Questions for Discussion. Introduction. What is ph? Neutralization

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1 Questions for Discussion Why do leaves change color in autumn? What are acids and bases? How does red cabbage fit in? How are all these things related? Introduction The questions above may not seem related, but they are. In this article, we ll find out how they re related, but first we need to discuss the idea of the ph scale (see def.). What is ph? In chemistry, ph is the measure of how acidic or Fun Fact basic an aqueous solution (see def.) is. A substance What is up with the abbreviation ph? that is neither acidic nor basic (alkaline) is neutral. Pure Well, the p is somewhat debatable, water is considered neutral on the ph scale and is but generally stands for power or assigned a value of 7. All other liquids are measured in potential of and the H is the atomic relation to pure water on the ph scale. The lower symbol for the hydrogen element. values are acids (the lower the number the more acidic) and higher values are bases (the higher the number the more basic). For example lemon juice (acidic) is approximately 2 on the ph scale, while bleach (alkaline) is 13. An acid is a compound that produces hydrogen ions (H+) in water, where a base produces hydroxide ions (OH-) in water. Neutralization Whenever you mix an acid with a base, they neutralize each other, and the products of the neutralization do not have characteristics of either acids or bases. Instead, a neutral salt (see def.) and water are formed. As noted above, pure water is neutral. But when chemicals are mixed with water, the mixture can become either acidic or basic. 1 Page

2 Determining ph Acids and bases are everywhere in nature. Indicators of ph also exist in nature. These indicators illustrate, qualitatively, the ph of a solution. For example, the pigments (called anthocyanins) that are responsible for the red color of cabbage are one such indicator. Other natural indicators are found in blueberries, beets, or radishes. We can use these natural indicators to tell us the relative ph of other solutions. Let s try it! Acid Rain A Case of ph and the Environment Acid rain is rain that has been made acidic by certain pollutants in the air. Normal precipitation such as rain, sleet, or snow reacts with alkaline chemicals and materials found in air, soils, bedrock, lakes, and streams to neutralize the natural acids. If precipitation becomes too acidic, these natural materials are not able to neutralize all of the acids, and, over time, the neutralizing materials can be washed away by acid rain. Damage to crops, trees, lakes, rivers, and animals can result. Government and industry regulations, plus some new technologies, are helping to reduce acid rain, but it is still a serious environmental concern. Because energy production creates much of the pollutants that cause acid rain, one step we can take is to conserve energy. You can learn more by researching acid rain on your own. Experiment Equipment/Materials: About 2 cups of chopped red cabbage Large pot 8 9 cups of water Strainer or colander An assortment of solutions to test (see Step 5 for list) Two coffee filters Several shallow bowls Protective eyewear Protective gloves. Caution! Be very careful when dealing with strong acids and bases. Always wear rubber gloves and use eye protection, as strong acids and bases are highly corrosive. This experiment should be performed under adult supervision. 2 Page

3 Procedure: 1. Boil the red cabbage in approximately 8 cups of water for 2 3 minutes on the stove. 2. Pour the liquid through a strainer or colander lined with the first coffee filter to remove any large particulates. The filtered solution will be our indicator liquid. We will use the coffee filter to create ph indicator strips, or we could also just use the liquid. 3. Soak the second coffee filter in the liquid for about ½ hour. Place the filter on a cutting board to dry (consider covering your cutting board with plastic wrap, so that it is not stained with the juice). 4. When the filter is dry, cut it into strips. Collect a few of the strips to use in our test; the rest can be saved for later use. 5. Under adult supervision, select some liquids/solutions you have on hand at home, because in the next step we will determine whether they are acids or bases. Some examples of solutions are: Baking soda mixed in water Vinegar Soda (diet or regular) Lemon juice Milk Liquid soap or shampoo Hand sanitizer. 6. Pour a small amount of a solution of your choosing into a shallow bowl (you don t need much). Remember, both strong acids and strong bases can be very dangerous and burn your skin. 7. Dip the indicator strip into the liquid. Pull the indicator strip out and use the color indicator above to view the relative ph of these solutions. Record your results. 8. Look at the ingredients for each liquid you tested. In your opinion, which ingredients contribute to each liquid s ph level? What can the color of the cabbage solution tell us? 3 Page

