Classifying Matter

An earlier version of this article appeared in Greg’s now-defunct chemistry website, Science Help Online for Chemistry.

If you have taken biology already, you are probably familiar with Taxonomy, which is the science of classification as applied to living organisms. Perhaps, you have also come to understand the purpose of classifying living organisms? With millions of species on our planet, no one is able to be an expert on all of them. However, if you know the characteristics of the specific categories (kingdoms, phylla, classes, etc.) you are able to infer much about an unknown organism by knowing what category it falls in. If I tell you that a particular animal is classified as a mammal, then you can infer that it is probably warm-blooded and that it likely has hair or fur.

If you have taken earth science, you might not remember the characteristics of gneiss rocks, but if I remind you that gneiss is a form of metamorphic rock, you can infer that it forms under extreme heat and pressure.

People classify things into groups, so we can describe and learn the characteristics of the groups rather than all of the individual things within these groups.

Like biology and earth science, chemistry has a classification system that you will want to become very familiar with. Knowing the characteristics of the various categories will help you identify and understand the different types of matter. Fortunately, the classification system for chemistry is far less complex than for Biology.

Matter - Matter is defined as anything that has the properties of mass and volume, which you might recall from a previous post. As a science teacher, I don’t like to use definitions that contain words that you might not know, so allow me to repeat the definitions of mass and volume.

Mass - An object has mass if it is made up of particles. Anything that is made up of atoms has mass. Even subatomic particles found on their own have mass. So, you can think of mass as a measure of the number of particles an object has.

Volume - Volume is how much space something takes up. It is simply the size of the object.

Therefore, we can define matter as anything that is made up of particles and takes up space.

The classification of matter

As you can see from the image above, we can first break matter up into two categories; Pure Substances and Mixtures.

Pure substance - A Pure Substance is a material with a constant composition. This means that the substance is the same no matter where it is found. If you were to find a sample of 100% pure gold, it would only be made up of gold atoms. It wouldn’t matter where you found the pure gold, it would be made up of gold atoms. Carbon dioxide is a compound made up of one carbon atom bonded to two oxygen atoms. No matter where you find carbon dioxide, even on another planet, it will always be composed of molecules with the same composition; one carbon atom bonded to two oxygen atoms.

Now, you may have noticed that I mentioned that gold is only made up of one type of atom, those being gold atoms. Yet carbon dioxide is made up of two types of atoms, carbon and oxygen. These are examples of the two types of pure substances, elements and compounds. As you can see in our image, the category called “Pure Substances” is made up of two sub-categories, Elements and Compounds.

Elements - Elements are substances that are made up of only one type of atom. At this time, there are 118 known elements, most of which are metals. This means that we only know of 118 types of atoms. The symbols shown on the periodic table represent these types of atoms, called elements. These elements are thought of as the elemental building blocks of matter. All of the materials that you encounter in your day to day life are made up of some combination of these building blocks.

Even atoms are made up of smaller particles, but they are not broken down by ordinary chemical means.

Carbon is classified as an element because it is made up of only one type of atom.

Compounds - Compounds are substances that are made up of more than one type of atom. You don’t see water on the periodic table, because there is no such thing as “water atoms.” Water is considered a compound because it is made up of more than one type of atom, chemically bonded together. A sample of water is made up of hydrogen and oxygen atoms. Water is still a substance because it has a consistent composition. Anywhere that you find pure water, it will be made up of molecules consisting of one oxygen atoms bonded to two hydrogen atoms.

Unlike elements, compounds can be decomposed or broken down into their constituent atoms by simple chemical reactions.

Carbon Dioxide is classified as a compound because it contains more than one type of atom chemically combined.

Now, we said that matter can be broken up into pure substances and mixtures. How do they differ again? Pure substances have a consistent composition and mixtures don’t. It is easy when you realize that chicken soup is a mixture. The composition of the chicken soup depends upon who makes it and when they make it. One time, I might put celery in my chicken soup. The next time, I might use peas instead. Either way, it is still soup, even if it doesn’t have a consistent composition.

Mixtures - Mixtures are made up of two or more substances that are physically combined, like the chicken, celery and carrots in my soup. The specific composition will vary from sample to sample. There are two types of mixtures which vary based on how well they are mixed. The important thing to remember about mixtures is that the parts of the mixture can be separated by physical means.

Heterogeneous mixtures - Chicken soup is considered a heterogeneous mixture, because it is not particularly well-mixed. Even if you stir the soup for quite some time, it doesn’t have a uniform composition. If I want a bowl of this soup, it makes a difference whether I take from the top of the pot or the bottom of the pot. No two bowls taken from this pot will ever be identical. What if I take my soup and put it in a blender? Sometimes, squash soup or pumpkin soup are well blended that they appear to have a consistent composition.

However, when you eat the soup, you will realize that it is not quite a homogeneous mixture. Spices are small enough to end up unevenly distributed, so one spoonful may taste more spicy than the next.

Salad is a Heterogeneous Mixture because it does not have a uniform composition.

Homogeneous mixtures - Some mixtures are so well blended that they are considered homogeneous mixtures. Homogeneous mixtures have such a consistent composition, that all parts of the sample are the same. If you stir a small amount of salt into a glass of water, the salt will distribute so well that the consistency is considered constant. Each sip that you take is just as “salty” as the next sip. Salt water is still a mixture because the salt and the water are physically not chemically combined, but the two substances mix so well that the resultant mixture is said to be uniform. Homogeneous mixtures are also called “solutions.”

Solutions - Solutions, which are also called homogeneous mixtures, are made up of two parts: The solute, which gets dissolved, and the solvent, which does the dissolving. In the case of salt water, salt is the solute and water is the solvent.

And, that’s all there is to the classification of matter. Students, do your best to understand and learn this system of classification. Your teacher will be using these terms in class all year, and you want to be able to understand what they are talking about. Investing some of your time early on will save you a great deal of confusion later. It is much harder to develop a love of chemistry when you are feeling confused and frustrated.

Check your understanding with this classification of matter crossword puzzle.

Teachers - Do you have any additional tips for our young readers? Join our team of teachers by joining the discussion. Comment in the section below.