Science Lab: Using Purple Cabbage Juice as an Indicator for Acid and Base Molecules (Chemical Reactions-Change)

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1 Science Lab: Using Purple Cabbage Juice as an Indicator for Acid and Base Molecules (Chemical Reactions-Change) CONCEPTS: What s a chemical reaction (change)? What happens during a chemical reaction (change)? Do all substances have a chemical change when mixed together? What s the difference between chemical and physical changes? 1. An indicator is a chemical which turns a different color when it comes in contact with a certain molecule. 2. Chemicals may be classified in many different groups, according to the way they behave. One group of chemicals are known as acids and bases. Acids and bases are the opposite of each other. In between acids and bases are the neutrals, which are neither acidic or basic in nature. Orange juice is an acid (citric acid), drinking water is neutral, and Maalox is a base. 3. Purple cabbage juice is an indicator for acid and base molecules. When purple cabbage juice (PCJ) comes in contact with acid molecules it turns pink. If PCJ is mixed with base molecules it turns a blue-green color. STANDARDS GLE Observe and measure the simple chemical properties of common substances. GLE 0507.Inq.1 Explore different scientific phenomena by asking questions, making logical predictions, planning investigations, and recording data. GLE 0507.Inq.3 Organize data into appropriate tables, graphs, drawings, or diagrams. GLE 0507.Inq.4 Identify and interpret simple patterns of evidence to communicate the findings of multiple investigations. GLE 0507.Inq.5 Recognize that people may interpret the same results in different ways. GLE 0507.T/E.4 Recognize the connection between scientific advances, new knowledge, and the availability of new tools and technologies.

2 EQUIPMENT NEEDED: 2 (or more) glass test tubes, plastic eyedroppers, 1/2 a purple cabbage, an electric burner, a cooking pan, 12 small plastic cups (or clean glass baby food jars or small 40 ml beakers), GOGGLES, a clean glass jar, 1/4 cup each of the following: orange juice, lemon juice, vinegar, Coca-Cola, Sprite, ammonia, baking soda, Maalox, Windex, and bleach. STUDENT JOBS: Chemists (everyone takes turns adding PCJ); gofer (goes to get equipment and samples); group manager (responsible for insuring that all data is gathered and recorded before the next sample is taken...a chance for that bossy kid to try to develop positive group skills); equipment manager (director of cleaning the equipment...this is good for the hyperactive, wiggly kid!) OPTIONAL EQUIPMENT: more glass test tubes, a test tube holder, ph test paper, ph Scale displayed on Promethean Board. TIME: 30 minutes - one hour PREVIOUS KNOWLEDGE: Not all chemicals have a chemical reaction (change) when they touch other chemicals. Sometimes nothing happens, such as when you drop a glass marble in water. There is no new or different molecule formed-it s still glass and water. Other times when molecules touch, changes occur. These changes are called chemical reactions or chemical change. For example, when a candle is lit, the wax molecule C25H52 is heated and combines with the air to form C25H O2 25 CO H2O. In other words when a wax molecule joins with exactly 38 O2 molecules during burning, things happen! The molecules form different molecules and voila, the wax is reorganized into 25 carbon dioxide molecules and 26 water molecules. None of the atoms were destroyed only changed into new molecules. (This is called the Law of Conservation of Matter and students will learn more about this in coming years.) So, everyone has seen a candle seem to disappear, but it s really turning into everyday carbon dioxide and water. Simple, right? Whenever atoms and molecules reorganize into new groups this is called a chemical reaction (or chemical change). Some common examples are a burning

