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Water and SolutionsIntroduction to Chapter 25

This chapter explores the chemistry of acids and bases and explains the pH scale. Inthe first Investigation, you will create a pH scale using a natural indicator solution.You will learn about the applications of acid-base chemistry in household products,medicine and the environment. In the second section and Investigation, you willlearn about and simulate the effects of acid rain.

Investigations to Chapter 25

In this Investigation, a natural indicator and household chemicals are used to createa color-based pH scale. You will use your pH scale to figure out the pH of additionalhousehold chemicals and two mystery solutions.

In this Investigation, you will model the effects of acid rain on a natural ecosystem.You will observe the effect of different dilutions of an acid on the activity of waterfleas (Daphnia magna). Water fleas are an important source of food for fish andother organisms in fresh water environments. In this Investigation, you will alsohave the opportunity to research topics related to acid rain on the Internet.

25.1 Acids, Bases, and pH What is pH?

25.2 Acid Rain What is acid rain?

Chapter 25Acids

andBases

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Chapter 25: Acids and Bases

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Learning Goals

In this chapter, you will:

Describe pH as a way to measure the strength of acids and bases.

Understand the definitions of an acid and a base.

Identify pH of common household chemicals.

Understand the cause of acid rain.

Understand the environmental effects of acid rain.

Demonstrate the effect of acid on a natural ecosystem.

Vocabulary

acid base pH smogacid precipitation electrolytes pH indicatoracid rain neutral pH scale

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25.1 Acids, Bases, and pH

Figure 25.1: Acids and bases play important roles in digestion.

25.1 Acids, Bases, and pHIn the Investigations, you measured several indicators of water quality, including the pH. You mayremember that pH values range from 0 to 14. Low values (0 to 6) indicate that a solution is acidicwhereas high values (8 to 14) indicate that a solution is basic. Your stomach secretes one of the strongeracids, hydrochloric acid (HCl). As acidic solutions of digested food leave your stomach, other organs inyour digestive system secrete bicarbonate, a base. The added base neutralizes the strong, corrosive acid.

As you can see, acids and bases play a significant role in how your body works. What exactly are acidsand bases? What does pH mean?

Why is pH important?

The pH of waterindicates its

quality

Water quality is evaluated using pH values for many reasons. For example, if thepH of your tap water is too high, it might indicate that calcium or magnesiumdeposits are forming in and may clog your water pipes. On the other hand, if thepH is too low, the water may be corroding your pipes.

The pH of water is important to life. The pH of natural bodies of water also has tobe just right—neither too high nor too low. For example, at a pH of 4 or 5, fishhave trouble reproducing. At even lower pH values, they die.

Acids and basesare defined by pH

values

The pH of water is related to the action of a class of chemicals called acids andbases. A solution with a pH value that is less than 7 contains an acid, and asolution with pH values greater than 7 contains a base. You will learn how pH isdetermined later on in this section.

What are acids and bases?

Acidic solutionshave more H+ions, and basicsolutions havemore OH- ions

An acid is a chemical that contributes hydrogen ions, H+, to a solution. A base is achemical that contributes hydroxyl ions, OH-, to a solution. Therefore, solutionscan be described as “acidic” or “basic” according to the concentrations of H+ andOH- ions in the solution. A solution with a high concentration of H+ ions and fewOH- ions is strongly acidic. A strongly basic (or alkaline) solution has a highconcentration of OH- ions and few H+ ions.

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HCl is an acid Hydrochloric acid (HCl) is a very strong acid which can seriously irritate yournose if you smell it and your skin if it spills on you! When HCl dissolves in water(as indicated by the symbol “aq”), it ionizes to become H+ ions and Cl- ions:

An acid such as HCl ionizes almost completely when dissolved in water andtherefore contributes many H+ ions to a solution. Because of this, HCl is a knownas a strong acid.

NaOH is a base Sodium hydroxide (NaOH) is a base commonly used for rigorous housecleaningand for unclogging drains. As you can see from the chemical formula, NaOH willrelease OH- ions when it dissolves in water:

Since NaOH ionizes almost completely when dissolved in water and contributesmany OH- ions to a solution, it is known as a strong base.

