Definitions:
Substance - A material with a constant
composition. This means that the substance is the same no matter where it is found.
NaCl, H2O, Ne, CO2, and O2 are all substances,
because their composition will be the same no matter where you find them. All
elements and all compounds are defined as substances.
Elements - Elements are substances that are
made up of only one type of atom. At this time, there are 113 known elements, most
of which are metals. The symbols shown on the periodic table represent the known
elements. Even atoms are made up of smaller particles, but they are not broken down
by ordinary chemical means.
Compounds - Compounds are substances
that are made up of more than one type of atom. Water, for example, is made up of
hydrogen and oxygen atoms. Carbon dioxide is made up of carbon and oxygen atoms.
Table salt is made up of sodium and chlorine. Compounds differ from
mixtures in that they are chemically combined. Unlike elements, compounds can be
decomposed, or broken down by simple chemical reactions.
Phase - A phase is any region of a
material that has its own set of properties. In a chocolate chip cookie the dough
and the chips have different properties. Therefore they represent separate phases. Pure gold, which is an element, would only contain
one phase. Italian dressing would clearly represent several phases, while a solution
of salt water may only contain one phase.
Homogeneous Materials - Any material that
contains only one phase would be considered homogeneous. Elements like hydrogen,
compounds like sugar, and solutions like salt water, are all considered homogeneous
because they are uniform. Each region of a sample is identical to all other regions
of the same sample.
Mixtures - Mixtures are made up of two or
more substances that are physically combined. The specific composition will vary
from sample to sample. Some mixtures are so well blended that they are considered
homogeneous, being made up of only one phase. Other mixtures, containing more than
one phase, are called heterogeneous.
Solutions - Solutions are a special type of
homogeneous material, because unlike compounds, the parts of a solution are physically,
not chemically, combined. When you mix a glass of salt water, the salt does not
chemically react with the water. The two parts just mix so well that the resultant
solution is said to be uniform. Ice tea, coffee, metal alloys, and the air we
breathe are some examples of solutions.
Solutions are made up of two parts: The solute,
which gets dissolved, and the solvent, which does the dissolving.
In the case of salt water, salt is the solute and water is the solvent.
Heterogeneous mixtures - Heterogeneous
mixtures are made up of more than one phase and they can be separated physically.
The aforementioned chocolate chip cookie, a tossed salad, sand, and a bowl of raisin bran
cereal are all examples of obvious heterogeneous mixtures.
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