3 match, a burning candle, a sliced apple turning brown, a rusting nail, baking soda plus vinegar, and a burning piece of paper. So how do scientists know if a chemical reaction has occurred? Remember, not everything is a chemical reaction (change); sometimes it s just a physical change, such as breaking a pencil, tearing paper, or melting ice. Chemists look for the following things to indicate (point out) that there has been a chemical change. THREE WAYS TO TELL THAT A CHEMICAL CHANGE HAS HAPPENED 1. A change of STATE 2. A change of COLOR 3. A change of TEMPERATURE Let s look at each of these briefly. 1. A change of state is the example of the candle. It changes from a solid wax candle into two gases. Baking soda (solid) and vinegar (liquid) turn into an invisible gas. 2. When iodine touches starch it turns black. When oxygen touches copper it turns green. This is why the Statue of Liberty is green! When oxygen touches a nail it turns orange (rust). 3. When some chemicals are mixed they either get hotter or colder on their own. Ever seen a cold pack. Hit it and mix the chemicals together and voila, it gets cold. A hand warmer generates heat when the molecules are mixed. When the chemicals in a match are ground together it gets VERY hot all by itself. These are some general guidelines. Students should learn them in identifying chemical reactions. Ask students to say the 3 ways to tell there s been a chemical reaction when you are talking about chemical reactions (change). This lesson focuses on ways to tell that there has been a chemical reaction (change) based on a color change. There are special chemicals which will help to find certain chemicals and these are called INDICATORS. Chemists use indicators all the time such as testing a swimming pool for chlorine, aquarium water to check for excess acidity or alkalinity (or base). When indicators touch the molecule they react with, a color change occurs. This is much safer that tasting all the chemicals! There is a family of chemicals called the ACIDS and BASES. They are in the same chemical family even though they are opposites of each other. Scientists measure how strong the acids and bases are with something called the ph Scale.

4 The Acids go from 0 to 6. Neutral is 7 on the ph scale. From 8 to 14 are the Bases (also called Alkaline). The strongest ACID is a 0. The strongest BASE is a 14. The increment from number to number is a factor of ten. The weird thing is that a 5 is ten times stronger than a 6. A 4 is ten times stronger than a 5. That would make a 4 on the ph scale one hundred times stronger than a 6! This is a neat math scale. More on this later. Purple cabbage juice (PCJ) is an indicator of ACIDS and BASES. When PCJ touches ACIDS it turns PINK. When it touches a BASE it turns GREEN. If it touches a NEUTRAL it stays PURPLE. This indicates that a chemical reaction (change) has occurred. In this lesson we will test some common substances to see it they are ACID, NEUTRAL, OR BASE. In each case the indicator (PCJ) will turn a color (which is a sign of a chemical reaction occurring). (See PRODUCTIVE TALK MOVES and STRUCTURING SCIENCE TALK packet. This gives you excellent ways to teach your students to move forward and get past yes/no answers and helps them develop strategies for thinking about the problems.)

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6 OBJECTIVES: 1. Students will test for acids, neutrals and bases using purple cabbage juice with 100% accuracy. 2. Students will record the data with 100% accuracy. 3. Students will determine the relative strength of the acids or bases relative to neutral with 100% accuracy. This objective can be done the next day if the teacher has a short class. THE LESSON PREPARATION AND/OR ADVANCED PREPARATION 1. Make purple cabbage juice (PCJ) by purchasing half a head of purple cabbage from the grocery. Cut the cabbage head in half and place it in a large one gallon soup pan and cover with water. Bring the cabbage to a boil. At first it will turn blue, then purple. You want purple cabbage juice. Then turn off the stove and allow the cabbage juice (which is now a lovely purple color) to cool. This makes about one gallon of PCJ. It is now ready to use--it doesn t have to cool down. Pour ONLY the cabbage juice into an old clean milk jug and refrigerate if you are making it in advance. Label the container PCJ. The PCJ will remain good for about a week or so if it is kept refrigerated. I make it in the classroom, but some may want to make it at home the night before. 2. Collect the following chemicals: orange juice, lemon juice, vinegar, Coca-Cola, Sprite, ammonia, baking soda, Maalox, Windex, and bleach. You will not need more than 1/4 cup of each. THE EXPERIMENT 1. Let students help you prepare the solutions to be tested. (STUDENTS AND TEACHER SHOULD WEAR GOGGLES) Put each of the following into a separate beaker, drinking glass, or baby food jars. (Label each mixture to avoid confusion later) : orange juice, lemon juice, vinegar, Coca-Cola, Sprite, ammonia, baking soda (mixed with a little water), Maalox, Windex, and bleach* (*EACH OF THESE IS A POWERFUL CHEMICAL AND SHOULD BE HANDLED WITH CARE. STUDENTS AND TEACHERS SHOULD ALWAYS EXERCISE PROPER CAUTION WHEN HANDLING CHEMICALS. NEVER PUT CHEMICALS INTO THE MOUTH OR EYES! No fooling around.

