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Red cabbage extract can indicate whether a substance is an acid (like vinegar) or a base (like ammonia). It can also show how strong an acid or a base a substance is. Chemists use the pH scale to express how acidic (like an acid) or basic (like a base) a substance is. A pH value below 7 means that a substance is acidic, and the smaller the number, the more acidic it is. A pH value above 7 means that a substance is basic, and the larger the number, the more basic it is. Red cabbage extract has different colors at different pH values. Acids are materials that have certain properties in common. Bases (also called alkalis) are other substances with a different set of properties. In these experiments, you will investigate some of these properties with materials that are found around your home. In addition, you will learn how chemists use the pH scale to describe acids and bases. Materials ListpH paperred cabbage ammonia vinegar Lemon juiceBaking sodaCans of veggies or fruits (look for fewest ingredients added)straws water beaker/colorless glasses/glass measuring cups teaspoontablespoonstirring rod strainer
PART 1:ï¿½ Make the indicatorBoil the red cabbage in a beaker or pan of water until the color has been well extracted. More specifically, To make red cabbage indicator first cut the cabbage into quarters and grate into pot. Cover the cabbage with enough water and boil for approximately 5 minutes. Strain the indicator into three equal portions.Alternative method:ï¿½ Place about 500 milliliters (2 cups) of red cabbage cut into 2.5-centimeter (1-inch) cubes into a blender or food processor. Add about 250 milliliters (1 cup) of water and blend the mixture until the cabbage has been chopped into uniformly tiny pieces. Strain the mixture by pouring it through a sieve.ï¿½ Divide into three equal portions.
PART 2:ï¿½ Test the indicatorSince red cabbage extract is an acid-base indicator, it should change color as pH conditions change.ï¿½ To one portion of the indicator perform the following:1. ï¿½Add a few drops of dilute ammonia. Indicator should turn green. 2.ï¿½ï¿½ Using a straw, blow into the indicator solution until the green color turns blue. 3.ï¿½ï¿½ Add a few drops of dilute vinegar….solution should turn pink.
PART 3:ï¿½ Set up references (standards)The second portion of the red-cabbage extract will be used for exploring acids and bases. Examine the label of a bottle of white vinegar. The label probably says that it contains acetic acid. This indicates that vinegar is an acid and has properties of an acid. Let’s see what effect an acid has on the color of the red cabbage extract. Pour 125 milliliters (ï¿½ cup) of vinegar into a labeled colorless drinking glass. Add 5 milliliters (1 teaspoon) of red cabbage extract, stir the mixture, and note its color. What is the color of the mixture? (Write your answer below.) Red
The color of the cabbage extract with vinegar is the color the extract has when it is mixed with an acid. Save the mixture in this glass to use as a reference in part 4. Now examine the effect of laundry ammonia on the color of red cabbage. Pour 125 milliliters (ï¿½ cup) of laundry ammonia into another labeled colorless drinking glass. Add 5 milliliters (1 teaspoon) of red cabbage extract and stir the mixture. Write the color of this below. Yellow
Laundry ammonia is a base (alkali). The color of this mixture is the color of cabbage extract when it is mixed with a base. The color of cabbage extract indicates whether something mixed with it is an acid or a base. Cabbage extract can be called an acid-base indicator. Save the mixture in this second glass to use as a reference in part 4. Now test the properties of a solid, baking soda. Place 5 cubic centimeters (1 teaspoon) of baking soda in a labeled glass and add 125 milliliters (ï¿½ cup) of water. Stir the mixture until the baking soda has dissolved. Then, add 5 milliliters (1 teaspoon) of red cabbage extract to the solution. Write the color of the mixture below. Green
The color obtained with baking soda is different from the color obtained with vinegar and from the color obtained with ammonia. ï¿½Save for use as a reference in part 4.These colors and approximate pH values are:
approximate pH: 2 4 6 8 10 12
color of extract: red purple violet blue blue-green green
Based on this information, what is the approximate pH of vinegar? 4 What is the approximate pH of ammonia? 2 What is the approximate pH of the baking soda mixture? 6
PART 4:ï¿½ Determine pH of household itemsUse the instructions for testing vinegar and ammonia to test the pH of several other nearly colorless liquids, such as lemon-lime soft drink (Sprite or 7-Up) and lemon juice. Record your observations. Liquids that are white, such as milk, can be tested in the same way. You can also test solids that dissolve in water by following the instructions for baking soda. This will also work with viscous liquids such as liquid detergents. Test other substances around the house, such as sugar, table salt, shampoo, hair rinse, milk of magnesia, antacid tablets, and aspirin.
