LESSON 9: Ageless Apples ESTIMATED TIME Setup: 5 10 minutes Procedure: The effects can be observed over a 24-hour time period. DESCRIPTION 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 MATERIALS o Lemon juice o 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 Always remember to use the appropriate safety equipment when conducting your experiment. Refer to the Safety First section in the Resource Guide on pages 391 393 for more detailed information about safety in the classroom. Jump ahead to page 117 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 Matter may be classified as either a pure substance or a mixture. A mixture is made of two or more substances that are combined physically. The different parts of a mixture have different properties. Scientists are able to separate mixtures into their original parts. A solution is a uniform mixture in which one or more substances (solutes) are dissolved in another substance (solvent). For example, salt may be dissolved in water to form a saltwater solution. The salt is the solute, and the water is the solvent. Pure substances and mixtures may experience changes, and these changes can be either physical or chemical. A physical change is any change in a substance s form that does not change its chemical makeup. The chemical formula of the substance stays the same before and after the change. Cutting an apple is an example of a physical change. A chemical change or chemical reaction is a change that takes place when the structure or composition of the materials changes. When a chemical change is complete, the resulting substance(s) is/are different from the original substance(s). The browning of an apple exposed to air over time is an example of a chemical change. You Be The LESSON Chemist 1: Activity Goofy Guides Putty page 114 114
LESSON 9: Ageless Apples 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 and changes can be found in the Classification of Matter section of CEF s Passport to Science Exploration: The Core of Chemistry. 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. To describe certain substances, chemists may use the terms acid and base. The ph scale measures how acidic or basic a substance is, and it ranges from 0 to 14. Acids generally have a ph below 7, have a sour taste (be sure to remind students that they should never taste an unknown substance to determine what it is!), and are corrosive. Common household acids include lemon juice, vinegar, soda pop, and orange juice. Bases generally have a ph above 7 and are slippery to the touch (again, remind students they should never touch an unknown substance without proper protection, like gloves). Common household bases include ammonia, baking soda, milk of magnesia, borax, and bleach. 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 compounds 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 those 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. Basic solutions, such as a baking soda solution, will not lower the ph level on the surface of the apple slices. Lemon juice is often used to keep fruit from browning because lemon juice has a very low ph level. 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. FORMULAS & EQUATIONS Lemon juice gets its acidity from citric acid. The chemical formula for citric acid is C6H8O7. 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, 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. You Be The Chemist Activity Guide page 115
LESSON 9: Ageless Apples DIFFERENTIATION IN THE CLASSROOM LOWER GRADE LEVELS/BEGINNERS Use the experiment to discuss physical and chemical properties. Physical properties can be observed by using our senses and taking measurements. Some examples of physical properties are color, texture, shape, melting point, and density. Chemical properties can be identified by observing how a chemical reacts with other substances. Some examples of chemical properties include acidity, toxicity, and flammability. During the experiment, students can observe the different physical and chemical properties of the substances. Likewise, use the same method to go over physical and chemical changes in more detail. For example, show a picture of cake batter and then a baked cake. Ask how they know it s a chemical change. Then, show a picture of a cake cut in pieces. It s a physical change! 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 the 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. OBSERVATION & RESEARCH 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 H + 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, baking soda, milk of magnesia, borax, and bleach. Solutions containing H + concentrations equal to that of pure water are neutral. The concentrations 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 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 to 14 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. Most foods contain proteins. Proteins are complex organic compounds that are involved in almost all cell functions. Enzymes are proteins that can help to increase the rate of chemical reactions. 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 certain type of chemical compound called a phenol. The rate of this oxidation reaction is increased by a certain enzyme, 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 lowering 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 the oxygen. Therefore, the apple slice will remain crisp and white in color for a longer period 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 changes can be found in the Classification of Matter section of CEF s Passport to Science Exploration: The Core of Chemistry. 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 proteins 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 116
EXPERIMENTATION LESSON 9: Ageless Apples 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. 5. Cut an apple or apples into evenly sliced pieces. Sharp objects like knives can be dangerous. For safety, 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. 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. NOTES 6. Place two to three apple slices into each of the three bags. Seal the bags tightly. Gently shake the 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 corresponding 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 the color of the apples. You Be The Chemist Activity Guide page 117
