• The formative assessment for this lesson occurs when students are answering questions posed in the various lab sections. If students cannot identify the mixture as a suspension, remind them of the last lesson, citing examples of suspensions. Ask them, “Does the mixture in front of you settle out over time? Is it easy to see that it is a combination of more than one thing?”

• While students are developing a proper lab procedure, move around the room, asking questions such as:

• “You have four components. Which one is soluble in water?”

• “Which component would be the easiest to remove based on size?”

• “What technique would you use to remove salt from salt water?”

• “Which metal is magnetic, iron or copper?”

• Students also have follow-up questions at the conclusion of the lab. Monitor their responses and correct misunderstandings, if necessary.

Scaffolding , Explicit Instruction

Conduct a brief review of the past material by asking the class the following questions:

• “How would you be able to differentiate between a suspension and a colloid?”

(Colloids exhibit the Tyndall effect, whereas suspensions do not. Suspensions often settle out, whereas colloids do not.)

• “How would you be able to differentiate between a solution and a colloid?”

(Colloids have medium-sized particles and their particles scatter light. Solutions have very small particles, cannot be separated through basic filtration, and their particles do not scatter light.)

• “List five physical properties…”

  (Answers may vary. Examples include viscosity, density, conductivity, magnetism, boiling/melting/freezing point, thermal expansion/contraction, malleability, ductility, surface tension, volatility, etc.)

Lab Activity

Tell students, “Today, you are going to demonstrate your knowledge of physical properties and mixtures by completing a lab activity. Each team (lab groups of two to four students) will be given the same substance. Your task is to separate the substance into its original components and in their original state. The substance is made from sand, copper shot (or copper pieces), iron filings, and salt.”

Have students proceed to the lab stations in teams of two to four. At each lab station, provide the equipment listed in the Materials section. Give each student the Separation of a Mixture activity sheet (S-8-5-3_Separation of a Mixture Student.doc).

Tell students, “You will be given 15 minutes to examine the mystery substance, look over the equipment provided to you, and fill out Part I of your Separation of a Mixture lab worksheet.” As a class, discuss the answers to Part I. Use the Separation of a Mixture–Teacher activity sheet for answers (S-8-5-3_Separation of a Mixture Teacher.doc). You can have students return to their seats or stay at their lab stations. They should all be aware that the mystery substance is a heterogeneous mixture that is classified as a suspension. When addressing the last question in Part I, say, “Which physical properties would help you separate this mixture? In what order should the separation techniques be done?”

Then direct students’ attention to Part II of the lab handout. In this section, the teams design their own procedure for separating the mixture. The flowchart will give them an idea of the order. After 10 minutes, write and discuss the correct answers on the board.

 

Once students understand the procedure, demonstrate proper lab protocol.

1. Obtain a small evaporating dish of the mixture.

2. Using tweezers, pick the copper shot from the mixture. Transfer it to an empty weigh boat.

3. Use a magnet to attract the iron filings. Sort through the mixture’s layers, as there are filings at the bottom as well. This may irritate your hands, so gloves will be available. Scrape the filings off the magnet and into a separate weight boat.

4. To separate the sand from the salt, you will need to add 10–25 mL of water to the evaporating dish. Use glass stir rods to dissolve all of the salt. Set to the side.

5. Using filter paper, make a funnel shape like the one shown below:

6. Put the funnel-shaped filter paper inside a funnel and place the funnel on an Erlenmeyer flask.

7. Pour the sand, water, and salt mixture through the funnel. Wait until all the water has gone through the filter and into the flask.

8. Remove the sand and filter paper from the flask and lay them on a paper towel to dry.

9. Pour the salt water into an evaporating dish and place it on a hot plate (on medium). Watch closely. The water will boil off, leaving the salt behind. Turn off the hot plate.

10. Remove the evaporating dish with tongs or a hot pad. Be careful as the dish is hot.

11. All glassware can be rinsed with water. Iron, copper, and sand can be recycled for the next class to use.

Extension:

• If you have covered phase changes, sublimation in particular, you can add ammonium chloride (NH₄Cl) to the mixture. After students have removed the copper shot and the iron filings, they can set up a ring stand with a Bunsen burner underneath. They should place the evaporating dish on a clay triangle and heat the mixture until it smokes. The white smoke is the ammonium chloride leaving the mixture. Students should use a glass rod to stir the mixture every once in a while. Once the smoking stops, the sublimation of NH₄Cl is complete. Let the dish cool until it is safe to be handled. The remaining separation techniques are the same.