LESSON 2: “Ageless” Apples
ESTIMATED TIME Setup: 5–10 minutes | Procedure: The effects can be observed over a 24-hour time period.
• DESCRIPTION
• MATERIALS
Place apple slices into solutions with different levels of
acidity to change the rate at which the apples turn
brown.
• OBJECTIVE
This lesson introduces acids, bases, and the pH scale
as a measure of acidity. Students apply solutions
with different levels of acidity to apple slices to
determine what level of acidity works best to keep
the apple slices from browning. This lesson can be
extended to introduce proteins, amino acids, and
enzymes.
• CONTENT TOPICS
Scientific inquiry; properties of matter; chemical
reactions; acids and bases; food chemistry; chemistry
in the human body
Lemon juice
Baking soda
o Water (preferably distilled)
o Apples
o Plastic sandwich bags with a snap or zip closure
(or small bowls with lids)
o Permanent markers
o Measuring cups and spoons
o Knife or apple slicer
o
o
Always remember to use the appropriate safety
equipment when conducting your experiment.
Refer to the Safety First section in the Resource Guide
on pages 421–423 for more detailed information about
safety in the classroom.
Jump ahead to page 31 to view the
Experimental Procedure.
NATIONAL SCIENCE EDUCATION STANDARDS SUBJECT MATTER
This lesson applies both Dimension 1: Scientific and Engineering Practices and Dimension 2: Crosscutting Concepts
from “A Framework for K–12 Science Education,” established as a guide for the updated National Science Education
Standards. In addition, this lesson covers the following Disciplinary Core Ideas from that framework:
• PS1.A: Structure and Properties of Matter
• PS1.B: Chemical Reactions
• ETS2.B: Influence of Engineering, Technology, and Science on Society and the Natural World
(see Analysis & Conclusion)
OBSERVATION & RESEARCH
BACKGROUND
To describe certain chemical compounds, chemists may
use the terms “acid” and “base.” In general, a solution
that contains a concentration of hydrogen ions (H+)
greater than the concentration in pure water is called an
acid. Common household acids include lemon juice,
vinegar, soda pop, and orange juice. Likewise, a solution
containing an excess of hydroxide ions (OH-) or an H+
concentration less than that of pure water is called a
base. Common household bases include ammonia,
Fun Fact
baking soda, milk of
magnesia, borax, and
bleach. Solutions
containing an H+
concentration equal to that
of pure water are neutral.
Bases can be used to
neutralize acids and
vice versa.
The concentration of hydrogen ions in acids and bases are
measured on the pH scale. The higher the concentration
of H+, the lower the pH will be. A substance with a pH
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LESSON 2: “Ageless” Apples
CONNECT TO THE YOU BE THE
CHEMIST CHALLENGE
FORMULAS & EQUATIONS
Lemon juice gets its acidity from citric acid.
The chemical formula for citric acid is C6H8O7.
For additional background information, please
review CEF’s Challenge study materials online at
http://www.chemed.org/ybtc/challenge/study.aspx.
• Additional information on acids and bases can
be found in the Acids, Bases, and pH section of
CEF’s Passport to Science Exploration:
Chemistry Connections.
• Additional information on food chemistry and
preservation can be found in the Applications
of Chemistry in Everyday Life section of CEF’s
Passport to Science Exploration: Chemistry
Concepts in Action.
lower than 7 is considered to be acidic. The lower the
concentration of H+, the higher the pH will be. A substance
with a pH higher than 7 is considered to be basic. Most
substances range from 0 to14 on the pH scale, with 0 being
the most acidic, 7 being neutral, and 14 being the most
basic. Pure water is a neutral substance with a pH of 7.
Apples (and apple juice) are acidic. They are also
perishable, which means that they will eventually go
bad or decay. When apple slices are exposed to oxygen,
certain organic substances in the apple, called enzymes,
cause the exposed flesh of the apple to turn brown. The
rate at which this browning process occurs can be slowed
down by making the enzymes less active. The enzymes
become less active at lower pH levels. Therefore,
applying an acid to the surface of the apple slice slows
the activity of the enzymes and keeps the apple from
browning. However, basic solutions, such as a baking
soda solution, will not lower the pH level on the surface
of the apple slices.
Ascorbic acid, commonly known as vitamin C, is also
found in many fruits.
The chemical formula for ascorbic acid is C6H8O6.
The vitamin C and citric acid found in citrus fruits, such as
lemons, oranges, and limes, all help to prevent browning.
