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Using the combining test to identify the unknown liquids

How can you use the characteristic way each liquid combines with water to identify the unknown liquids?

In this investigation, students have seen that liquids absorb into different surfaces and combine with water in characteristic ways. As a culminating challenge, students will use their recorded observations from Activity 3.3—Using color to see how liquids combine as they test four unknowns and try to identify them. The characteristic way each of the household liquids combines with water will allow students to correctly identify these same liquids, relabeled A, B, C, and D.

Materials needed for each group

Notes about the materials

  • Be sure you and the students wear properly fitting goggles.
  • When using isopropyl alcohol, read and follow all warnings on the label.
  • Only the teacher will need yellow food coloring when preparing the solutions.
  • To prevent spills, tape cups to the desk or table so that the cup and dropper do not fall over.
  • Reuse solutions and droppers from Activity 3.3.

Preparing Materials

  • Make another set of solutions for the class according to the following procedure. These recipes make 1/4 cup of each solution, which is enough for 8 groups to conduct the activity.
    • Water—Use 1/4 cup regular tap water.
    • Salt water—Add 1 tablespoon salt to 1/4 cup tap water.
    • Alcohol—Use 1/4 cup 70% isopropyl alcohol. This is a common household strength.
    • Detergent—Add 1 teaspoon clear, colorless, liquid hand soap or detergent to 1/4 cup tap water. Stir gently.
  • Add 2 drops of yellow food coloring to each solution
  • Label 4 cups and 4 droppers A, B, C, and D. These will be used with the “unknown” liquids.
  • Decide which yellow solutions will be A, B, C, and D, and write down your choice. Be sure to keep the identity of each solution a secret until students have completed this activity. The following is an example:
    • A = Salt water
    • B = Detergent
    • C = Alcohol
    • D = Water
  • Place about 1 teaspoon of each newly made yellow solution into its labeled cup ?(A, B, C, or D) for each group. Make more blue water if necessary.

Testing Sheet

Download the Chapter 3 PDF and make copies of the testing sheet on page 151. (This page contains two testing sheets, and each group will need only one.) Cut along the dotted lines and laminate each testing sheet. If you do not have access to a laminating machine, place each chart in a sandwich-sized zip-closing plastic bag, seal it, and have student groups tape each bag to the desk or table. Students will also need the testing sheet on page 146 of of the Chapter 3 PDF.

Activity sheet

Download the student activity sheet, and distribute one per student when specified in the activity.


An assessment rubric for evaluating student progress during this activity is via download on this page. For this formative assessment, check a box beside each aspect of the activity to indicate the level of student progress. Evaluate overall progress for the activity by circling either “Good”, “Satisfactory”, or “Needs Improvement”.

Question to investigate: How can you use the characteristic way each liquid combines with water to identify the unknown liquids?

  1. Introduce the unknown liquids and discuss strategies to identify each.

    Introduce students to the yellow unknown liquids labeled A, B, C, and D. Explain that each of these liquids is one that they combined with blue water in the previous activity. Ask students what they might do to identify each of these unknown liquids. Students might suggest pairing each yellow liquid with blue water and joining each pair with a toothpick as they did in the previous activity. Then they can compare what they see with the results they recorded for Activity 3.3. Give groups a few minutes to discuss how they might go about identifying each unknown liquid. Then have groups share basic plans. One plan is described in the procedure below. Let all students know that they may test the liquids more than once, if necessary.

    Distribute the student activity sheet so that students can record the identity of each of the unknown liquids as they discover them. Students will also need laminated testing sheets found on pages 146 and 151 of the Chapter 3 PDF.

  2. Have groups test and identify all four unknown liquids.

    The following procedure is not provided for students in the activity sheet. Students should have an idea of how they will test each liquid from their group and class discussions. If students need guidance, explain the testing strategy described in the procedure.

    1. Use drops of yellow liquid labeled A to fill its circle and then fill the opposite circle with blue water. Combine this pair with a toothpick the way you did in the last activity.

      A student uses a toothpick to combine samples of unknown liquids with water
    2. To figure out the identity of unknown A, compare the way these two liquids combine to your drawings and captions from the previous activity. What do you think is the identity of unknown A?
    3. When you think you might know, test that liquid with blue water on your other chart and compare it with the way unknown A combines with blue water. Conduct the test as many times as you need to.
    4. Repeat Steps 1–3 for each of the remaining unknowns. Record the identity of each unknown liquid on your activity sheet and include what made you think that was the identity of the unknown.
  3. Have students compare their results and conclusions.

    Have students discuss their results and say what they think are the identities of the unknown liquids. Ask students what evidence they used to help them decide the identity of each liquid. Reveal the identities of all four of the unknown liquids.