LESSON 9: 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 acids and bases and explain the differences between them. Identify common household acids and bases. Explain ph and the ph scale. Define and identify chemical reactions. Understand the process of oxidation as a chemical reaction (see Differentiation in the Classroom). Understand the role of enzymes in chemical reactions (see Differentiation in the Classroom). Fun Fact Our stomachs contain gastric acid, which is mainly hydrochloric acid. It has a ph varying from 1 to 3. The high level of acidity is one of the causes of heartburn. Fun Fact An antacid is taken to relieve heartburn. It is comprised of basic solutions that neutralize the stomach acids to provide relief. MODIFICATIONS/EXTENSIONS 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. 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 on 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. You Be The Chemist Activity Guide page 118
OBSERVE & RESEARCH 1. Write down the materials you see. 2. How might these materials 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) Mixture Solution Physical change Chemical reaction Acid Base 4. Consider what will happen if apple slices are coated with the different solutions and why. uwrite your hypothesis. You Be The Chemist Activity Guides page 119
PERFORM YOUR EXPERIMENT 1. Use a marker to label three plastic bags. Label one lemon juice. Label the second baking soda. Label the third water. 2. Pour ¼ cup of lemon juice into the bag labeled lemon juice. 3. Create a baking soda solution. Mix 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. Get six to nine apple slices from your teacher. 6. Place two to three apple slices into each of the three bags. Seal the bags tightly. Gently shake the 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. 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. 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 You Be The Chemist Activity Guide page 120
2. What effect did bases have on the apple slices? What effect did acids have? 3. List other household substances that you think are acids. 4. List 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 is 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 Guides page 121
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) ph scale Protein Enzyme 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. You Be The Chemist Activity Guide page 122
ANSWER KEY Below are suggested answers. Other answers may also be acceptable. OBSERVE & RESEARCH 1. Write down the materials you see. Lemon juice, baking soda, water, apples 2. How might these materials be used? Lemon juice and baking soda may be used for cooking. Water is used for drinking, cleaning, and many other things. Apples are a healthy food to eat. The different materials may be combined so the browning of apple slices (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 Example (write or add image) Mixture A physical combination of two or more substances that can be physically separated. Solution A homogeneous (uniform) mixture in which one or more substances (solutes) are dissolved in another substance (solvent). Physical change Chemical reaction Acid A change that alters the form or appearance of a substance but does not change its chemical makeup or create a new substance. A change that takes place when atoms of one or more substances are rearranged, and the bonds between the atoms are broken or formed to produce new substances; also known as a chemical change. 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. 4. Consider what will happen if apple slices are coated with the different solutions and why. uwrite your hypothesis. Acids and bases will have different effects on the rate at which apple slices turn brown because of the differences in their ph levels. You Be The Chemist Activity Guide page 123
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. Label the second baking soda. Label the third water. 2. Pour ¼ cup of lemon juice into the bag labeled lemon juice. 3. Create a baking soda solution. Mix 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. Get six to nine apple slices from your teacher. 6. Place two to three apple slices into each of the three bags. Seal the bags tightly. Gently shake the 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. 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. 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 124
ANSWER KEY Below are suggested answers. Other answers may also be acceptable. 2. What effect did bases have on the apple slices? What effect did acids have? The bases did not have any effect on the 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. 3. List other household substances that you think are acids. Soda pop, vinegar, orange juice, milk, coffee... 4. List other household substances that you think are bases. Ammonia, bleach, milk of magnesia, antacids (such as Tums )... 5. Why is it useful for a chemist to know if a chemical is an acid or a base? Acids and bases react differently with other 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. 6. If you have an apple for lunch but don t finish it, what is the best way to keep it fresh and crisp? To keep the 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 125
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 Example (write or add image) 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. Protein A group of complex organic compounds made up of amino acids and involved in almost all cell functions; proteins help the body to grow and repair damage. Enzyme A type of protein found in living cells that acts as catalyst by increasing the rate of a chemical reaction in living organisms. 2. Where are proteins found in the human body? What is their role? Proteins can be found in human hair, nails, organs, muscles, ligaments, and skin. They help the body to grow and to repair damage. 3. List some chemical reactions in the human body that use enzymes. Enzymes help with digestion and metabolism, as well as cellular respiration. You Be The Chemist Activity Guide page 126