Pure Substances – Elements and Compounds
Atoms are the most basic particle that make up everything in the universe.
If a material is made of only one type of atom, we call it a pure substance. It will
have certain properties by which it can be identified. No matter where a sample
of a pure substance comes from, its properties will always be the same because
it is made just from that one kind of atom. Examples of this type of pure
substance are oxygen, hydrogen, carbon and gold. These pure substances made
of only one type of atom are called elements. They are the most elementary, or
most basic of pure substances. There are 92 naturally occurring elements in the
universe. That’s it – just 92.
Pure substances can be made of more than one kind of atom if, and only if,
the different kinds of atoms are combined in identical sets. These sets of atoms
bonded together are called molecules. For example, the formula H2O represents
one molecule of water. Every molecule of water is identical. Even though it is
made of more than one type of atom, water is a pure substance because all its
molecules are the same. Because of this, any sample of water, no matter where
in the world it comes from, will be exactly the same as long as it is just pure
water with nothing mixed in it. Pure substances made of molecules that have
more than one type of atom (like water) are called compounds.
Kool-Aid is a mixture, not a pure substance. The difference between a
mixture and a pure substance is that a pure substance is made of only one type
of particle (either one type of atom or one type of molecule), and mixtures are
made of different types of particles together in the same place. The particles off
a mixture are not combined chemically in any particular ratio. They do not make
identical sets like molecules; therefore, the different types of particles can be
present in different amounts. If this is true, different samples of a mixture can
have different properties. For example, sugar is a pure substance because all the
molecules of sugar are exactly the same, and water is a pure substance because
all its molecules are the same. But, if you mix sugar and water together, you will
have a mixture containing two different kinds of particles. Before we even
consider color and flavor, we know that Kool-Aid is not a pure substance
because it has two different kinds of particle in it: water molecules and sugar
molecules.
pure substance: matter made of only one type of particle (one type atom or one type of molecule);
or matter made of more than one type of atom, but the type of atoms are chemically combined in
a set ratio
element: a pure substance that cannot be broken down into a simpler substance by physical or
chemical means; made of only one type of atom
atom: the basic particle from which all elements are made
compound: a pure substance made from two or more elements chemically combined in a set ratio
molecule: two or more atoms bonded together (covalently); can be the same or different types of
atoms; the most basic particle in from which compounds are made
mixture: a combination of different substances
ratio: relationship in amount between two different things
1. What is a pure substance? (A pure substance is….)
2. Why is oxygen considered a pure substance? (Oxygen is considered a pure
substance because…)
3. What is an element? (An element is…)
4. How many elements exist naturally in the universe? (There are… naturally
existing elements.)
5. When can a pure substance be made of more than one kind of atom? (A pure
substance can be made of more than one kind of atom if…)
6. What is an example of a pure substance made of more than one element? (An
example of a pure substance made of more than one element is…)
7. What is a compound? (A compound is…)
8. How are compounds different from elements? (Compounds… while elements….)
9. How are compounds and elements alike? (Compounds and elements are both
made…)
10. Compounds and mixtures are both made of more than one thing. Why are
compounds considered pure substances while mixtures are not? Give two reasons.
(Compounds… while mixtures…; compounds… while mixtures…)