4 What Can We Conclude? The red cabbage extract, which itself it neutral, makes a good indicator of the acidity or basicity of solutions or common household items. We could test any aqueous solution to see where it lies on the scale using the indicator strips produced in this experiment. Liquids all around us have either acidic or basic (alkaline) properties. Strong acids and bases can cause serious illness if ingested. However, it is noteworthy information that, in general, acids taste sour, while bases taste bitter and feel slippery. Fun Fact Chlorophyll absorbs light so strongly and there is so much of it in an active leaf that it can mask other pigments. Some of the more delicate colors (like yellows) are revealed when the chlorophyll molecule slows and decays in autumn and trees in certain regions turn red, orange, and gold. Chlorophyll can also be damaged when vegetation is cooked. This affects the energy levels within the molecule, causing its light absorbance spectrum to alter. Thus, cooked leaves change color, often Post-Experiment Discussion becoming a paler, more yellowy green. Pigments like those in red cabbage, beets, blueberries, etc., are also present in the leaves of bushes, plants, and trees. These pigments include carotenoids, anthocyanins, and chlorophyll. Carotenoids are yellow, orange, and red organic pigments. In addition to coloring leaves, they produce color in such things as corn, carrots, daffodils, rutabagas, and bananas. Although carotenoids are present all year long, they are masked by chlorophyll during the growing season. In early autumn when chlorophyll decreases, carotenoid pigments begin to show. Sometimes they show earlier, depending on how quickly the plant or tree can regenerate chlorophyll. Anthocyanins give color to leaves and such things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. Anthocyanins are pigments that appear red, purple, or blue, depending on the cell ph, which makes them different from carotenoids. Anthocyanins are not present during the growing season, but are produced at the end of summer in the sap of a leaf's cells. Because bright light supports the production of anthocyanins, areas with bright and brisk autumn days will have the largest color variation in their leaves (meaning an abundance of red). 4 Page

5 Finally, chlorophyll, the most dominant pigment, is responsible for the green coloration in leaves. It is also the molecule that absorbs sunlight and uses its energy to synthesize carbohydrates from CO 2 and water a process known as photosynthesis, which is the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period. Fun Fact Anthocyanin pigments are formed when sugars combine with complex compounds called anthocyanidins. There can be great variety in the pink to purple colors and this is influenced mainly by cell ph. The unique indicator compounds in the leaves react differently depending on whether the sap is acidic or basic. Sap is usually acidic in trees with red leaves, and basic in trees with purple or blue leaves. Inside the Tree As the daytime hours grow shorter in late summer, the veins that carry fluid in and out of the leaf are gradually closed off. Water and mineral intake are reduced, and chlorophyll begins to decrease. That s when a new color palette emerges. Certain autumn colors are characteristic of particular tree species, as the pigments in the leaves react differently depending on whether the sap is acidic or basic. Oaks turn red, brown, or russet; hickories turn a golden bronze; aspen and yellow poplar become golden yellow; dogwood change to purplish red; and beech trees turn a light tan. Leaves of some species, such as the elms, simply shrivel up and fall, exhibiting little color other than drab brown. Maples differ by species with most turning crimson red or yellow. Leaves of the striped maple become almost colorless. 5 Page

6 Conclusion Acids and bases are all around us, and ph is the measure of how acidic or basic an aqueous solution is. Indicators of ph are abundant in nature. Unique pigments (called anthocyanins) that are responsible for the red color of cabbage are one such indicator. Other natural indicators are found in blueberries, beets, or radishes. The experiment we performed by creating our own red cabbage extract allowed us to test the ph of a variety of household solutions and draw conclusions based on ingredients in those solutions. We found milk to be neutral. Acidic solutions included lemon juice and vinegar, while basic solutions included liquid soap or baking soda mixed with water. Pigments like those in fruits and vegetables are also present in the leaves of bushes, plants, and trees. Those pigments include carotenoids, anthocyanins, and chlorophyll. Chlorophyll is the most dominant pigment. A number of factors contribute to the changing colors of autumn leaves, such as cooler temperatures, reduced moisture, and fewer hours of sunlight. However, cell ph and sap ph also play major roles, most significantly in the display of red shades influenced by anthocyanins. Definitions Aqueous solution. An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending (aq) to the relevant formula. For example, a solution of ordinary table salt, or sodium chloride (NaCl), in water would be represented as NaCl(aq). ph. The ph scale measures how acidic or basic a substance is. The ph scale ranges from 0 to 14. A ph of 7 is neutral. A ph less than 7 is acidic. A ph greater than 7 is basic. The ph scale is logarithmic and as a result, each whole ph value below 7 is ten times more acidic than the next higher value. For example, ph 4 is ten times more acidic than ph 5 and 100 times (10 times 10) more acidic than ph 6. The same holds true for ph values above 7. Salt. A salt is any compound which can be derived from the neutralization of an acid and a base. The word "neutralization" is used because the acid and base properties of H+ and OH- are destroyed or neutralized. In the reaction, H+ and OH- combine to form HOH or H 2 O or water molecules. A neutralization is a type of double replacement reaction. A salt is the product of an acid-base reaction and, in this context, is a broader term than common table salt. 6 Page

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