Sodium bicarbonate is commonly known as baking soda.
The chemical formula for sodium bicarbonate
is NaHCO3.
The baking soda solution used in this lesson has a higher
concentration of hydroxide ions and a pH level of about 9.0.
Distilled water (pure water) is neutral. It has a pH level of
7.0 and is, therefore, neither acidic nor basic.
The chemical formula for pure water is H2O.
Regular tap water may be slightly acidic. If you
use regular tap water, you may want to discuss
the difference between tap water and distilled water
with your students.
HYPOTHESIS
uAcids and bases will have different
effects on the rate at which apple slices turn
brown because of the differences in their pH levels.
Lemon juice is often used to keep fruit from browning
because lemon juice has a very low pH. Applying lemon
juice to the surface of sliced apples increases the acidity
on the surface to a pH level below 2. This low pH
reduces the activity of the enzymes in the apple and
slows down the browning process.
You Be The Chemist® Activity Guide | page 29
LESSON 2: “Ageless” Apples
DIFFERENTIATION IN THE CLASSROOM
LOWER GRADE LEVELS/BEGINNERS
Conduct the experiment as described on page 31, but
spend more time on acids, bases, and physical and
chemical properties. Discuss other properties of acids—a
sour taste (be sure to remind them that they should never
taste an unknown substance to determine what it is!) and
corrosive. Discuss other properties of bases—slippery to
the touch (again, remind students they should never touch
an unknown substance without proper protection, like
gloves). Then, use pictures and have students write down
or state their answers of whether a certain substance is an
acid or base. For example, show a picture of orange juice
—an acid. Show a picture of laundry detergent—a base.
Likewise, use the same method to go over chemical
reactions in more detail. For example, show a picture of
cake batter and then a baked cake. Ask how they know a
chemical reaction occurred. Then, show a picture of a
rusted nail. Again, ask how they know a chemical reaction
occurred.
HIGHER GRADE LEVELS/ADVANCED STUDENTS
DESCRIPTION
Use solutions of varying pH levels to affect the enzyme
activity in an apple and change the rate at which apple
slices turn brown.
OBJECTIVE
This lesson introduces acids, bases, and the pH scale as
a measure of acidity. Students apply different solutions
to apple slices to determine what pH level works best
to keep the apple slices from browning and learn how
acidity relates to enzyme activity. This lesson also
introduces proteins, amino acids, and catalysts.
OBSERVATION & RESEARCH
Most foods contain proteins. Proteins are complex
organic compounds that are involved in almost all cell
functions. Enzymes are proteins that act as a catalyst by
increasing the rate of chemical reactions. A catalyst is a
substance that helps to change the rate of a reaction.
During a reaction, a catalyst is not consumed or changed.
Enzymes, like all proteins, are made up of amino acids—
the building blocks of proteins. Enzymes are formed by
stringing together between 100 and 1,000 amino acids in
a very specific and unique order. The chain of amino acids
then folds into a unique shape that allows the enzyme to
carry out specific chemical reactions very quickly.
When apples and many other fruits are cut, they may turn
brown quickly. This browning occurs as a result of the
exposure to oxygen in air. When a fruit is cut or bruised,
its cells become damaged, allowing oxygen in the air to
react with a type of chemical compound in the fruit
called phenols. The rate of this oxidation reaction is
increased by the enzyme polyphenol oxidase (PPO) or
tyrosinase, which is also present in the fruit.
To slow down the rate at which a fruit turns brown,
chemical compounds are used to control the activity of
the enzymes. The activity of enzymes can be reduced by
reducing the pH on the surface of the exposed fruit.
The most common treatment is to apply ascorbic acid
(vitamin C) to the fruit. The increase in the acidity
decreases the rate of the chemical reaction between the
phenols in the fruit and oxygen in the air (an oxidation
reaction). Therefore, the apple slice will remain crisp
and white in color for a longer amount of time.
CONNECT TO THE YOU BE THE
CHEMIST CHALLENGE
For additional background information, please
review CEF’s Challenge study materials online at
http://www.chemed.org/ybtc/challenge/study.aspx.
• Additional information on physical and chemical
properties can be found in the Classification of
Matter section of CEF’s Passport to Science
Exploration: The Core of Chemistry.
• Additional information on chemical reactions and
catalysts can be found in the Chemical Reactions
section of CEF’s Passport to Science Exploration:
Chemistry Connections.
