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LAB in Observations, Mixtures, Inferences, Intermolecular Forces, Scientific Method. Last updated January 04, 2019.

Summary

In this lab, students will investigate the addition of detergent to a mixture of whole milk and food coloring. Students will attempt to explain the cause of their observations. Also, students will have the opportunity to manipulate the experiment and determine how other variables may impact the results.

Grade Level

High School

Objectives

By the end of this lab, students should be able to

  • Determine how soap/detergent impacts a solution containing fat.
  • Manipulate an experiment in order to test a specific variable, while holding all other factors constant.
  • Explain the meaning and purpose of an emulsifying agent.

Chemistry Topics

This lab supports students’ understanding of

  • Scientific Method
  • Inferences
  • Observations
  • Solutions
  • Mixtures
  • Intermolecular Forces

Time

Teacher Preparation: 10 minutes

Lesson: 30-40 minutes

Materials

  • Petri dish
  • Liquid Food colorings (4 different colors)
  • Toothpicks
  • Dishwashing detergent (example: Dawn, Joy, Ivory)
  • Milk (whole milk is best)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Do not consume lab solutions, even if they’re otherwise edible products.

Teacher Notes

  • I use this lab experiment as a take home lab assignment. Find out more about my take home labs in the March issue of Chemistry Solutions or in the AACT Webinar archive.
  • The materials are commonly found at home, so have students to check for availability. If this is difficult, allowing students to complete the lab in the classroom before or after school is a good option.
  • Teachers should provide the petri dish for students to use.
  • This lab can be used as a reminder to students about creating valid experiments, and the implementation of the scientific method. It also presents an opportunity to discuss the importance of scientific reasoning and making inferences.
  • Teachers might find this lab applicable to a unit on Intermolecular Forces and/or Solutions.

For the Student

Background

Is what you see really what you get? Observations and inferences are important to the science of chemistry. Careful and accurate observations are used to make inferences. Inferences are explanations or interpretations of observations. In this activity you will make a variety of observations and then attempt to explain what you see.

Materials

  • Petri dish
  • Liquid Food colorings (4 different colors)
  • Toothpicks
  • Dishwashing detergent (example: Dawn, Joy, Ivory)
  • Milk (whole milk is best)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Do not consume lab solutions, even if they’re otherwise edible products.

Procedure

  1. Add whole milk to the Petri dish until it’s almost to the top. The milk should be about room temperature to start with.
  2. In a clockwise fashion, put one drop of each food coloring sample in your Petri dish at 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock.
  3. Dip the end of a toothpick in liquid dishwashing detergent.
  4. Touch the tip of the toothpick (with the detergent on it) to the center of the milk in the petri dish to introduce the detergent to the system. Record your observations.
  5. Repeat this experiment by manipulating one of the following variables 2 times (so you have 3 tests in all):
    a. Fat content of the milk (increase / decrease it)
    b. Temperature of the milk (increase / decrease it)

    *Be sure to clean your Petri dish between tests to remove all traces of detergent.

Observations

Variable

Observations

Room temperature whole milk (follow experiment as given)


Manipulated Variable 1 (explain):


Manipulated Variable 2 (explain):


List 3 factors that you help constant in the experiment:

Conclusions

  1. Based on your observations what affect do you believe fat content or temperature has on this system?
  2. Oil, the fat in milk, and grease belong to a class of chemicals called lipids. The advertisement says that Dawn takes grease out of your way. Discuss the validity of that statement using your observations to reinforce your argument.
  3. Was there one color that appeared to mix more quickly than others? If you can, sort the rates at which the colors mixed, listing the fastest mix first. Propose a reason why the colors mix at different rates.
  4. The dishwashing liquid contains ingredients that help to emulsify fats. Look up the term emulsify in the dictionary. What evidence did you see of emulsification taking place in the samples you tested?
  5. Have your parent sign your work: