How Do Scientists Measure Acidity?

Transcription

1 4.5 Investigate How Do Scientists Measure Acidity? ph scale: a measure of the concentration of hydrogen ions in a substance. neutral: a solution with a ph of 7. ph 7 has an equal number of hydrogen ions and hydroxide ions. acid: a solution that tastes sour, has more hydrogen ions than pure water, and has a ph of less than 7. base: a solution with a bitter taste, a slippery feel, and a ph more than 7. hydroxide ion: one oxygen atom, one hydrogen atom, and an extra electron. You know that the more acidic the solution is, the higher the concentration of hydrogen ions. Scientists communicate how acidic a solution is using numbers on a ph scale. The ph scale tells you the concentration of hydrogen ions in a solution. The name of this scale, ph, stands for power of hydrogen. The scale ranges from 0 to 14. In the middle of the scale is 7. Solutions with a ph of 7 are called neutral. Pure water, or distilled water, is neutral and has a ph of 7. Keep in mind that water from a faucet and rainwater are not pure water. They have substances dissolved in them, so their ph is not 7. Acids are solutions that have a ph less than 7. The lower the ph, the more acidic a solution is. This may sound confusing, but it is important to remember. When ph is lower, a solution is more acidic. Normal rainwater is slightly acidic, with a ph of about 5.6. The ph of acid rain is closer to 4.3. You might wonder about the numbers that are higher than 7 on the ph scale. Solutions with a ph higher than 7 are called bases. Bases have a bitter taste, and they tend to feel slimy or slippery. When placed in water, a base produces hydroxide ions (OH ). A hydroxide ion consists of one oxygen atom, one hydrogen atom, and an extra electron. The minus sign ( ) shows that the ion that has gained an electron and has a negative charge. Pure water has fewer hydrogen ions than acids, and bases have fewer hydrogen ions than pure water. If a solution has a ph less than 7, then it has a higher concentration of hydrogen ions than pure water. If it has a ph greater than 7, then it has fewer hydrogen ions than pure water. The ph scale ranges from 0 to 14. Pure water is neutral and has a ph of 7. The ph of an acid is less than 7, whereas the ph of a base is greater than 7. ph increasing acidity neutral decreasing acidity AQ 208

2 4.5 Investigate The interval between numbers on the ph scale is different from the interval on a number line. On a number line, the interval between each number is one. On the ph scale, each number indicates a change of ten times. For example, a solution with a ph of 4 is ten times more acidic than a solution with a ph of 5 and one hundred times more acidic than a solution with a ph of 6. Car battery acid is a strong acid. It measures 1 on the ph scale. The acid in oranges is a weaker acid. It measures 4 on the ph scale. Acid-base Indicators You know that ph measures the acidity of a solution, but how do you measure ph? After all, you cannot directly measure the concentration of hydrogen ions. Scientists have found many different indicators for measuring ph. An indicator is a tool that can be used to determine the condition of something that cannot be measured directly. Red Cabbage Juice Boiled red cabbage juice has been used as an acid indicator for thousands of years. People who dyed cloth in ancient times often used the juice from boiled red cabbage leaves to produce a pale bluish-purple dye. They noticed that when sour-tasting substances (acids) were added to the cabbage juice, the dye turned red. When bitter-tasting substances (bases) were added to the cabbage juice, the dye turned green. Red cabbage juice became one of the first ph indicators. But it can determine only whether a solution is an acid or a base. It cannot measure exactly how acidic or basic a solution is. indicator: a tool that can be observed to determine the condition of something. Red cabbage juice was one of the first ph indicators. Acids turn red cabbage juice red. Bases turn red cabbage juice green. AQ 209 AIR QUALITY

3 lichen: two distinct organisms, a fungus and an alga, living as one. litmus paper: a paper made from wood, lichen, and other compounds that is used to determine whether a solution is acidic or basic. universal indicator: a test used to measure a range of ph s. ph paper: a universal indicator that determines how acidic a solution is. Litmus Paper Ancient people also observed that sour substances changed the color of a substance called litmus. Litmus is a dye extracted from lichen. Lichen is an odd kind of organism. It is both a fungus and an alga, living as one. Lichen was often called dyer s weed, and people used it to dye cloth. Early scientists also used litmus to indicate if a solution was acidic or basic. They knew that litmus changed color according to the acidity of a substance. In the 1800s, a scientist developed litmus paper made from wood, lichens, and other compounds. An acid turns blue litmus paper red. A base turns red litmus paper blue. Litmus paper is still used today to determine whether a solution is acidic or basic. Lichen, a fungus and an alga living as one, are used to make litmus paper. ph Paper: A Universal Indicator Litmus paper and red cabbage juice can indicate whether a solution is an acid, but they cannot measure the strength of the acid. A different type of test is needed for that. Modern scientists have developed what they call universal indicators. A universal indicator is a mixture of substances that each changes color at different ph values. You test how acidic a solution is by dipping the universal indicator in the solution and observing what color it turns. An indicator for ph can be a liquid or a solid. The most commonly used universal indicator for ph is called ph paper. When you dip ph paper ph paper turns different colors to indicate the ph of a solution. AQ 210

