Substances

What is a "substance," in chemical language?

• matter of particular or definite chemical constitution

To a chemist, a substance can be either a pure element (e.g. gold, silver, hydrogen, mercury)  or a pure compound (e.g. water, ammonia gas, hydrogen chloride, sodium chloride).  The common meaning of substance is much broader.  In the Chemical Tests unit, the chemical meaning is always to be taken.

Element

An element is composed of just one kind of atom.   There are approximately 106 elements.  Each element corresponds to one kind of atom.  What makes an atom one kind and not another?  The number of protons in the atom's nucleus determines what element it is.  (This number must be matched by the number of electrons surrounding the nucleus of the atom, since protons have a + charge and electrons have a - charge.)

Compound

A substance composed of more than one kind of atom, bonded together in a fixed ratio.  A given substance consists of either molecules or ions.

A molecule is formed from atoms bonded together, where the electrons of the atoms circle all of the atoms.

An ion is an atom or group of atoms from which either some electrons have been removed (leaving a + charge) or some electrons have been added (creating a - charge.)

Pure substances versus mixtures

A mixture consists of more than one pure element or compound mixed together.  But in a mixture they are mixed in such a way that they do not chemically combine; that is, they do not form a new compound.  Mixing is an example of a physical change.

In contrast to a compound, a mixture does not have a fixed ratio of the substances it is composed of.  For example, water (a pure compound) always has a ratio of 2 grams of hydrogen to 16 grams of oxygen.  But we can make mixtures of hydrogen and oxygen without allowing them to react chemically, and these mixtures can have any composition we want.*

For another example, both water and sodium chloride (table salt) are compounds.  Each has a fixed ratio of elements.**  But salt and water can be mixed in any ratio we want.  Thus salt water is a good example of a mixture.   This is true of every solution.  Although a solution may look clear and homogeneous, it is actually a mixture. 

Another example of a mixture is air.  Although the air we encounter has a pretty fixed ratio of substances such as nitrogen, oxygen and a few others, these substances are not chemically combined, and in fact the ratios of these substances can and does vary.  The best-known examples are the water vapor and carbon dioxide in the air, which can vary considerably.  Furthermore, our atmosphere did not always have the composition it does now.

As noted above, when either a compound or an element is unmixed in this way, it is said to be pure.

*We'd have to be very careful mixing hydrogen and oxygen.  The slightest spark will cause them to violently react, combining in a 1/8 weight ratio, forming water and releasing a tremendous amount of energy.