The difference between mixtures and compounds is whether there is
more than one kind of molecule or not. A sample of something that is
made up of one kind of molecule is called a "pure substance", and all
other collections of different kinds of molecules are called mixtures.
Mixtures may be held together by weak bonding forces between the
molecules, such as surface tension, electrostatic forces, or just plain
gravity. Some mixtures are better mixed than others.
An example of a mixture from which you can draw samples of varying
properties is ice cream with chocolate chips embedded in it. We call
this a "suspension". If you use a small scoop, you may or may not get a
chocolate chip, and the ice cream you get will taste different
depending on what came up. On a smaller scale, picking tiny things out
of the ice cream may give you an ice crystal or a blob of congealed
cream. On smaller scales yet, you might get a long oil molecule, a
sugar molecule, or a water molecule, each of which has very different
In a compound, the atoms are bonded to each other to form
molecules, and all molecules are the same as each other. For example,
water is a compound where the simplest component is made of two
hydrogens bonded to one oxygen. In any sample of water (assuming it is
pure water), I will always have exactly the same ratio of hydrogen to
oxygen (2:1). If the proportion is different, then it isn't really a
In compounds, the ratio of the components never changes (water is
always 2 hydrogens to 1 oxygen), and the simplest component is always
two hydrogens bonded to one oxygen (not 4 hydrogens bonded to 2 oxygens
in one big molecule) so all samples will have the same properties.
Here's a special case (it's really not all that special) which
breaks the rules you assume in your question: Suppose your mixture is
icewater. It's made entirely of water molecules, but the properties of
a sample of it will depend on how much ice you pick up. It is therefore
possible to make a mixture out of just one compound.
math dan and Tom
(republished on 07/13/06)