• Additional information on proteins and amino
acids can be found in the Organic Chemistry
section of CEF’s Passport to Science Exploration:
Chemistry Concepts in Action.
You Be The Chemist® Activity Guide | page 30
LESSON 2: “Ageless” Apples
EXPERIMENTATION
As the students perform the experiment, challenge them to identify the independent, dependent, and controlled variables, as
well as whether there is a control setup for the experiment. (Hint: If the level of acidity of the solutions changes, will the
results change?) Review the information in the Scientific Inquiry section on pages 14–16 to discuss variables.
EXPERIMENTAL PROCEDURE
1. Use a marker to label three plastic bags.
Label one “lemon juice,” the second “baking
soda,” and the third “water.”
2. Pour ¼ cup of lemon juice into the bag labeled
“lemon juice.”
3. Create a baking soda solution by mixing ¼ cup
of water with one tablespoon of baking soda.
Pour the baking soda solution into the plastic bag
labeled “baking soda.”
4. Pour ¼ cup of water into the plastic bag
labeled “water.”
Although common household acids and bases
are diluted, they can still pose risks. Proper
safety procedures should be followed to protect your
eyes, skin, clothing, and work surfaces.
Never use the sense of taste in the lab. Do not
allow students to eat the apples or taste the
solutions.
DATA COLLECTION
Have students record data in their science notebooks
or on the following activity sheet. For example, which
substances were acids and which were bases? How long
did the apple slices take to brown in each bag? You can
use the table provided in the activity sheet (or a similar
one of your own) for students to record their data.
5. Carefully cut an apple or apples into evenly sliced
pieces.
NOTES
Sharp objects like knives can be
dangerous. You may want to cut the
apples for the students, and then have them come
to the front of the class to get their apple slices once
they have prepared their bags.
6. Place two to three apple slices into each of the three
bags. Seal the bags tightly. Gently shake each bag
to make sure the apple slices are completely coated
by the solution in the bag.
7. Take the apples out of the lemon juice bag, and
place them on a table on top of the bag. Do the
same for the apples in the other bags. Be sure to lay
them on top of the right bag so you know which
apples were coated with which solution.
8. Observe the apple slices immediately following the
treatment. Then, set the apple slices aside for a few
hours (or a day or two) and observe the differences
in color of the apples as the time passes.
You Be The Chemist® Activity Guide | page 31
LESSON 2: “Ageless” Apples
ANALYSIS & CONCLUSION
Use the questions from the activity sheet or your own
questions to discuss the experimental data. Ask students
to determine whether they should accept or reject their
hypotheses. Review the information in the Scientific
Inquiry section on pages 14–16 to discuss valid and
invalid hypotheses.
ASSESSMENT/GOALS
Upon completion of this lesson, students should be able
to …
• Apply a scientific inquiry process and perform an
experiment.
• Define and understand the differences between acids
and bases.
• Identify common household acids and bases.
• Understand pH and identify the pH of different
solutions.
• Describe the chemical reaction (an oxidation reaction)
and how they know a chemical reaction took place
(see Differentiation in the Classroom).
• Understand the role of enzymes (a group of catalysts)
in chemical reactions (see Differentiation in the
Classroom).
MODIFICATIONS/EXTENSIONS
• Instead of water, use plain apple slices as a control.
(Do not put them in any substance. Just cut them and
leave them out.) Then discuss the use of controls in
experiments.
REAL-WORLD APPLICATIONS
• Chemistry is important in keeping food fresh.
Chemicals are added to foods to preserve their
freshness, shelf life, and flavor. Challenge your
students to look for a list of chemical preservatives
on the labels of their favorite foods. Also discuss the
importance of certain preservation methods to their
health. For example, pasteurization kills bacteria in
milk so that it is safer to drink.
• The acidity of foods can also affect your health.
Discuss the discovery of citrus fruits as a means of
preventing scurvy among explorers, pirates, and other
early sailors. Talk about how it is important to consume
sources of vitamin C daily.
COMMUNICATION
Discuss the results as a class and review the activity sheet.
Review the information in the Scientific Inquiry section on
pages 14–16 to discuss
the importance of
communication
to scientific
progress.
Our stomachs contain gastric
Fun Fact
acid, which is mainly hydrochloric
Modifications and extensions provide alternative methods
for performing the lesson or similar lessons. They also
introduce ways to expand on the content topics presented
and think beyond those topics. Use the following
examples, or have a discussion to generate other ideas as
a class.