Weak acids andbases

Many acids and bases do not ionize completely in water and are known as weakacids and bases.

Vinegar, also known as acetic acid, is a weak acid because it contributes a few H+ions to solution. Vinegar is sometimes used for cleaning but is not very irritating tothe skin. Your nose may not like it, however!

A weak acid or base ionizes incompletely when dissolved in water as shown by thedouble arrows in the chemical equation above.

Household ammonia is an example of a weak base because it does not completelyionize in water.

Hydronium Ions

In an acid solution, separatehydrogen ions (H+) do notexist by themselves for verylong. Each hydrogen ion isattracted to the oxygen end ofa water molecule. The twocombine to form a hydroniumion. In chemistry, it iscommon to refer to H3O+ ionsas H+ ions.

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25.1 Acids, Bases, and pH

Figure 25.2: The pH scale is based on the concentration of hydrogen ions in solution.

pH and the pH scale

pH is based on theconcentration of

H+ ions insolution

Just as centimeters describe length, pH describes the exact concentrations of H+ions in a solution. Most instruments that measure pH use a pH scale that runs from0 to 14. At pH 1, a solution is strongly acidic. At pH 14, a solution is stronglybasic, or alkaline.

pH 7 means equalH+ and OH- ions

A solution that has a pH of 7 has equal numbers of H+ and OH- ions and is calleda neutral solution.

Low pH meanshigh H+

concentration

Everyone is familiar with using a ruler to measure things. However, a pH scale isdifferent from a ruler in that the more H+ ions there are in a solution, the lower thepH value. Conversely, fewer H+ ions in a solution means a higher pH value.

The pH scale isbased on powers

of 10

A pH scale is based on powers of ten. A solution that is pH 1 has 10 times morehydrogen ions than a solution of pH 2. In turn, the pH 2 solution has 10 times morehydrogen ions than a solution of pH 3.

Another way of looking at the scale is to see that as the pH numbers increase, theamount of H+ ions decreases by powers of 10. Figure 25.2 illustrates the pH scale.For example, a solution at pH 1 has 10 million times more hydrogen ions than asolution at pH 7, and a solution at pH 7 has 10 million times more hydrogen ionsthan a solution at pH 14!

You now know that the amount of H+ in a solution at pH 6 is 10 times the amountin a solution at pH 7. But a solution at pH 7 is neutral. What is the amount of OH-ions in a neutral solution? What is the amount of OH- ions in a solution at pH 6?

Acids have moreH+ ions, bases

have more OH-ions

The answers to these questions are that the amount of OH- ions in a neutralsolution is equal to the amount of H+ ions. If you go down the pH scale to 6, theamount of OH- ions decreases by a power of 10 whereas the amount of H+increases by a power of 10. Remember:

• pH values below 7 means H+ ions outnumber OH- ions, and• pH values higher than 7 means OH- ions outnumber H+ ions.

Since each pH is an increase or decrease by tenfold, there is a big difference in ionconcentration between each pH value.

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The ionization of water

Water is both aweak acid and a

weak base

Water plays an important role in acid and base chemistry. Water itself is both aweak acid and a weak base. A water molecule can split or ionize into H+ and OH-ions. Because the H+ ions immediately attach themselves to water molecules, theequation for the ionization of water is written as:

2H2O ↔ H3O+ + OH-

What does thedouble-headed

arrow mean?

The double-headed arrow in the equation means that water, if left alone, is alwaysin equilibrium with its ions. The rate at which water breaks down into ions is thesame as the rate at which the ions re-form into water. However, water ionizes soslowly that most water molecules exist whole, not as ions.

