47 Acids, Bases, and the ph Scale R EA D I N G Wherever chemical solutions are involved, ph matters. Some important chemical reactions, such as those involved in corrosion of iron or digestion of food, will only take place within a specific ph range. In agriculture, the soil s ph affects crop yields of fruits, vegetables, and grains, such as wheat, barley, and corn. The control of ph is also important within your body. For example, the ph of your blood is maintained at a slightly basic level of approximately 7.4. In your stomach an acidic ph between 2 and 4 helps you digest your food. The ph of water in the environment is also important because it affects the survival of fish and other organisms. Scientists measure ph when they study acid rain and when they work to maintain drinking water quality. CHALLENGE What is ph, and how does it affect the quality of water? Populations of brook trout have been drastically reduced by the effects of acid rain on freshwater lakes and streams in the northeastern United States. C-86
Acids, Bases, and the ph Scale Activity 47 MATERIALS 1 Student Sheet 47.1, Directed Reading Table: Comparing Acidic, Basic, and Neutral Liquids concept map from Activity 38, Dissolving Duel Battery acid READING Use Student Sheet 47.1, Directed Reading Table: Comparing Acidic, Basic, and Neutral Liquids, to guide you through the following reading. The ph scale As you observed in the last activity, the ph of a solution is a number that expresses how acidic or basic it is. The ph scale ranges from 0 for very acidic to 14 for very basic. At the middle of the scale, 7, is a neutral liquid, such as distilled water. The scale is not linear with equal increments like a ruler or a thermometer. The ph scale is a lot like the Richter scale used to measure the strength of earthquakes. An 8.0 earthquake is 10 times stronger than a 7.0 earthquake, and a ph of 3 is 10 times more acidic than a ph of 4. For every increase or decrease in ph of 1.0 unit, the acidity or basicity of the solution changes tenfold. Lemon juice Vinegar Orange juice Boric acid Corn Milk Distilled water Blood Sea water Baking soda Milk of magnesia 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 Borax Ammonia Bleach Household lye Very acidic Common range for most natural waters ph Scale Very basic C-87
Activity 47 Acids, Bases, and the ph Scale The color change of a ph indicator results from a chemical reaction between the indicator and the acid or base dissolved in water. An acidic solution contains acid and turns ph paper deep orange or red. A basic solution contains a base, and turns ph paper blue or violet. Acids are recognizable by their sour taste and because they can dissolve some metals. Bases from soap to bleach to drain cleaner have other properties. They tend not to react with metals and are bitter in taste. They are slippery to the touch because they react with your skin oils to form a substance a lot like soap. However, both acids and bases can be corrosive to human skin, if the ph is below 2 or above 12. Acids, Bases, ph, and Industry Controlling ph is important in the production of foods, paper products, and chemicals. The low ph of vinegar preserves foods, such as pickles, because it slows the growth of bacteria and other microbes. Because they are chemically reactive, acids and bases are also important in the manufacture of many products. For example, sulfuric acid is by far the largest single product of the chemical industry and is used to make fertilizer, refine petroleum, and clean and process metals. Nitric acid is used to make explosives and dyes. The bases sodium hydroxide and potassium hydroxide are used to make soaps. Sodium hydroxide is also the main ingredient in drain cleaners. Ammonia, a base that contains nitrogen, is used both as a fertilizer and to produce other fertilizers. Orange juice and vinegar are two familiar liquids that contain acids. C-88
Acids, Bases, and the ph Scale Activity 47 Drain cleaner and household ammonia are two familiar liquids that contain bases. ph and the Environment If the ph of water in rivers, streams, and lakes becomes too high or too low, many animals and plants cannot survive. Wastes released by human activity may disturb the acid base balance in bodies of water. Even a small change from the normal range can harm some aquatic life and cause changes in ecosystems that are hard to predict. Most organisms that live in freshwater are healthiest at ph values from 6 to 9. Variations in ph outside of this range reduce populations of organisms by decreasing survival of adults and reducing reproductive rates. ph and Drinking Water The federal government s standards for drinking water require a ph between 6.5 and 8.5. Acidic or basic tap water may react chemically with water pipes and release contaminants into the drinking water. The biggest hazard is that acid may dissolve lead out of the metal of some pipes into the water flowing through them. Lead can have serious health effects that vary with dosage and among different individuals. Young children are much more sensitive than adults, but adults can also be affected. Lead acts on the brain, nervous system, and other organs, causing effects from behavioral and learning problems to seizures and even death. C-89
Activity 47 Acids, Bases, and the ph Scale ANALYSIS 1. In what ways can the ph or acidity of water affect: a. living things, such as fish? b. people? 2. Fill in the blanks of the following sentences with the correct number. a. A solution with a ph of 5 is times as acidic as a solution with a ph of 6. b. A solution with a ph of 4 is times as acidic as a solution with a ph of 6. 3. Add the following words to your concept map from Activity 38, Dissolving Duel. acid base neutral ph 4. Compare and contrast acids and bases by completing a Venn diagram. In your science notebook, make a larger version of the Venn diagram shown below. Compare acids and bases by recording common features in the space where the circles overlap. Contrast acids and bases by recording unique features of each kind of substance on the far side of each circle. Acids Bases 5. What do you predict will happen to ph when an acid and a base are mixed together? Explain your prediction. C-90