• Consider using different household acids and bases
to test their effects on the apple slices. You could
bring in various substances, such as lemon juice,
vinegar, cola, a baking soda solution, and milk of
magnesia. Then, allow students to choose two
substances. Once they observe the reactions, have
them guess whether the substance is an acid or base.
acid. It has a pH varying from
1 to 3. The high level of acidity is
one of the causes of
heartburn.
Fun Fact
Antacids are taken to relieve
heartburn. It is comprised of
basic solutions that neutralize
the stomach acids to
provide relief.
You Be The Chemist® Activity Guide | page 32
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
OBSERVE & RESEARCH
1. Write down the materials you observe. ________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
2. Predict how these materials may be used. ______________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Example (write or add image)
Acid
Base
pH scale
4. Consider what will happen if apple slices are coated with the different solutions and why.
uWrite your hypothesis. ______________________________________________________________
______________________________________________________________________________________
______________________________________________________________________________________
You Be The Chemist®Activity
ActivityGuides
Guide | page 33
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
PERFORM YOUR EXPERIMENT
1. Use a marker to label three plastic bags. Label one “lemon juice,” another “baking soda,” and the third “water.”
2. Pour ¼ cup of lemon juice into the bag labeled “lemon juice.”
3. Create a baking soda solution by mixing ¼ cup of water with one tablespoon of baking soda. Pour the baking soda
solution into the plastic bag labeled “baking soda.”
4. Pour ¼ cup of water into the plastic bag labeled “water.”
5. Collect the apple slices from your teacher.
6. Place two to three apple slices into each of the three bags. Seal the bags tightly. Gently shake each bag to make sure
the apple slices are completely coated by the solution in the bag.
7. Take the apple slices out of the lemon juice bag, and place them on a table on top of the bag. Do the same for the apple
slices in the other bags. Be sure to lay them on top of the right bag so you know which apple slices were coated with
which solution.
8. Observe the apple slices immediately following the treatment. Then, set the apple slices aside, and observe them
later at your teacher’s direction.
ANALYZE & CONCLUDE
1. Record your observations of the reactions in the table below at different time intervals. For example, the first row of
the table might read: lemon juice, 30 minutes, white.
Coating Substance
Time Passed
You Be The Chemist® Activity Guide | page 34
Color of Apple Slices
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
2. What effect did bases have on the apple slices? What effect did acids have? __________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. List some other household substances that you think are acids. ____________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
4. List some other household substances that you think are bases. ____________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
5. Why is it useful for a chemist to know if a chemical is an acid or a base? ____________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
6. If you have an apple for lunch but don’t finish it, what would be the best way to keep it fresh and crisp? ______________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
7. Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
You Be The Chemist®Activity
ActivityGuides
Guide | page 35
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
EXPAND YOUR KNOWLEDGE—ADVANCED
1. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Example (write or add image)
Protein
Amino acid
Enzyme
Catalyst
2. Where are proteins found in the human body? What is their role? __________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. List some chemical reactions in the human body that use enzymes as catalysts.
______________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
You Be The Chemist® Activity Guide | page 36
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
OBSERVE & RESEARCH
Lemon juice, baking soda, water, apples …
1. Write down the materials you observe. ________________________________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
Lemon juice and baking soda may be used for cooking. Water is used for drinking,
2. Predict how these materials may be used. ______________________________________________________________
cleaning, and many other things. Apples are a healthy food to eat. The different materials may be combined, so the browning of apples
______________________________________________________________________________________________________
(a chemical reaction) occurs at different rates.
______________________________________________________________________________________________________
3. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Acid
A solution that contains an excess of hydrogen ions
(H+); acids have a higher concentration of hydrogen
ions than pure water.
Base
A solution that has an excess of hydroxide ions
(OH-); bases have a lower concentration of
hydrogen ions than pure water.
pH scale
A scale that is used to measure the acidity of
(concentration of hydrogen ions in) a solution;
the pH scale generally ranges from 0 to 14.
Example (write or add image)
4. Consider what will happen if apple slices are coated with the different solutions and why.
Acids and bases will have different effects on the rate at which apple slices turn brown
uWrite your hypothesis. ______________________________________________________________
because of the differences in their pH levels.
______________________________________________________________________________________
______________________________________________________________________________________
You Be The Chemist® Activity Guide | page 37
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
PERFORM YOUR EXPERIMENT
1. Use a marker to label three plastic bags. Label one “lemon juice,” another “baking soda,” and the third “water.”
2. Pour ¼ cup of lemon juice into the bag labeled “lemon juice.”
3. Create a baking soda solution by mixing ¼ cup of water with one tablespoon of baking soda. Pour the baking soda
solution into the plastic bag labeled “baking soda.”