4 4.5 Investigate into a solution, it changes color to indicate the ph of the solution. ph Meters ph meters can also test the strength of an acid or a base. A ph meter measures hydrogen ions in the solution tested. On most ph meters, the ph of the solution is displayed on a digital readout. This makes ph meters the most accurate way to measure ph. A ph meter is the most accurate way to measure the ph of a solution. How Acidic Is Each Solution? Each group in the class will measure the ph of distilled water and 5 different liquids. You will use ph paper or a ph meter to measure the ph of each of the liquids. Then you will report to the class. Predict Record the names of the 5 solutions you will be testing in the first column of your Acids and Bases page. You are familiar with some of these solutions. For each of your solutions you are familiar with, predict where it will fall on the ph scale. Use information you know about the ph scale and about each solution to make your decision. Keep in mind the characteristics of acids and bases while you are making your predictions. Record your predictions on your Acids and Bases page. Materials well plate ph paper tape water in a tea in dropper bottle coffee in baking powder (dissolved in water) in milk in dropper bottle power drink in pickle juice in window cleaner in dropper bottle stain remover (liquid) in liquid antacid in AQ 211 AIR QUALITY

5 Materials aspirin (dissolved in water) in buffered aspirin (dissolved in water) in clear carbonated beverage in household bleach in vinegar in mineral water in liquid soap in rubbing alcohol in hydrogen peroxide in liquid shampoo in mouthwash in safety glasses Acids and Bases page Some of these solutions can burn your eyes or skin. Some are poisonous. Be sure to wear safety glasses. Do not taste any of the solutions. When you are finished, allow your teacher to dispose of the solutions. Procedure 1. One member of your group should take the well plate to the solution station. Place 4 5 drops of water in well A1. Place 4 5 drops of each of your solutions into wells A2 through A6 in Row A of your well plate. Analyze Your Data 2. Each member of your group should have 6 pieces of ph paper. Take turns testing the ph of each of the solutions in your well plate. Use a fresh strip of ph paper for each solution. After you see the color the paper turns, match its color to the colors on the ph scale, and record the ph of each solution on your Acids and Bases page. 3. Allow your test strips to dry, and then tape them on your Acids and Bases page. Work with your group to analyze your data, and make sure you all agree on the ph of each solution. If group members determined a different ph for any of the solutions, measure the ph of those solutions again. Retest the liquids until you are satisfied that you have accurately determined their ph. 1. Which of your solutions were neutral? How do you know? 2. Which of your solutions were acidic? How do you know? Which was most acidic? How do you know? 3. Which of your solutions were basic? How do you know? Which was most basic? How do you know? 4. Compare your predictions with your results. Which solutions were you able to predict accurately? Which solutions were difficult to predict? Why? AQ 212

6 4.5 Investigate Communicate Share Your Results As a class, arrange all the solutions in order from most acidic to most basic. Each group measured the ph of different solutions. Each member of the group measured the ph of each of the group s solutions. As you are arranging the solutions in order, compare the results of those measurements. For each liquid for which members of the group recorded different ph results for the same solution, retest the liquid until you are satisfied that you have accurately determined the ph. Earlier in the Unit, you saw that, when you diluted colored water, you produced solutions that had fewer parts per million of the dye. Dilution is also a way to change the acidity of a solution. Choose one of your solutions, and predict what would happen to its ph if you added water to the solution. What Is the ph of Acid Rain? You cannot directly measure the acidity of acid rain, because you most likely do not have a sample of acid rain in front of you. You can, however, model acid rain and measure the acidity of the water in your model. You can model acid rain by capturing smoke from combustion and dissolving it in water. It will not have all of the products of combustion from a coal-burning power plant, but it will have some of them. Your teacher has burned some paper, captured the smoke, and then dissolved the smoke in water. You will measure the ph of the resulting solution. Procedure Slowly open your bag containing the solution of smoke dissolved in water, and pour the solution into the cup. Dip a strip of ph paper in the water. Record your observations. Analyze Your Data 1. What was the color of the ph paper after it was dipped in the smoke/water solution? Use the ph scale to determine the ph of the smoke/water solution. Materials ph paper clear plastic cup bag with smoke/ water solution 2. How did the ph of the smoke/water solution differ from the ph of the distilled water you tested in the previous investigation? What does that tell you about the smoke/water solution? AQ 213 AIR QUALITY

7 Reflect 1. Compare the acidity of the smoke/water solution to the acidity of the household liquids you tested earlier. Which liquid is closest in acidity to the smoke/water solution? 2. Normal rainwater is slightly acidic (ph = 5.6). How much more acidic is the smoke/water solution? Why do you think rainwater that contains the products of combustion is more of a problem for the environment than normal rainwater? 3. Your smoke/water solution had only some of the products of combustion dissolved in it. How do you think the ph of the smoke/water solution compares to the ph of acid rain? Why? 4. Earlier in the Unit, you learned about measuring concentration using parts per million (ppm) and parts per billion (ppb). ph is also a measure of concentration. How do you think the concentration of hydrogen ions in ppm or ppb is different when ph is higher than when it is lower? Update the Project Board Acid in the air is a big problem in the Adirondacks. As air pollutants mix with water, acid rain is formed. Add your understanding of acids and acid rain to the Project Board in the What are we learning? column. Be sure to include evidence from your reading and from your investigations that help you to support your learning. You may also have some new questions to add to the Project Board. For example, you still do not know exactly how it causes damage to the plants and animals. What s the Point? All substances can be classified as acids, bases, or neutral. Acids produce hydrogen ions when placed in water. The concentration of hydrogen ions in a solution is a measure of its acidity. Hydrogen ions cannot be measured directly, so indicators are used to determine whether a solution is an acid, a base, or neutral. Acid-base indicators include red cabbage juice, litmus paper, and ph paper. A color change in the indicator shows the presence of an acid or a base. No color change indicates a neutral substance. The ph of a substance is a measurement of the concentration of hydrogen ions in a solution. The ph scale has a range of 0-14 and indicates the strength of an acid or a base. When the ph changes by one number on the scale, the level of acidity changes by a factor of ten. AQ 214