter

The physical form of a sample of matter depends on its temperature and pressure. The most commonly observed states of matter are solid, liquid, gas, and plasma.

Material Composition (purity)

A sample of matter is ‘pure’ if it has one fundamental composition throughout (there is one and only one ‘thing’ in the container). A mixture has two or more different substances physically blended together. Although added together, each component of a mixture retains its own identity and fundamental characteristics. Pure things are rather rare; nearly everything that occurs naturally is a mixture. In order to have pure substances, we have to spend time, energy, and money to remove the matter we don’t want from the matter that we do want. When we manage to accomplish this, we have purified that sample.

Fundamental Make-up

An element is a sample of matter that cannot be simplified (note that is ‘simplified’ instead of ‘purified’). A compound is a sample of matter that can be changed into two or more elements. Since it contains more than one of the simple types of matter, it is more complicated than a substance with only one. Gold is an element. We cannot change gold into anything simpler than gold. Water is a compound. We can change water into hydrogen and oxygen, both of which are simpler than water.

If you are truly a beginner to the study of chemistry, you may be amazed and discouraged when the students in your class can volunteer other examples, such as aluminum is an element and rust is a compound. How can you tell the difference between an element and a compound? You can’t, unless you have access to fairly sophisticated chemical equipment. How can the other students tell the difference? They can’t. They just memorized some examples before they got to class. The good news is that there are only about a hundred elements to learn. The bad news is that there are an almost infinite number of compounds.

Concept Check

What is the difference between a mixture and a compound?

They are both things that have more than one kind of stuff in them. The difference lies in how the different kinds of stuff are combined. In a mixture, two or more things are blended but each retains its own identity. The properties of the mixture are similar to the properties of the things in the mixture. So if I add salt to water and stir, I get a mixture that looks like water but tastes salty. A compound has two or more elements that are combined so that they make a thing with a completely new identity. This new stuff has properties that are different from those of the elements that were used to make it. Iron is an element; it is shiny, strong, metallic, and magnetic. Oxygen is an element; it is a colorless, odorless gas. When iron rusts, it combines with oxygen in the air to form the compound iron oxide. Iron oxide not only has a different name, it has different properties from those of iron and/or oxygen. It is reddish brown, a solid, crumbly rather than strong, and non-magnetic.