4. Pour ¼ cup of water into the plastic bag labeled “water.”
5. Collect the apple slices from your teacher.
6. Place two to three apple slices into each of the three bags. Seal the bags tightly. Gently shake each bag to make sure
the apple slices are completely coated by the solution in the bag.
7. Take the apple slices out of the lemon juice bag, and place them on a table on top of the bag. Do the same for the apple
slices in the other bags. Be sure to lay them on top of the right bag so you know which apple slices were coated with
which solution.
8. Observe the apple slices immediately following the treatment. Then, set the apple slices aside, and observe them
later at your teacher’s direction.
ANALYZE & CONCLUDE
1. Record your observations of the reactions in the table below at different time intervals. For example, the first row of
the table might read: lemon juice, 30 minutes, white.
Coating Substance
Time Passed
Color of Apple Slices
Lemon juice
30 minutes
White
Baking soda solution
30 minutes
Slightly brown
Water
30 minutes
Slightly brown
Lemon juice
1 day
White or slightly brown
Baking soda solution
1 day
Completely brown (or dark brown)
Water
1 day
Completely brown (or dark brown)
You Be The Chemist® Activity Guide | page 38
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
The bases did not have any effect on the
2. What effect did bases have on the apple slices? What effect did acids have? __________________________________
browning reaction of the apple slices, so the apples still turned brown quickly. The acids slowed the rate at which the apples turned brown, so
______________________________________________________________________________________________________
they stayed white and crisp longer.
______________________________________________________________________________________________________
Soda pop, vinegar, orange juice, milk, coffee ...
3. List some other household substances that you think are acids. ____________________________________________
______________________________________________________________________________________________________
______________________________________________________________________________________________________
Ammonia, bleach, milk of magnesia, antacids (such as
4. List some other household substances that you think are bases. ____________________________________________
______________________________________________________________________________________________________
Tums®) ...
______________________________________________________________________________________________________
Acids and bases react differently with other
5. Why is it useful for a chemist to know if a chemical is an acid or a base? ____________________________________
substances and can be dangerous if handled incorrectly. Acids and bases should also never be stored together because they can react
______________________________________________________________________________________________________
violently. Determining the pH of a substance is important to lab safety.
______________________________________________________________________________________________________
To keep the
6. If you have an apple for lunch but don’t finish it, what would be the best way to keep it fresh and crisp? ______________
apple crisp, add some lemon juice or another edible acid to the apple.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
7. Is your hypothesis valid? Why or why not? If not, what would be your next steps? ____________________________
Answer 1: Valid because the data support my hypothesis.
______________________________________________________________________________________________________
Answer 2: Invalid because the data do not support my hypothesis. I would reject my hypothesis and could form a new one, such as …
______________________________________________________________________________________________________
You Be The Chemist® Activity Guide | page 39
LESSON 2 ACTIVITY SHEET: “Ageless” Apples
ANSWER KEY: Below are suggested answers. Other answers may also be acceptable.
EXPAND YOUR KNOWLEDGE—ADVANCED
Have students complete this section if you used the advanced differentiation information, or challenge them to find the answers to these
questions at home and discuss how these terms relate to the experiment in class the next day.
1. Define the following key terms. Then, provide an example of each by writing the example or drawing/pasting an
image of the example.
Term
Definition
Protein
A type of complex organic compound made up of amino acids and
involved in various cell functions; proteins help the body to grow
and repair damage.
Amino acid
A substance that makes up proteins; amino acids are the building
blocks of proteins.
Enzyme
A type of protein found in living cells that acts as a catalyst by
increasing the rate of a chemical reaction in living organisms.
Catalyst
A substance that helps to change the rate of a reaction but is not
consumed or changed during the reaction.
Example (write or add image)
Proteins can be found in human hair, nails, organs,
2. Where are proteins found in the human body? What is their role? __________________________________________
muscles, ligaments, skin, etc. They help the body to grow and to repair damage.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
3. List some chemical reactions in the human body that use enzymes as catalysts.
______________________________
Enzymes help with digestion and
metabolism, as well as cellular respiration.
______________________________________________________________________________________________________
______________________________________________________________________________________________________
You Be The Chemist® Activity Guide | page 40