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9-12 Open-Ended Inquiry • Design Your Own Lab
Additional Lab 2
Enzymes in Detergents
Problem
Are all detergents equally effective at removing protein stains?
Introduction
Every detergent maker claims that its products are the best at removing stains from
clothing. Is there really that much difference among laundry detergents? All
detergents have ingredients that help remove oils and grease, which do not dissolve
easily in water. Detergents have other ingredients that are designed to remove stains
with a high protein content, such as blood and milk. Protein stains are among the
hardest stains to remove. The enzymes in detergents help break down the molecules
in stains.
In this lab, you will use gelatin to compare the effectiveness of different brands of
detergent. Gelatin is the main ingredient in a common fruit-flavored dessert. It is a
protein that is prepared from animal tissue.
Skills Focus
Form a Hypothesis, Design an Experiment, Compare and Contrast
Materials
• knox gelatin
• 8 oz plastic cups
• powder detergents
• balance
• weighing paper
• plastic knives
• plastic forceps
• 10-mL graduated cylinder
• distilled water
• metric ruler
Safety
Wear safety goggles and gloves when you prepare solutions. Check glass containers
for cracks and chips. Alert your teacher if you break a glass object. Do not touch
broken glass. Wash your hands thoroughly with soap and warm water before leaving
the lab.
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Procedure
In Part A, you will observe the effect of a detergent on gelatin. In Part B, you
will design and perform an experiment to compare the effectiveness of three
different powder detergents.
Part A: Observe the Effect of a Detergent on Gelatin
1. Yesterday, your teacher prepared the setup for Part A. He or she started
with three similar containers of gelatin cubes that are 2cm x 2cm x2cm.
Your teacher mixed 0.5 g of a detergent with 4.5 mL of distilled water. He
or she added the solution to one container of gelatin, 5 mL of distilled water
to the second container, and nothing to the third container. Record your
observations of the three containers.
Observations:
Pre-Lab Questions
1. Design an Experiment In Part A, what is the purpose of the gelatin to
which no liquid has been added?
2. Control Variables Why will you use distilled water to prepare your
solutions?
3. Form an Operational Definition How will you decide whether a
detergent is “effective” at removing protein stains?
Part B: Design an Experiment
2. Form a Hypothesis Are all powder detergents equally effective at
removing protein stains? You will be testing three different powder laundry
detergents. Record the hypothesis you will test to answer this question.
Hypothesis:
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3. Control Variables What will your independent variable be? What will your
dependent variable be?
Independent Variable:
Dependent Variable:
4. Describe Your Plan Record the details of your plan in the space below. Assume
you will have three containers of gelatin to work with. Which variables will you
need to control? Before you begin, have your teacher review your plan.
Experimental Plan:
5. Organize Data Construct a data table in the space below. Include a row for each
detergent you plan to test.
6. Disposal When you are done, follow your teacher's instructions for cleanup and
disposal of materials. Then, wash your hands thoroughly with soap and warm
water.
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Analyze and Conclude
1. Draw Conclusions Which detergent would be most effective at dissolving
protein stains, and why?
2. Use Analogies How is what happens when a person eats a gelatin dessert
similar to what happens when a detergent is used to clean clothing with
protein stains?
3. Control Variables Why was it necessary to compare powder laundry
detergents rather than liquid laundry detergents in this experiment? Hint:
What variable were you trying to control when you made the detergent
solutions.
4. Infer An auto mechanic needs to remove some grease stains from clothing.
Would it make sense for the mechanic to base the choice of detergent on the
results of this experiment? Why or why not?
5. Predict Would you expect the effectiveness of a detergent to increase if
you increased the concentration of detergent in a solution but used the same
volume of solution? Explain your answer.
Extend Your Inquiry
Grocery stores must provide both the shelf price of an item—what the consumer
will pay for the item—and the unit price of the item. Knowing the unit price,
which is the “cost per unit,” makes it easier for shoppers to compare the cost of
different brands. The unit may be a mass, a volume, or the number of items in a
package.
Suppose you pay $6.39 for 26 ounces of Brand A detergent and $5.29 for 33
ounces of Brand B. You know that Brand A is twice as effective as Brand B at
removing protein stains. Use unit prices to determine which brand of detergent
is more cost effective.
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