http://www.ck12.org Chapter 2. Matter and Change
MEDIA
Click image to the left for more content.Pure Substances
As described in the lesson, “Properties of Matter,” pure substances have a definite and fixed composition. Pure
substances can further be classified as either elements or compounds.
Elements
Anelementis the simplest form of matter that has a unique set of properties. Examples of well-known elements
include oxygen, iron, and gold (Figure2.9). Elements cannot be broken down into simpler substances. Additionally,
one element cannot be chemically converted into a different element.
FIGURE 2.9
Chemical elements are the simplest of pure substances. (A) An oxygen tank like this is used by people with a
need for breathing assistance. (B) A simple skillet can be made from cast iron. (C) Gold bars are formed and
used for monetary purposes.Compounds
Acompoundis a pure substance containing two or more elements that have been chemically combined in a fixed
proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest
is called methane, which always has four times as many hydrogen particles as carbon particles. Methane is a pure
substance because it always has the same composition. However, it is not an element because it can be broken down
into simpler substances (the elements carbon and hydrogen). The process of breaking down a compound into its
elements is sometimes called decomposition.
Recall that the components of a mixture can be separated from one another by physical means. This is not true for
a compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine. Salt cannot