2.2. Classification of Matter http://www.ck12.org
FIGURE 2.5
Pure substances: (left) the element sulfur
and (right) the compound water.The image on the left shows elemental sulfur in the solid state. The image on the right shows water in its liquid form.
Sulfur is a pure element, and water is a compound comprised of the elements hydrogen and oxygen. Both of these
substances have a constant composition, but water can be broken down into its elements, whereas sulfur cannot be
decomposed into a simpler substance. Water can be broken down into its elements by passing electricity through a
salt solution.
Periodic Table of Elements
Chemists have classified and organized all of the known elements into what is called the periodic table. All known
substances are made of some combination of these elements. The periodic table is a tool that we use to help identify
and describe the composition of a given substance. All pure substances which cannot be broken down further, which
we have called elements, are displayed in the periodic table.Figure2.6 shows our modern periodic table. We will
study the periodic table in more detail in the chapterThe Periodic Table.
Mixtures
When two or more pure substances are combined together, amixtureis formed. Unlike pure substances, mixtures
have a variable composition. Variable composition indicates that the relative proportions of the mixtures components
may vary, and they can be separated by physical methods. There are two main types of mixtures.
Homogeneous Mixtures
Ahomogeneous mixtureis one in which the composition is uniform throughout the mixture. A glass of salt water
is a homogeneous mixture because the dissolved salt is evenly distributed throughout the entire sample. It is often
easy to confuse a homogeneous mixture with a pure substance because they are both uniform, and it can be difficult
to tell which type you have by the naked eye. The difference is that the composition of the pure substance is always
the same, while the composition of a homogeneous mixture can vary. For example, you may dissolve a small amount
or a large amount of salt into a given sample of water. Although the ratio of salt to water will differ, the mixtures
will both be homogeneous. However, pure water will always have the same ratio of elements that make it a pure
substance (two hydrogen atoms per oxygen atom).
Wine, air, and gunpowder are other examples of common homogeneous mixtures (Figure2.7). Their exact
compositions can vary, making them mixtures rather than pure substances. Wine is a liquid mixture of water,