Cabbage juice

Red cabbage juice, a natural acid-base indicator. The picture shows (left to right) its colors at pH 1,4,7,10. and 13. 

Litmus is not the only plant material that turns a different color in response to acidic or basic conditions. For example, when red cabbage or beets are boiled, the solids can be separated from the liquid. The liquid is then cooled for use as an acid-base indicator. Red cabbage juice is red or purple in acidic conditions, while bases cause it to turn blue or yellow. When a solution is neutral, the juice is a bluish-purple. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

The change in color of a pH indicator is not permanent. Red cabbage juice brought to a pH of 4 turns red. This same solution brought to a pH of 8 turns green and then red again as it is brought back to a pH of 4. 

What color does the cabbage juice turn when you add the vinegar What color does the cabbage juice turn when you add the baking soda What color does the cabbage juice turn when you add lemon juice ... 

Some molecules, like those in the red cabbage juice, can be used to Identify other molecules as acids or bases. These identifying molecules are called indicators because they will change color. They have a different color in an acid and a different color in a base. 

Next, you will need one or more of the following phenolphthalein solution (less than an ounce of 1% solution), bromothymol blue solution (about an ounce of 1% solution is enough, and this can be diluted with water in a 1 to 4 ratio and it will work fine), or cabbage juice (just boil a small amount of red cabbage for a few minutes in a cup of water, remove the cabbage, and use... 

Using bromothymol blue (BTB) or cabbage juice. If you use these liquids instead of, or in addition to, phenolphthalein, expect to see two or more color changes. Also, with BTB you should add vinegar to start and then use sodium hydroxide to reverse the color changes. 

Bromothymol blue and cabbage juice work in much the same way as phenolphthalein. As the acidity of the solution they re in changes, their molecular structure changes as well. These different structures, three kinds in each case, re-radiate different colors of light. Of course, there are many more kinds of acid-base indicators, and each one is either a weak, low-concentration acid or a weak, low-concentration base. That s why they interact with the acids and bases... 

You have learned that certain substances, called indicators, change color when the pH of a solution changes. The juice from red cabbage is a natural indicator. How do the pH values of various solutions compare to each other How can you use red cabbage juice to determine the relative pH of several solutions ... 

Design a data table to record the names of the solutions to be tested, the colors caused by the added cabbage juice indicator, and the relative strengths of the solutions. 

Add ten drops of the cabbage juice indicator to each of the soiutions to be tested. Gently agitate or wiggle each test tube to mix the cabbage juice with the solution. 

Cabbage Juice Color Relative Strength of Acid or Base... 

Universal indicators are mixtures of several acid-base indicators that display a continuous range of colors over a wide range of pH values. Figure 18-2 shows concentrated solutions of a universal indicator in flat dishes so that the colors are very intense. The juice of red (purple) cabbage is a universal indicator. Figure 19-2 shows the color of red cabbage juice in solutions within the pH range 1 to 13. 

To each of the four bags, add 5 mL of white vinegar, 10 mL of water, and enough cabbage juice indicator (probably 30 to 40 drops) to impart a distinct color. 

Indicators often are used to determine the approximate pH of solutions. In this ChemLab, you win make an indicator from red cabbage and use the indicator to determine the approximate pH values of various household liquids. The cabbage juice indicator contains a molecule, antho-cyanin, that accormts for the color changes. 

Set the beaker on a hot plate, and heat until the water has boiled and become a deep purple color. Remove the beaker from the hot plate using beaker tongs, and allow it to cool. Pour off the cabbage juice indicator liquid into a clean beaker. 

Draw the cabbage juice indicator solution into a clean pipet, and add 5 drops to each of the solutions in wells H1-H8. Stir the solution in each well with a clean toothpick. 

Hypothesize about how other solutions at home would react with the cabbage juice indicator. Explain your predictions. 

Some chemicals can be identified by their interaction with water, vinegar, iodine, red cabbage juice, and heat. 

Some chemicals can be classified as acids, bases, or neutral substances by their reactions with red cabbage juice. 

Finally, Lesson 16 is an embedded assessment of the skills students have been developing throughout the unit. Here, students face a challenging reversal instead of using known test liquids (water, vinegar, iodine, and red cabbage juice) to identify an unknown solid, they use the... 

Appendix D explains how to make the red cabbage juice for Lesson 9 and the iodine, ammonia, alcohol, and detergent solutions for Lessons 8 and 15. 

Concepts Common household chemicals have different physical and chemical properties. Chemicals undergo changes in form, color, or texture when they are mixed together, separated, or heated. Some chemicals can be identified by their interaction with water, vinegar, iodine, red cabbage juice, and heat. Different types of mixtures, such as solutions or suspensions, are created when solids are combined with water. Evaporation and filtration are methods for separating mixtures of solids and liquids. Some chemicals can be classified as acids, bases, or neutral substances by their reactions with red cabbage juice. Observations... 

Note In Lesson 9, students will test the unknowns with red cabbage juice. Because the Juice requires ad ranee preparation on your pan, please read the Background and Preparation sections on pgs. 99 and 100 at this lime. 

Testing Unknown Solids with Red Cabbage Juice... 

In this lesson, students use a third chemical liquid, red cabbage juice, to explore further the chemical properties unique to each unknown. Following the completion of this test, students analyze all the data they have collected over the past seven lessons and summarize what they now know about the physical and chemical properties of each of the five unknown solids. This is a good time for the class to add to the What We Think about Chemicals list from Lesson 1. 

Students predict the results of testing the five unknowns with red cabbage juice. 

Red cabbage juice is a natural indicator that is, a substance that, through its color, reveals the presence of certain chemicals. Because this is a plant juice and contains no preservatives, its shelf life is short. BHT, a nontoxic food preservative, has therefore been added to the red cabbagejjuice in this ki... 

Freshly made red cabbage juice has a blue-purple tint. However, after a while, it may assume more of a bluish tint. This slight color change will not interfere with its indicator properties. 

Your students may not be as familiar with red cabbage Juice as they are with water, iodine, and vinegar. For your information, a property of red cabbage Juice is that it is an acid-base indicator. Students will discover this property in Lesson 15, when they test a variety of household chemicals with the Juice. 

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