Summary

In this demo students will learn about how to identify and control variables to conduct a fair test by observing apple slices. You will do two separate demo experiments emphasizing the experimental design of each.

The first experiment is to determine if something in the air causes the apple to turn brown. The second experiment is to see if a liquid added to an apple slice can prevent it from turning brown even if it is exposed to air.

Grade Level

Elementary School

Objectives

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

• Understand the meaning of variables and a control in a scientific experiment.
• Understand the basics of designing a fair test. Record observations.

Chemistry Topics

This lab supports students’ understanding of

• Designing a fair test
• Observations

Time

Teacher Preparation: 15 minutes

Lesson: 30 minutes

Materials (per group)

• Plastic Wrap
• 1 apple
• Water
• Salt
• Sugar
• Lemon juice (1 lemon squeezed into a cup)

Safety

• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.
• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.

Teacher Notes

• To prepare the solutions:
  • Mix half a cup of water with one tablespoon of salt, place solution in a cup and label it.
  • Cut lemon and squeeze juice into a cup and label it.
  • Fill one cup will have half a cup of plain water.
• The average time it takes for an apple to oxidize with no solution is 5-10 minutes. Apple wedges with solutions can vary and can take up to 20 minutes.
• When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue, this produces a chemical reaction that creates the brown color commonly seen. This change in color is the indication that oxidation is occurring.
• I recommend that teachers show this video after the lab is completed to explain the science behind the chemical changes the students will be observing.

Procedure

Demo Experiment 1

1. Use a knife to carefully cut part of the apple into 4 slices.
2. Tell students that you want to find out if something in the air causes apple slices to turn brown.
3. Use plastic wrap to wrap 2 slices so that air cannot get to them. Leave the other two slices exposed to air.
4. Ask students about the experimental set-up: Why do you think I wrapped some slices but not others?
   
   Explain that we want to see if air affects the apple slices, so we have one sample exposed to air and another sample not exposed to air. Everything else about the slices is the same. They are from the same apple, they were cut at the same time, they are at the same temperature, and exposed to the same amount of light.
   
   Explain that all these different things that could affect the apple are called variables. For an experiment to be fair, all variables need to be kept the same except for the variable you are testing. We are testing whether the air makes the apple slice turn brown, so the only thing different about the slices is their exposure to air.

The apple slice that was left unwrapped is called the control. In a valid experiment, there is always a control to compare to the other samples. If anything is different between the control and the other samples, it must have something to do with how they were treated differently.

Demo Experiment 2

While the first experiment is running, discuss the second experiment with students as you set it up.

1. Explain to students that if there is something in the air that causes apple slices to turn brown, an experiment can be done to see if a liquid can be added to the apple slice like water, salt water, sugar water, or lemon juice that will prevent the color change.
2. Ask students how to set up the experiment. Students should suggest adding the different liquids to the apple slices. Remind students that there should also be a sample that gets no liquid added to it at all. This is the control.
3. Use the knife to carefully cut 10 slices of apple. Place 5 paper towels on a table or desk. Label them Control, Water, Saltwater, Sugar water, and Lemon Juice. Place two apple slices on 5 separate paper towels. Do not add anything to the slices on one paper towel. Add about ½ teaspoon of each liquid to the slices on its labeled paper towel.

By the time the second experiment has been set up, have students look back to make observations of the first experiment.

If there are no observable changes, have students make observations every half hour or hour throughout the day.

Expected Results:

Experiment 1:

The apple exposed to air should be browner than the apple enclosed in plastic wrap.

Experiment 2:

The apple with lemon juice should be less brown than the apple slices with the other liquids.

Ask students:

• What do your observations from the first experiment tell you?
  Something from the air is making the apple slices turn brown. Tell students that it is the oxygen in the air that makes the apple slices turn brown. The oxygen has a chemical reaction with substances in the apple causing the color to change. If the apple is wrapped well enough and little air can get to it, the reaction can’t happen, and the apple should not turn brown.
  
• What do your observations from the second experiment tell you?
  There is something about lemon juice that prevents the apple from turning brown. Lemon juice is an acid, so there may be something about an acid that prevents or slows down the chemical reaction that turns the apple brown.

Students could do a follow-up experiment using another acid like vinegar to see if it has similar results.