A substance or material can be an ELEMENT, a COMPOUND, or a MIXTURE.
ELEMENTS and COMPOUNDS are both pure substances. The difference between them is that ELEMENTS consist of only one type of atom (such as oxygen atoms or carbon atoms) whereas COMPOUNDS consist of more than one type of atom combined together in a specific way.
For example, the formula O3 represents the substance ozone. The 3 means that this subtance exists as molecules, groupings of 3 oxygen atoms joined together. Even though there is more than one atom, all of the atoms are the same, they are all oxygen. Therefore, ozone is an ELEMENT.
Oxygen gas, is represented by the formula O2. This means that oxygen gas consists of molecules each having 2 oxygen atoms joined together. Again, since all of the atoms are the same type of atom, oxygen gas is an ELEMENT.
An example of a COMPOUND is carbon dioxide, CO2. This formula represents the molecule consisting of one carbon atom and two oxygen atoms all joined together. Since there are two different types of atoms, carbon dioxide is a COMPOUND.
Another COMPOUND containing carbon and oxygen is carbon monoxide, CO. These molecules each have one carbon atom and one oxygen atom joined together.
Yet another COMPOUND is sucrose, table sugar, C12H22O11. These molecules are much larger, with each one containing 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms.
A different type of COMPOUND is sodium chloride, table salt. Its formula is NaCl. But, there are no molecules here. Instead, when a COMPOUND is formed from the combination of metal atoms and nonmetal atoms, the formula represents only the ratio of the atoms to each other, not the actual number of each atom in a molecule. There is no particular size for a grouping of sodium and chlorine atoms in table salt, whereas the only grouping of carbon and oxygen atoms in carbon dioxide is that 3-atom molecule (1 carbon, 2 oxygens). Sodium chloride is just one pure substance, though, and more than one type of atom make up sodium chloride, and that is why it is a COMPOUND.
Another metal-nonmetal COMPOUND is the active ingredient in chlorine bleach - sodium hypochlorite, NaOCl sometimes written as NaClO.
A MIXTURE contains more than one pure substance combined together.
What makes a MIXTURE different from a COMPOUND, which after all is a combination of different types of atoms (different ELEMENTS) ?
| (1) | When two ELEMENTS combine to form a COMPOUND, they undergo a complete transformation in identity and properties. In other words, a chemical change occurs. For example, sodium is a metal and chlorine is a poisonous yellow-green gas. When they combine, they form sodium chloride, a white crystalline solid. When two substances (ELEMENTS or COMPOUNDS) combine to form a
MIXTURE, they retain their identities. So salt water is a combination of salt and
water, but the water still boils at 100 C and the salt does not. You can separate a
MIXTURE into its components using only physical processes. You can boil off water,
leaving the salt behind. |
| (2) | In a COMPOUND, there is always a specific ratio of the different
ELEMENTS. Water is H2O, with a 2:1 ratio of hydrogen:oxygen,
always. And sodium chloride always has a 1:1 ratio of sodium to chlorine. A MIXTURE of two substances does not contain any specific amounts of the two substances. Salt water contains salt and water, but it does not really matter how much salt and how much water. |
A MIXTURE can be HOMOGENEOUS or HETEROGENEOUS.
HOMOGENEOUS means that the properties and composition are the same, uniform throughout the material. ELEMENTS and COMPOUNDS are pure substance, so they must always be HOMOGENEOUS. Salt water is an example of a MIXTURE which is HOMOGENEOUS. You cannot visually see the different substances in there, and there is no difference between one portion of the salt water and another portion of the same salt water.
HETEROGENOUS means that the properties and composition are not the same throughout. ELEMENTS and COMPOUNDS cannot be HETERGENEOUS. A combination of sand and water would be HETEROGENEOUS. You can distinguish the sand from the water, and one scoop of the mixture might be mostly water, while another scoop might be mostly sand. The composition and properties vary (change) throughout the MIXTURE.
It can be very difficult to distinguish a COMPOUND from a HOMOGENEOUS MIXTURE. Usually you need some background knowledge, which you can obtain from a variety of sources, including your textbook, the CRC Handbook, the dictionary, ...
For example, which type of matter is brass? It is not an ELEMENT, because it is not on the Periodic Table. It is not a HETEROGENEOUS MIXTURE because it appears uniform throughout. To find out whether it is a COMPOUND or HOMOGENEOUS MIXTURE, you might need to do some research. According to Webster's New Collegiate Dictionary (1975), brass is "an alloy consisting essentially of copper and zinc in variable proportions". Even if you do not know what an alloy is, you can tell that brass is a MIXTURE because of the phrase "in variable proportions". By the way, an alloy is essentially a homogeneous metal mixture (produced when the metals are in the molten state).
What about orange juice? If it has pulp in it, then it must be a HETEROGENEOUS MIXTURE. But, suppose there is no pulp, and it appears HOMOGENEOUS. Is it an ELEMENT? No, because orange juice does not appear on the Periodic Table. Is it a COMPOUND? Well, what is in orange juice, what is it made of? You drink orange juice for the vitamin C, and vitamin C is a molecule (a COMPOUND) with the formula C6H8O6 (look it up in the index of your textbook, or an ordinary dictionary). Is there anything else in orange juice? Of course. For sure there is water, and some sugar, plus probably lots of other things. That means nonpulp orange juice is a HOMOGENEOUS MIXTURE.
last updated 14 December 2008
