BE PREPARED

Materials

Class materials:

• 80 plastic cups, 9 oz
• 240 g baking soda
• 200 g salt
• 320 g sugar
• 1200 ml hydrogen peroxide
• 600 ml vinegar
• 600 ml water
• 3 100 ml beakers
• 1 liter pitcher
• 5 100 ml graduated cylinders
• 3 – 4 potatoes
• 1 lab scoop
• 1 triple beam balance
• 3 weigh dishes
• 1 glass stir rod
• 20 15 ml centrifuge tubes
• 1 roll of masking tape
• 1 marker
• 1 knife

Group materials

• 3 weigh dishes
• 1 lab scoop
• 3 400 ml beakers
• 1 liter pitcher
• 500 ml water
• 1 100 ml graduated cylinder
• 1 glass stir rod
• 1 stopwatch/timer
• 7 pH indicator strips
• 1 pH chart
• 1 tongs
• 1 triple beam balance
• Paper towels

Individual materials

• 1 Scientist Data Record

Note: The use of plastic cups is suggested to decrease post lab clean-up. However, 100 ml beakers may replace the plastic cups if desired.

 

TEACHER PREPARATION

1. For each group, prepare the following materials:

a. Fill a plastic cup with 40 g of salt. This can be approximated by filling a 100 ml beaker up to the 30 ml mark with salt. Label the plastic cup “A.”

b. Fill a plastic cup with 40 g of baking soda. This can be approximated by filling a 100 ml beaker up to the 30 ml mark with baking soda. Label the plastic cup “B.”

c. Fill a plastic cup with 40 g of sugar. This can be approximated by filling a 100 ml beaker up to the 40 ml mark with sugar. Label the plastic cup “C.”

d. Using a graduated cylinder, fill a plastic cup with 120 ml of vinegar. Label the plastic cup “D.”

e. Using a graduated cylinder, fill a plastic cup with 120 ml of hydrogen peroxide. Label the plastic cup “E.”

f. Using a graduated cylinder, fill a plastic cup with 120 ml of water. Label the plastic cup “F.”

g. Fill four 15 ml centrifuges tubes up to the 2 ml mark with baking soda. Label the falcon tube “baking soda.”

h. Slice the potato into small pieces approximately 2 cm x 2 cm. Each group will need seven pieces of potato. Place the pieces of potato in a plastic cup labeled “potato.”

2. Prepare the unknown solution for each group.

a. Obtain a 120 g sample of sugar. This can be approximated by using a graduated cylinder to obtain 120 ml of sugar. Pour the sample into a liter pitcher.

b. Obtain a 600 ml sample of hydrogen peroxide using a graduated cylinder. Pour the sample into the same liter pitcher.

c. Stir the sugar and hydrogen peroxide until all of the sugar is dissolved.

d. Using a graduated cylinder, pour 120 ml of unknown solution into a plastic cup. Label the plastic cup “unknown solution.”

3. Place one triple beam balance on each table.

4. Place materials required by student groups at the distribution point.

5. Divide class into five cooperative groups.

 

 

INSTRUCTION

1. Direct each student group to obtain the following necessary materials from the distribution point: six (6) plastic cups labeled A – F, one (1) plastic cup labeled “unknown solution,” three (3) weigh dishes, one (1) lab scoop, three (3) empty 400 ml beakers, one (1) liter pitcher, one (1) 100 ml graduated cylinder, one (1) glass stir rod, one (1) stopwatch/timer, seven (7) pH indicator strips, one (1) pH chart, one (1) pair of tongs, one (1) plastic cup containing potato, four (4) falcon tubes containing baking soda, and paper towels. Instruct one student from each group to fill the liter pitcher with 500 ml of room temperature water from the liter pitchers at the distribution center. The water should be at room temperature.

PROCEDURE

1. Tell students that their main goal in this Investigation is to use what they know about properties of compounds to determine the identity of an unknown solution. The solution is made of one of the three solids (solute) and one of the three liquids (solvent).

In addition, students will examine three solid compounds labeled A, B, and C, and three liquid compounds labeled D, E, and F. Based on their observations, students will determine which solute and which solvent was used to make the unknown solution.

2. Briefly outline the specific goals of this project to the class. Read aloud the goals from the list on the Student Data Record (below):

“You are going on a job interview to become a lab technician. During the interview, a scientist at the lab gives you a beaker that contains an unknown solution. The scientist gives you plastic cups that contain the possible compounds that may be found in the unknown solution. The unknown solution will contain one solid solute and one liquid solvent. You must perform tests to determine the identity of the solid compound and liquid compound that make up the unknown solution.”

a. Prepare solid compounds for testing and test their saturation points.

b. Obtain and observe the unknown solution. Use a test to compare the unknown solution to the other compounds.

c. Predict which solvent was used to create the unknown solution.

d. Use a potato and baking soda to test your prediction and find out which liquid compound was used as the solvent in the unknown solution.

e. Predict which solid compound was used to make up the unknown solution.

f. Obtain saturation point data for the unknown solution from the instructor. Use it to prove or disprove your prediction.

Emphasize that the performance assessment requires problem solving, and problems can often be solved if you use what you already know about the topic – in this case, properties of compounds. Encourage students to ask themselves two questions to guide them through the performance assessment:

• What do I know about properties of compounds?
• How can I use what I know to solve each part of the problem?

3. With the students, walk through the six steps of the project. Encourage students to draw on their understanding of the properties of compounds from the first five Investigations.

Note: For Problem 1, tell students that they only need the amount of each compound provided at the beginning of the performance assessment to complete the test. No additional compound will be provided.

 

4. In order for students to complete Problem 6, they will need to compare saturation points to the saturation point of the unknown solution. The students should approach the instructor for this information when they reach Problem 5 in their Student Data Record. The saturation point of the unknown solution should be reported to students as “greater than 50 g of solute in 100 ml of solvent.”

5. Allow students to complete their Performance Assessments. You may wish to informally or formally evaluate students as they work on their Performance Assessments. An evaluation rubric for the performance assessment has been provided to aid in the evaluation process.

6. Upon completion of the project, permit enough time to clean up the lab and return the materials and equipment to their assigned location.

CLEAN UP

Let students know your expectations and instruct them to clean up their lab bench after the Performance Assessment.