Show the pictures of common mixtures and
pure substances such as:
Ask students to classify each photo as
either a pure substance or a mixture. Write their answers down under
each photo. Tell them that you will revisit these pictures at the end
of the lesson and see if their answers change.
Tell students, “In the last lesson you
learned that all matter can be described using physical properties.
We will now look at specific categories in which all matter can be
classified. There are two major categories that encompass all matter:
pure substances and mixtures.” Put the following definitions on
Mixture: Two or more different substances that are not
chemically combined and can be physically separated.
Pure substance: A substance that cannot be physically
Say, “Can anyone think of an example of a mixture?” If
students give a pure substance as their answer, explain that it
would not be a mixture because one cannot separate it using physical
means. Tell them there are two types of mixtures:
Homogeneous mixture: A mixture with a uniform composition.
Heterogeneous mixture: A mixture with a nonuniform
Give examples of homogeneous mixtures, including: cola, coffee,
and iced tea. Ask, “What do cola, coffee, and iced tea have in
common? Focus on the way they look, rather than their function or
ingredients.” Guide them to notice that all
three homogenous mixtures look uniform throughout. They cannot see
any particles floating or sinking. Say, “Notice that they all
look alike. Just by looking at them, you may not even know they are
made from more than one substance.” Follow that with, “If
cola, coffee and iced tea are all examples of homogenous mixtures,
what would a heterogeneous mixture look like?” Students may
offer that heterogeneous mixtures will look like they are made of
different substances. Say, “What about Jell-O? It looks
uniform. Would it surprise you to know that Jell-O is not a
homogeneous mixture? It is heterogeneous! We need to dive into more
specific descriptions about mixtures. Heterogeneous and homogenous
mixtures have subgroups. All homogenous mixtures are called
solutions. Heterogeneous mixtures can be either colloids or
suspensions.” The general rules are:
Students may have a difficult time understanding the Tyndall
effect. A demonstration works to alleviate this problem. Fill a
flask with 1000 mL of water. Add 10 to 20 drops of milk. With the
lights off, shine a laser (generic laser pointers work) through the
flask. They will see the laser go through the colloid. This is proof
that colloids scatter light. Do the same procedure with plain water.
They will not see the laser light within the liquid.
Say, “Now that we have talked about mixtures, what about the
other category of matter? Pure substances can also be further
described as either compounds or elements.” Ask students to
give you examples of elements. Refer them to the Periodic table.
Define element on the board.
Element: Pure substance consisting of one type of atom.
Element symbol: An abbreviation for an element’s name,
found on the Periodic table.
Ask, “What do you have if you have more
than one type of element?” Define compound on the board.
Compound: Pure substance consisting of two or more different
Compound formula: Represents the combination of two or more
elements in fixed proportions. Subscripts designate the number of
atoms of each element.
Put the following list on the board and ask students to
categorize each as either an element or a compound:
For students who might need additional practice, reinforce that
compounds must have two or more different elements.
- Use the Mixtures vs. Pure Substances–Teacher sheet (S-8-5-2_Mixtures vs. Pure Substances Teacher.doc) to set up the Mixtures vs. Pure Substances lab and to
correct and assess students’ work when they have finished the lab.
Assign students to teams. Hand out Mixtures vs. Pure
Substances–Student (S-8-5-2_Mixtures vs. Pure Substances Student.doc) to students. In their groups, they
should complete the information table for the examples, giving a
designation and reason for each.
For students performing above and
beyond the standards, have them fill out a flowchart similar to the
one shown at http://www.shschem.info/Classifying%20Matter.htm
to help them organize information throughout the lesson.
Students requiring more practice with
the standards may find it helpful to express the difference between
a homogenous and heterogeneous mixture on a molecular level, as
shown below. Students can use the pictures throughout the lesson as
a reference or if needed, express answers and definitions in picture
form, as shown below.