Prior to this lesson, pour a small amount of salt water into a heat-proof container (sauce pan or laboratory grade glassware). Set it over a heating apparatus (hot plate, stove…etc) with a small bowl or jar next to it. Secure a piece of plastic or kitchen wrap at the top of the container to catch the water and funnel it down into the bowl or jar. Once the water has evaporated, refer to this demonstration to provide an example of a mixture.
In this lesson, students learn the difference between pure substances and mixtures. Students recall information previously learned on the three states of matter and tie in that content with pure substances and mixtures. At the end of the lesson, students should be able to define pure substances and mixtures, as well as give examples of both.
Begin this lesson by revisiting the previous lesson on physical and chemical changes. Hold up a pencil in front of the students. “Let’s review physical and chemical changes. Take out your science journals. Write a physical change that you can make to this pencil. Next, write a chemical change that you can make to this pencil.” Give students about 5 minutes to brainstorm. Copy the T-chart below on your board and ask student volunteers to give you answers. Use the correct answers to fill in your T-chart. Students should report answers such as sharpening the pencil (physical change) and burning the pencil (chemical change). After students have shared all answers, ask students to copy the completed T-chart in their science journals.
Provide the purpose for the lesson by saying, “Today we will be learning and discussing pure substances and mixtures. We will compare and contrast pure substances and mixtures. By the end of this lesson, you will be able to define these two new terms and give examples of both.”
Ask students to take out their science journals and turn to their Matter Vocabulary page (from previous lessons). Have students write in pure substance and mixture in the vocabulary column. Allow students about 5 minutes to brainstorm what they believe each term means in the “This is what I think this word means” column. (While students are working, prepare the blocks for the next activity.) After students finish, ask them to put away their science journals.
Have 9 or 16 blocks of the same color stacked together in the shape of a square. In another square, use the same number of blocks but of different colors. Ask students, “What are the differences between these two stacks of blocks?” Students should note that one stack has all red blocks, while the other stack has different colors. Next say to students, “I told you that today we would be talking about pure substances and mixtures. Which set of blocks do you think represents a pure substance and which do you think represents a mixture?” Students should respond by saying that the red blocks represent a pure substance and the multi-colored blocks represent a mixture. Explain to the class that each of the blocks represents a particle. Refer back to the previous lesson in which students acted as particles and represented solids, liquids, and gases. Make clear to students that when all the particles in a substance are alike it is a pure substance. When all the particles in a substance are different, it is a mixture. To extend this activity, arrange the blocks in different formations and ask students to tell you if the blocks represent a solid, liquid, or gas as a way to review prior material.
Have six to eight student volunteers stand in front of the classroom. Say to students, “Each student will represent a particle. We have different types of particles here, so this is a mixture. How can we sort these particles to end up with a pure substance?” Start by giving students an example. Point out that there are boys and girls in the group. Separate the girls and boys and explain that now it is a pure substance. (Have students focus on one attribute as a commonality.) Ask the class what other ways the students may be sorted to create pure substances. Students can be separated by different hair color, by those who wear glasses, or colors of clothing or shoes. As students suggest commonalities to make the students into pure substances, have students move into the suggested groups.
Refer back to the salt water evaporation demonstration started at the beginning of the lesson. Be sure the water has evaporated completely, leaving salt crystals behind. Discus the properties of salt water and review that the mixture was able to be separated by applying heat.
Ask students to take out their science journals and turn to the Matter Vocabulary page first used in lesson 1 (S-5-1-1_Matter Vocabulary Worksheet.doc). Give students the correct definition for pure substances and mixtures in the “This is what the word really means” column. Explain to students that mixtures are everywhere they look. Rocks, air, water, trees, and even people are considered to be mixtures. Inform students that mixtures do not involve chemical changes. Sometimes the components can be seen in a mixture, like a pizza. In other mixtures, like salt water, you cannot see all the components. Assist students in using the words in a sentence in the correct column.
Hand out to each student a copy of the Comparing and Contrasting Pure Substances and Mixtures worksheet (S-5-1-3_Substances and Mixtures Worksheet and KEY.doc). Also have a copy of this worksheet projected on your board or your Smart Board. Have students brainstorm similarities and differences between pure substances and mixtures. Allow students to share their ideas and fill in the graphic organizer with the correct answer. Have students glue this worksheet into their science journal.