Classification of Matter
Chemistry is the study of the composition, structure, and changes in
Matter is anything that has mass and occupies space. From these
definitions, we see that the scope of chemistry is going to be quite
broad. Just look at the world around you, and you will see a
virtually endless variety of kinds of matter. How will we ever
classify them all?
One simple idea that developed early in the history of chemistry is
that there are only limited "fundamental" kinds of matter that are put
together in various ways to make all the variety of matter we observe in
our world. An analogy we can draw here is that of letters and words. We
have only 26 letters in our alphabet, but just think how many words can
be "built" using these letters. Further these words can be arranged
into sentences to communicate complex concepts.
We will first classify matter on a physical basis, because the physical
properties and physical changes can be readily observed. Physically
speaking, every kind of matter we can consider is either a substance
or a mixture of substances. A substance is a kind of matter that
can not be separated into simpler kinds of matter by physical processes.
For example, pure water (distilled water) is a substance. There is
nothing you can physically do with water to separate it into 2 or more
other substances. Notice I said physically here.
Certainly, we know that water is composed of hydrogen and oxygen.
Almost everyone, even those with no formal exposure to chemistry, have
seen the formula H2O. We can separate water into
hydrogen and oxygen, but this involves a chemical change, not
just a physical one.
Physically, I can put water in a jar and shake it, I can boil it, I can
freeze it, and so on. None of these processes changes the chemical
identity of the water. Liquid water is a collection of water
molecules that are close together and have some mobility. If we
freeze this water, we cause the water molecules to become locked into
relatively fixed positions, and obtain the material we call ice. But
ice is still composed of water molecules, just like liquid water was.
We have changed the physical state of the substance, but not its chemical
Suppose we look at salt water, however. Salt water is a mixutre.
A mixture is a kind of matter than can be separated into two or
more simpler kinds of matter by physical processes.
Here, we can recognize two
distinct substances that are mixed together to make this kind of matter we
call "salt water". It is possible to separate the salt water using
only physical processes -- that is, whihout changing the chemical
identity of anything. Suppose we boil the water into steam. This
does not change the chemical identity of the water, it just changes the
physical state. When all the water has been boiled away, the salt
will remain in the container, thus it will have been separated from the
water. At no time during this process was the chemical identity of
the salt changed. It only changed in form -- from being in solution
to being solid. As for the water we boiled away, if we want to
recover the water, we can carry out the process in an apparatus that will
allow us to condense the steam back to water, and collect the now pure
water (steam condensate).
Of all the matter around us then, everything we could choose to study is
either a substance or a mixture. The mixtures we study can be
separated into all the individual substances that make them up.
But what about these individual substances? Can they be broken down
any further? Sometimes yes and sometimes no. As we have seen,
substances are as simple as they can be in a physical sense.
However, they might not be as simple as they can be in a chemical sense.
A kind of matter that is chemically as simple as it can be is an
element. An element is a kind of matter that can not
be separated into simpler kinds of matter by chemical changes.
All elements are substances, because if a kind of matter can not be broken
down by chemical changes, it certainly can not be broken down by physical
changes. If a substance CAN be broken down by chemical changes,
it is a compound. A compound is a kind of matter
that consists of two or more elements in chemical combination.
We can also say that a compound is a kind of matter that can be
separated into two or more simpler kinds of matter by chemical changes.
Both elements and compounds are substances. For those of you who
have had set theory in mathematics, we can say that the set of all elements
is a subset of the set of all substances. Likewise, the set of all
compounds is a subset of the set of all substances. The set of all
substances is the union of the set of all elements and the set of all
compounds -- that is, these two sets taken together comprise the
larger set. The set of all mixtures is totally separate from any of
the sets discussed previously. If something is classified as a
mixure, it can not be a substance, which means it can be neither an
element nor a compound. If something is classified as a substance,
it could be either an element or compound. If something is
classified as a an element, it must definitely be a substance and is
definitely NOT a mixutre. The same statement can be made of
something classified as a compound. You will see at the end of these
notes, some sample questions that test your understanding of the relationships
between elements, compounds, substances, and mixutres.
Compounds and mixtures have something in common: they are both built up
from simpler kinds of matter. Individual substances become
physically combined (mixed) to form mixtures, and elements
chemically combine to form compounds. But there are some
important distinctions that can be made between these two types of matter.
