Matter has been defined as any substance that has mass and occupies space.
There are three forms of matter: solids, liquids, and gases. Depending upon the
temperature, some substances may occur in each of these forms. For example, water
is a liquid under normal conditions; when exposed to freezing conditions, it becomes a
solid (ice); and when boiled, it evaporates into a gas (steam).
a. All forms of matter are characterized by certain physical and chemical
properties. Physical properties are characteristics such as density, solubility, color, and
conductivity. Many of these properties are easily detected by one or more of our
senses (sight, smell, touch, taste, and hearing). Chemical properties are characteristics
that are shown during chemical interaction of one substance with another. When a
substance undergoes a physical change, its composition is not altered. For example,
the freezing of water is a physical change. However, when a substance undergoes a
chemical change, its composition is changed, and one or more new substances are
formed. For example, the burning of wood is a chemical change.
b. On the basis of their composition, substances may be classified as elements,
compounds, or mixtures.
(1) Element. An element is a simple substance that cannot be
decomposed by chemical means. The smallest unit of an element that still retains the
characteristic properties of that element is an atom. An element is commonly
designated by a symbol made up of one or two letters of its English or Latin name. For
example, oxygen is designated by 0, hydrogen by H, mercury by Hg, and so forth. In
general, all elements may be grouped into two classes: metals and nonmetals.
Examples of metals are gold, magnesium, iron, lead and copper. Examples of
characteristic nonmetallic properties.
(2) Compound. A compound is a complex substance formed by a chemical
union of two or more elements in definite proportions by weight. The smallest unit of a
compound that still retains the characteristic properties of that compound is called a
molecule. A compound exhibits properties different from the elements of which it is
composed. For example, the elements that comprise the liquid water are the two gases
hydrogen and oxygen (in a ratio of one to eight by weight). A molecule of water is
composed of two atoms of hydrogen and one atom of oxygen. This relationship is often
designated by the chemical formula H2O. Further consideration of the structure of
atoms and molecules will be found later in this lesson.
(3) Mixture. A mixture is made up of substances (in any proportion), each
retaining its own physical and chemical properties. For example, salt and sand may be
combined to form a mixture. These components are unchanged and can be separated
by physical means, such as sorting or dissolving in solution.