Students can be assessed on this lesson in the following ways:

• After explaining the difference between elements and compounds, have students classify a list of pure substances. This will provide an opportunity to reteach the differences and similarities between elements and compounds.

• Both students and teacher can monitor understanding of the lesson with the Mixtures vs. Pure Substances lab activity. Students will be able to revisit an earlier activity and apply their acquired knowledge. This activity allows for revision and reflection.

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 the board:

• Mixture: Two or more different substances that are not chemically combined and can be physically separated.

• Pure substance: A substance that cannot be physically separated.

  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 composition.

  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:

• Heterogeneous mixtures (suspensions and colloids):

  • Suspensions do not look uniform throughout.

  • Suspensions have particles that are large enough to settle to the bottom and can therefore be filtered with a funnel and filter paper.

  • Colloids have medium-sized particles.

  • Colloids often look uniform throughout (like solutions).

  • Unlike solutions, colloids’ particles scatter light. This property is called the Tyndall effect.

• Homogenous mixtures (solutions):

  • Solutions look uniform throughout.

  • Solutions have very small particles.

  • Solutions cannot be separated through basic filtration (funnel and filter paper).

  • Solutions’ particles are so small they do not scatter light.

Demonstration

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.



http://www.silvermedicine.org/dark_tyndal_with_h2o2.jpg

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 atoms.

• 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:

• Calcium (element)

• Calcium oxide (compound)

• H₂O (compound)

• Na (element)

• NaCl (compound)

• Lithium nitride (compound)

• Sodium (element)

For students who might need additional practice, reinforce that compounds must have two or more different elements.

Lab Activity

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.

Extension:

• 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.