2.2. Classification of Matter http://www.ck12.org
dissolves in water, but salt water cannot be classified as a pure substance because its composition can vary. You may
dissolve a small amount of salt or a large amount into a given sample of water. Amixtureis a physical blend of
two or more components, each of which retains its own identity and properties. Only the form of the salt is changed
when it is dissolved into water. It retains its composition and chemical properties.
An activity that makes it easy to understand the differences between elements, compounds, and mixtures can
be found at http://www.bbc.co.uk/schools/ks3bitesize/science/chemical_material_behaviour/compounds_mixtures/activit
y.shtml.
Homogeneous Mixtures
Ahomogeneous mixtureis a mixture in which the composition is uniform throughout the mixture. The salt water
described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water
sample. It is often easy to confuse a homogeneous mixture with a pure substance because they are both uniform.
The difference is that the composition of the pure substance is always the same. Air is a mixture because the
different gases that make up air may be present in varying amounts. A troublesome aspect of modern society is that
the burning of fossil fuels in large amounts has increased the amount of carbon dioxide in our atmosphere compared
to the levels that were present hundreds of years ago before the industrial revolution. Carbon dioxide acts as a
greenhouse gas, trapping heat in our atmosphere. The increase in carbon dioxide levels has led to a corresponding
increase in average global temperatures (Figure2.5).
FIGURE 2.5
Carbon dioxide levels in Earth’s atmo-
sphere (measured in parts per million)
have increased dramatically since 1900.
The average global temperature has also
seen an increase, as seen on the left-
hand axis.Heterogeneous Mixtures
Aheterogeneous mixtureis a mixture in which the composition is not uniform throughout the mixture. Vegetable
soup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetables
and other components of the soup.
Aphaseis any part of a sample that has a uniform composition and properties. By definition, a pure substance or
a homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. When
oil and water are combined, they do not mix evenly (Figure2.6). Instead, they form two separate layers, each of
which is considered a phase.