Characteristics of Mixtures
When individual substances are put together to make a mixture, the original
properties of the component substances are retained. Suppose we mix
sugar and salt together, for example. We will get a solid material
that would have both a sweet and a salty taste. That is, the
characteristics of the individual substances are still recognizable in the
Another important characteristic of a mixture is that it can have a
variable composition. Let's return to our example of the sugar and
salt mixture in the previous paragraph. We could sprinkle a little
salt into a large bowl of sugar and get a mixture that is predominately
sugar, but contains traces of salt. Going to the other extreme, we
could add just a few tiny crystals of sugar to the salt in a salt
shaker and get a mixture that is predominately salt, but which contains
traces of sugar. Or, we could make a mixture that falls anywhere
between these two extremes by adding significant amounts of both
ingredients. When making mixtures, the chemist has considerable
controll (often complete control) over the relative proportions of the
substances that make up the mixture.
Mixtures can be either hetergeneous or homogeneous. The solid mixutre
of sugar and salt that has been the subject of the last two paragraphs is
an example of a hetergenous mixture. A hetergenous mixture is
a mixture that does not have a uniform composition.
Each tiny crystal within our solid mixture is either a crystal of sugar or a
crystal of salt. If we sample our mixture grain by grain, sometimes
we get sugar and sometimes we get salt. If we draw 2 separate tea
spoons of solid from the mixture, we may find the contents of one
teaspoon may be saltier than the other.
Now suppose you have a glass of salt water. This is an example of
homogenous mixture. All parts of the water are equally salty, whether you
draw your solution from the top, middle, or bottom of the glass.
All samples taken from this mixture will have the same relative
proportions of salt and water. A homogeneous mixture
is a mixture that has a uniform composition.
Characteristics of Compounds
The characteristics of a compound are opposite to those cited above for
When elements react to form a chemical compound, the properties of the
elements are not retained. The compound will typically have
properties that are very different from the elements that make it up.
As an example, consider salt. Chemically, salt is known as sodium
chloride (NaCl). Let's consider the properties of sodium and
chlorine and compare them to those of sodium chloride.
Sodium is a
chemically reactive metal. Because it reacts readily with oxygen and
with water, elemental sodium can not be kept out in the open. It must
be stored in a protective environment, such as under a liquid with which it
does not react. You can not handle sodium with your bare hands, because
it will react with the moisure in your skin and cause a skin burn.
Sodium reacts so vigorously with water that it will ignite if dropped in
water. If the piece of sodium used is large enough, it may generate
enough hydrogen gas (formed in the reaction of sodium with water) to cause
Chlorine is a pale greenish-yellow gas that is toxic. We take
advantage of this toxicity by using chlorine in low concentrations to
disinfict the water in swimming pools. In high concentrations, the
gas is toxic to humans.
Table salt, or sodium chloride, is nothing like sodium or chlorine, as you
can see from the above characteristics. Salt does not react with
oxygen or water, will not ignite when put in water, is not a greenish-yellow
gas, and is not toxic. When elements combine to form a compound, the
properties of the elements are lost and replaced with new properties of
Another way compounds differ from mixtures is in their composition. We
saw earlier that the composition of a mixture can be varied at will, by
the person making the mixture. We don't have this freedom with a
compound, however. In any given compound, we find the elements are
always present in the same relative amounts. This is called the Law of
Definite Proportions. It says that all samples of a pure
compound will have the same elemental composition, regardless of where or
how the sample was obtained.
For example, the compound known as hydrogen fluoride is 5% hydrogen and 95%
fluorine, by mass. If you have 1 g of hydrogen, it will require 19 g
of fluorine to react with all of it. By the law of conservation of
mass, you will get 20 g of hydrogen fluoride. We can summarize this
in the form of a chemical equation as follows:
The 20 g of compound contains 1 g of hydrogen, or 5%.
Now suppose you tried to get "hydrogen enriched hydrogen fluoride" by
using 5 g of hydrogen, instead of only 1g. If you stll used 19 g of
fluorine, you would find that only 1 g of hydrogen was used, and the other
4 grams would be left over unreacted. There is no way to make hydrogen
fluoride have anything other than 5% hydrogen and 95% fluorine.
||A compound is __________ a substance.|
||A substance is __________ an element.
||A mixture is __________ a substance.
||A __________ can be separated into
2 or more simpler kinds of matter by physical processes.
||How would you classify a kind of
matter that can not be separated into
simpler kinds of matter by physical processes but CAN be
separated by chemical processes?
||How would you classify a kind of
matter that can not be separated into simpler kinds of matter by either
physical or chemical processes?||