CAUTION: Some household products can cause skin irritations. Do not allow these to contact skin; rinse thoroughly with water if they do.
Part 5:ï¿½ titrationLAB: Acidity of Food INTRODUCTION: Commercial canning of food involves the sterilization and cooking of the food by pressure-cooking at a temperature of 250ï¿½ F. The food is then sealed in cans. Today, people are also canning their own food at home. One major factor that determines the temperature needed to kill bacteria is the amount of acid present in the food because bacteria grow well in low-acid foods, such as green beans. This is a major problem for home canners, since a bacteria called botulinum is extremely harmful to humans. In fact, even a small amount as that in a single bean can kill an adult. These bacteria exist as spores, which are resistant to heat and therefore require an increased boiling time in order to sterilize the beans. In this experiment, the acidity of canned foods is measured using a red cabbage juice indicator. This indicator acts like litmus paper and indicates the pH of a solution quickly, but less precisely than a pH meter. The red cabbage indicator is called an acid-base indicator, which is a colored substance that can exist in either an acid or a base form. The two forms are different colors. The red cabbage juice exists normally as a purple solution. It can react and change color when in contact with acids to pink and with bases to green. Titration is the progressive addition of a solution (the titrant) usually from a graduated tube called a burette to a known volume or mass of a second solution or substance until the end-point.ï¿½ The end-point is indicated by a color change of an added indicator or by some other detectable change, is reached. PURPOSE: To measure the acidity in canned fruits and vegetables by the method of titration using red cabbage juice indicator. PROCEDURE:
Create controls showing the endpoint colors of the indicator by placing 25 mL lemon juice in one 100 mL beaker and add enough indicator to get the color to change to pink. Count and record the number of teaspoons of indicator added.
Repeat step 2. with the baking soda to produce a color change of purple to green. Count and record the number of teaspoons of indicator added.
For each type of canned fruit/vegetable place 2 tablespoons of the red cabbage indicator in a 100 mL beaker.
In each beaker add the juice from the canned food one teaspoon at a time. Count and record the number of teaspoons added until there is a color change (end-point of the indicator).
OBSERVATIONS: Rank the fruits and vegetables according to acidity, from most acidic to least acidic. Differences could be caused by the presence of flavoring and other juices added during the canning process. You should check the labels of the fruits/vegetables for these additives. ï¿½Record any here.ï¿½ Acids often end with ate (Ex: benzoate). Below is a list of common items that have been organized in order of decreasing hydrogen ion concentration and increasing pH.
[H+] pH Substance
1.0 x 10-1 1.0 battery acid
1.0 x 10-2 2.0 lemon juice
6.3 x 10-3 2.2 vinegar
1.0 x 10-3 3.0 apples
3.2 x 10-4 3.5 soft drink
1.0 x 10-4 4.0 wine
3.2 x 10-5 4.5 tomatoes
2.5 x 10-6 5.6 unpolluted rainwater
2.5 x 10-7 6.6 milk
1.0 x 10-7 7.0 pure water
4.0 x 10-8 7.4 human blood
5.0 x 10-9 8.3 baking soda solution
4.0 x 10-9 8.4 sea water
3.2 x 10-11 10.5 milk of magnesia
1.0 x 10-11 11.0 household ammonia
4.0 x 10-13 12.4 lime
1.0 x 10-13 13.0 lye
LAB REPORT:1. ï¿½Include a picture of your standards with you in the picture.2. Make a table including the material tested, the extract color, the corresponding observed pH, and the expected pH (may have to look up).ï¿½ Three of the items will also have results from the pH paper.ï¿½ Include both household items and canned food.ï¿½ Comment on the results.ï¿½ 3. Submit to the dropbox.
Material Extract color pH
. Sugar . Purple . 7
. Salt . Purple .67777
. Milk of magnesia . Green . 10.5
. Antacid Tablets . Blue . 9
. Asprin. Pink . 4
. Milk . Violet . 6
Toothpaste Green 10
Household Ammonia Yellow 12
Pure water Purple 7
Tomato Juice Pink 4
Lemon Juice Red 2
Canned beets Pink 5
Canned Corn Pink 6