Classes of Matter

Elements are the most basic forms of matter. Aluminum, sulfur, iron and oxygen are all examples of elements. These are pure substances that cannot be broken down into simpler substances. Scientists have identified over 100 elements that are present in our universe. Each element is made up of one type of atom which determines its properties. The Periodic Table groups elements by their atomic number. Each element is represented by a chemical symbol.

Atoms are the building blocks of elements. Every element is made up of atoms that have a unique structure. For example, an oxygen atom contains eight protons and eight neutrons in its nucleus. Eight electrons circle the nucleus in an electron cloud.

A molecule is the chemical combination of at least two atoms. Many elements occur in nature as molecules, not as single atoms. Oxygen typically occurs as a molecule made up of two chemically bonded atoms, and is written in a chemical formula as O2. A molecule of water is composed of three chemically bonded atoms. The formula for water is written as H2O.

A compound is a pure substance made up of two or more different elements that are chemically combined in an exact ratio. The chemical formula of a compound tells you which elements make up the compound and how many atoms of each element are found in a single molecule of the compound. Water is a compound that has a ratio of two hydrogen atoms to every one oxygen atom. If this ratio is changed to two hydrogen atoms to every two oxygen atoms, a different compound is formed. This compound, hydrogen peroxide, has different properties than water. It is used as cleaning agent, and you would not want to drink it!

A mixture is the physical combination of two or more substances. Mixtures can be made of compounds and elements. The substances in a mixture are not chemically combined, and they are not found in an exact ratio. Imagine a mixture of nuts in a bowl. The ingredients keep their individual properties and can be separated. This nut mixture is called a heterogeneous mixture, because you can see the individual substances. In a homogeneous mixture, like paint, you cannot see the individual parts that make up the mixture.

A solution looks like a single substance, but it is actually a homogeneous mixture made up of two or more evenly distributed substances. For example, when salt is mixed with water, the salt particles dissolve, or mix evenly, with the water particles. The salt, which is the substance that dissolved, is called the solute. The water, which caused the salt to dissolve, is called the solvent. Solutions can be found in all the different states of matter. Solutions can be liquids like anti-freeze, or gases like the air we breathe, and even solids, like brass.

Some solutions are considered to be concentrated while others are considered to be dilute. The concentration of a solution is the amount of solute in a given amount of solvent. Concentrations are often measured in grams per milliliter. A concentrated solution has more solute in it than a dilute solution.

If the particles that are dispersed throughout a mixture are large enough to be seen and they scatter or block light, then the mixture is called a suspension. Eventually these particles will separate and settle out. For example if you mix sand in a beaker of water, and then let it sit for a while, the sand particles will settle on the bottom of the container. Colloids are a type of mixture in which particles are too small to see, but they are large enough to scatter light and be filtered. Milk, gelatin, and whipped cream and many of the foods we eat are considered colloids.

The substances in a mixture are not chemically bonded together and therefore can be separated using a variety of techniques, depending on the compounds and elements that make up the mixture. Magnets can be used to separate a mixture of nails that is made of iron and aluminum. Some mixtures made up of liquids and solids can be separated using filters. Distillation is a process used to separate substances that have different boiling points, like water and salt. A centrifuge is a machine that spins mixtures at high speeds. It can be used to separate substances by their densities.