Classification of Matter

Chemistry is the study of the composition, structure, and changes in matter.  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 identity.

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 mixture.

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 mixtures.

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 an explosion.

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 the compound. 

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:

hydrogen     +      fluorine     ----->      hydrogen fluoride

     1 g                     19 g                             20 g

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.

Sample Questions

1. A compound is __________ a substance.

a) always b) sometimes c) never
2. A substance is __________ an element.
a) always b) sometimes c) never
3. A mixture is __________ a substance.
a) always b) sometimes c) never
4. A __________ can be separated into 2 or more simpler kinds of matter by physical processes.
a) compound b) element c) mixture d) substance

5. 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?
a) compound b) element c) mixture d) substance

6. How would you classify a kind of matter that can not be separated into simpler kinds of matter by either physical or chemical processes?

a) compound b) element c) mixture d) substance



ANSWERS:

1 a      2 b      3 c      4 c      5 a      6 b