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Elements, Compounds, and Mixtures
Elements
Over time, scientists discovered that some matter is composed of pure chemicals. Pure substances are
homogeneous and have certain unchanging chemical compositions. For example, a pure sample of highly
condensed carbon, diamond, will always have the same crystalline structure. The repeating structural unit
of diamond consists of eight atoms in cubic shapes. Using this cubic form and its highly symmetrical
arrangement of atoms, diamond crystals form several different shapes. We will discuss this in more detail
in Chapter 15. This cubic form and its light reflectivity make diamond one of the most desired substances
on Earth.
Chemicals that have the same type of matter all through the sample are said to be pure elements . Oxygen,
potassium, mercury, and nickel are pure elements.
During his research in 1789, Antoine Lavoisier defined an element as a substance that could not be
decomposed by a chemical reaction into simpler substances. Lavoisier identified 33 elements that he
thought were pure and indivisible. Of those 33, 20 of the 109 elements currently identified, are still
considered pure elements.
An element is made up of a pure sample with all of the same kinds of atoms and cannot be further
separated into simpler elements.
Mixtures
Mixtures can be separated into two or more substances manually. No chemical reaction is needed. In
nature, salt water can be separated into its components of water and salt by allowing the water to
evaporate. Mixtures are found in two forms: heterogeneous and homogeneous .
A heterogeneous mixture is one with physically separate parts that have different properties. An easy
example is salt and pepper. A heterogeneous mixture has separate phases . A phase represents the number
of different homogeneous materials in a sample. Salt is all one phase and pepper is one phase. They do
not have a wide variety of characteristics, but are physically separate.
A homogeneous solution has one phase (liquid) but may have more than one component within the
sample. Again, salt water is an example of a homogeneous mixture. It is the same throughout, but has two
parts: water and salt. Figure 3.3 compares matter and its different parts.
Fig. 3.3. Matter can be further
broken down into different
divisions.
Compounds
Pure chemicals that can be broken down into simpler chemicals are known as compounds . Commonly,
chemical compounds are made up of two elements in set proportions to each other. Water provides an
easy example of a compound. It is composed of the elements hydrogen and oxygen. There are always two
parts of hydrogen to one part oxygen in every molecule of water. If the water sample is from the sea or
polluted, there may be other chemicals added, but basic water always has the same proportion of
hydrogen to oxygen by mass.
Elements
Compounds
Mixtures