Summary

In this lab, students will become more familiar with the scientific method as they investigate what type of cleaning product is best. Students will identify variables as they carry out a controlled test to compare the ability of different products to remove a ketchup stain.

Grade Level

Elementary and Middle School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• 3-5-ETS1-2: Generate and compare multiple possible solutions to a problem based on how well each is likely to meet the criteria and constraints of the problem.
• 3-5-ETS1-3: Plan and carry out fair tests to which the variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.
• MS-ETS1-3: Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations

Objectives

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

• Identify variables in a scientific investigation.
• Conduct an experiment, make and interpret observations, and draw conclusions based on their observations.
• Understand the purpose of the scientific method.

Chemistry Topics

This lab supports students’ understanding of

• Scientific Method
• Experimental Design
• Physical Properties
• Observations

Time

Teacher Preparation: 30 minutes
Lesson: 45-60 minutes

Materials for class:

• Cotton t-shirt/material squares/samples
• Metric ruler
• Scissors
• Ketchup
• Toothbrushes
• Liquid laundry detergent
• Liquid hand soap
• Dishwashing liquid
• Water
• Small cups/containers
• Sink/Plastic containers for rinsing
• Measuring spoons

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.
• Gloves should be available for student use in this experiment.

Teacher Notes

• Prior to starting the lab, it might be helpful to show the short video, How Do Soap and Water make us Clean? From SciShow Kids, at the beginning of the class. This will help students think about the purpose of soap and water, and think about what is actually happening at the molecular level when something is cleaned.
• Explain to the students that you will be trying to answer the question “What is the best substance for removing a stain?” As a class, discuss how to design an experiment to see which cleaning product is best at removing stains.
  • Discuss the terms: independent variable, dependent variable, controlled variable and constants.
  • Direct students to read and complete the “Pre-lab” section of the student handout.
• Prior to the lab, discuss the importance of “constants” with students. For example, they will need to apply each cleaning solution to the stain in a similar way so that the test is as fair as possible.

Teacher Preparation Tips:

• You can ask students to bring in old used (clean) toothbrushes for the experiment. You only need one for each group. Alternatively, you can buy toothbrushes for very cheap at discount/dollar stores.
• You can use old sheets, t-shirts, materials samples, or rags for the stained samples. Teachers should cut the materials to be the same size prior to the start of the lab. I suggest 8 cm x 32 cm rectangle strips of white cotton. Each group needs four samples.
• Teachers should fill small (labeled) cups or containers with small amount of each cleaning product and a small amount of water (mix together) for students groups to use.
• Depending on your students, and time available you can pre-make the stained samples in advance of the lab, or students can create the stains themselves. This would be a good opportunity to discuss how to design a controlled experiment.
  • Stains should be the same in size and amount of ketchup used so that the test is as fair as possible.
  • As a class, students could discuss how to apply the stains in the same way so that the ketchup stain is the same for each sample.
• Extension opportunity: Repeat the same tests using different stains, such as mustard or lipstick. Students can investigate if certain products are more successful on a particular type of stain.
• An answer key document is provided for teacher reference.