7 2. Use an eye dropper to put each of the above samples into separate test tubes (or baby food jars). (Use a separate eye dropper for each beaker of solution in order to avoid contaminating them) Put TWO eye droppers full of the solution into each test tube. Either label the test tubes or do the tests one at a time in order to avoid confusion! 3. Ask students to form a HYPOTHESIS for each of the chemical samples. Is it an acid, neutral, or a base? Record the HYPOTHESIS for each in the DATA Table. 4. Give each group of students a beaker with 200 ml of PCJ in it. Put an eye dropper in each beaker of PCJ in order to avoid contamination. 5. Students should squirt 3 eye droppers of PCJ into the sample test tubes, one at a time (The EXPERIMENT). What color does it turn? (Pink- it s an acid; Purpleit s neutral; Blue or green- it s a base) Record the DATA. 6. Continue testing all the substances one by one and record the DATA in the DATA Table. 7. Before you test each chemical, make a HYPOTHESIS as to whether the substance is an ACID, NEUTRAL OR BASE. This is how scientists do experiments, and it makes learning more fun and interesting, too.

8 DATA TABLE ACIDS, NEUTRALS, AND BASES EXPERIMENT DATA Chemical Sample Hypothesis Color it turned Acid/Neutral/ Base What s the ph?

9 8. Write the results on the DATA sheet. Use the ph chart at the end to determine the ph. It is possible (and preferred) to use ph paper to find the ph. 9. BONUS MATH: How much stronger is each ACID and BASE than NEUTRAL = 7? Remember that each number, moving away from 7 in either direction is 10 times stronger than the number next to it. So a 4 acid would be 10 x 10 x 10 = 1,000 times stronger than 7 or neutral. A base, such as ammonia with a ph 11, is 10 x 10 x 10 x 10 = 10, Think how powerful battery acid is (see chart at bottom) then compared to water (ph 7 neutral)! It s super strong. That means that battery acid (ph 0) is 1,000,000 times stronger than milk (ph 6). Milk has lactic acid in it, making it barely an acid. Unless you let it ferment into yogurt (ph 4.5 to 5). 11. This is a strange scale. It s NOT exactly like a normal number line, is it? In this lab students learned that a CHEMICAL INDICATOR will turn a color if it comes in contact with a certain type of molecule. Chemists know about dozens of different chemical indicators. These help scientists test for poisons, harmful chemicals, helpful chemicals, and even test for chemicals in the human body. Chemical indicators will show if a person has been drinking alcohol, using marijuana or other illegal drugs, is pregnant, or has diabetes, just to mention a few common indicator tests. So you see, chemical indicators are very useful. VARIATIONS AND EXTENSIONS OF THE LESSON Are there other everyday chemicals which students would like to test? Students may want to perform experiments on other chemicals to find out if they contain acid, neutral, or base molecules. With your parent s permission, make your own purple cabbage juice and test other substances, just be safe and have an adult help you. QUESTIONS and CONCLUSIONS: 1. What patterns did you see in your results and data? 2. How might technology help in testing for acid and base molecules? 3. How does the data chart assist you in making logical predictions and plan addition investigations?

10 4. Were these physical or chemical changes? Can you justify why these experiments were chemical or physical changes? 5. What TEN substances would you MOST like to test if you had the opportunity? (e.g. stomach acid, tears, the cafeteria food...) 6. Which of these chemicals do you think is most dangerous? Why? Justify your answer with data! (See PRODUCTIVE TALK MOVES and STRUCTURING SCIENCE TALK packet. This gives you excellent ways to teach your students to move forward and get past yes/no answers and helps them develop strategies for thinking about the problems.) 2013 Ronald Boston ph papers which test from 1-11 can be purchased from NASCO science for $ One container will last for years. ph plastic strips from 1-13 can also be purchased from NASCO for $