Elements Compounds and Mixtures
1. Elements
From the beginning of chemistry, men have searched to find the simplest, most basic
substances which make up our world. Such substances are called elements. In the days of
alchemy there were thought to be only four elements which could be used, in various
proportions, to make up anything. These four elements were earth, air, fire and water. But as
time went on we learned that earth, air and water could all be broken down into simpler
substances; and that fire was not a substance at all, but rather a release of energy.
Today we have identified over a hundred true elements which cannot be broken down.
Such elements are substances which are homogeneous, meaning the same throughout, and are
made up of only one kind of atom. Being homogeneous, they are considered pure substances,
in that all of their particles are identical to one another.
Most of the elements are made up of individual atoms like the carbon
shown at the right. A few of the elements are made of molecules like the oxygen
shown below. But in all cases there is only one kind of atom present in any
element. An atom is the smallest particle of an element that retains the properties
of that element.
carbon
oxygen
2. Compounds
When two or more elements are chemically combined together they lose their original
properties to form a compound with new properties. Such compounds are made up of
molecules, each of which contains two or more kinds of atoms. Since all of the molecules in a
compound are identical to one another, compounds are homogeneous and are therefore
considered to be pure substances. Compounds cannot be separated into their elements by simple
physical means such as filtering or magnetic separation; but they can be
sodium chloride
broken down by chemical reactions. Common table salt, sodium
chloride is an example of a compound.
3. Mixtures
When elements and/or compounds are mixed together with no chemical reactions
occurring, we have created a mixture. Mixtures are heterogeneous, not the same throughout, in
that the individual properties of the elements or compounds mixed together are retained. Since
the particles (atoms and/or molecules) that make up a mixture are not identical to one another,
mixtures are NOT pure substances. Since each component of a mixture retains its individual
properties, mixtures can be separated by simple physical means. Mixtures can be categorized
into three types: a mixture of elements, a mixture of compounds, and a mixture of elements and
compounds. These three types are illustrated below.
mixture of elements
Three different elements
have been mixed together.
mixture of compounds
Two different compounds
have been mixed together.
mixture of elements and compounds
One element and one compound have
been mixed together.
To Summarize: All matter can be divided into two categories: pure substances and
mixtures. Pure substances include elements and compounds. Mixtures include: mixtures of
elements, mixtures of compounds, and mixtures of elements and compounds.
Atoms are the smallest particle of an element that still have the properties of the element.
Atoms combine to form a molecule.
A molecule of two or more kinds of atoms is a compound.
A sample of a compound is formed from identical molecules. Since the molecules are identical,
the elements appear in a fixed proportion.
Combinations of elements and/or compounds are mixtures.
Chemistry Matters
A1
A2
A3
B1
B2
B3
C1
C2
C3
D1
D2
D3
1. List the pure substance(s) in line D?
2. List the mixture in line B?
3. How many molecules are in B1?
4. How many atoms are in C3?
5. How many molecules are in C3?
6. How many molecules are in C1?
7. Which space could represent carbon dioxide, CO2?
8. Which space could represent the major components in air?
9. Classify box D2 as homogeneous or heterogeneous.
10. Classify box B2 as an element, compound, or mixture.