Examples of acid and base chemistry

Acids and basesplay a role in

digestion

Many reactions, such as the ones that occur in your body, work best at specific pHvalues. For example, acids and bases are very important in the reactions involvedin digesting food (see figure 25.1). As you may know, the stomach secreteshydrochloric acid (HCl), a strong acid (pH 1.4). The acidity of our stomachs isnecessary to break down the protein molecules in our food so that they can beabsorbed. A mucus lining in the stomach protects it from the acid produced. Asfood and digestive fluids leave the stomach, however, other organs in the digestivesystem also need to be protected from the acid. This is accomplished by two partsof the system—the pancreas and liver. These two organs secrete bicarbonate toneutralize the stomach acid before it reaches other organs.

Acids and baseshave many uses

The reactivity of acids and bases means that these chemicals have many uses.They react with each other and other chemicals. Metals and glass can be etchedwith corrosive acids. Lye (pH 13) is used to unclog drains, and sulfuric acid (pH 1)is used throughout industry. Millions of tons of sulfuric acid are produced eachyear (40 million tons in 1990). Its uses range from making rayon to cleaningimpurities from gasoline.

Figure 25.3: .Water ionizes into H+ and OH- ions. Therefore, it acts like both a weak acid and a weak base. Pure water has a neutral pH (pH 7) because the concentrations of H+ and OH- ions are equal.

Figure 25.4: Carbon dioxide gas (CO2) escaping from carbonated water. The CO2 molecules are chains of three atoms, and the water molecules have a triangular shape. CO2 makes soda fizzy and reacts with water to form carbonic acid, which gives the soda a sharp, or acidic, taste. This and other acids in soda, like phosphoric acid, can dissolve tooth enamel. Be sure to brush your teeth after drinking soda!

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25.1 Acids, Bases, and pH

�Electrolytes and your body

When you perform astrenuous activity, your bodycools itself by sweating.Sweat contains water anddissolved salts (orelectrolytes) like sodium andpotassium. Before, during,and after exercising, you canreplenish fluids and yourbody’s electrolytes bydrinking diluted fruit juice,slightly salty water, or byconsuming a sports drinks.The water in these fluidshelps your body continue tocool itself so that you don’tget overheated. By replacingelectrolytes, you may behelping your body speed upresorption of fluids. Dilutedfruit juice or a sports drinkcontains small amounts ofcarbohydrates to give yourbody the energy boost it mayneed during strenuousexercise.

Electrolytes and nonelectrolytes

Electrolytesconduct current

Current is the flow of charge. When a solution contains dissolved ions (chargedparticles), it can conduct current. Chemicals that conduct current when dissolvedin water are called electrolytes. These chemicals form ions when dissolved.

Salt dissociates inwater

Ionic compounds, molecular compounds, and even atoms can contribute ions to asolution. When an ionic compound is dissolved in water, the polar ends of thewater molecule attract the positive and negative ions in the solution. In previoussections, you learned how water dissolves table salt (NaCl). Recall that theattraction of the polar ends of the water molecule is strong enough to break theweak ionic bonds of NaCl. When an ionic compound is brought into solution bywater it is said to dissociate. The term ionization is used if a molecular compoundor atom forms an ion.

For example, when NaCl dissolves in water we say it dissociates. When theelement Na loses an electron, it ionizes to Na+.

Acids and basesare electrolytes

All acids and bases are electrolytes because they contribute ions to a solution.Some chemicals, like salt (NaCl), dissociate to form ions in solution but are notacidic or basic. Acids, bases, and salt water are examples of electrolytes.

Non-electrolytesdo not have ions

and are not acidicor basic

Other chemicals do not form ions when they are dissolved in solution. They arecalled non-electrolytes. Non-electrolytes are not acidic or basic. Sugar dissolvedin water is an example of a non-electrolyte.

Electrical appliances and water

Because tap water contains small amounts of dissolved ions, it is anelectrolyte. Remember that even a small amount of current is dangerous ifit enters your body directly? Water provides a way for electric current toenter your body, so always take care when using electrical appliances nearwater!

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The pH of substances you use or consume

Many foods areacidic and many

cleaning productsare basic

Table 25.1 contains a list of some common chemicals and their pH values. Whatdo you notice about this list of substances? Where would you find acids in yourkitchen? Where would you find bases?

Table 25.1: The pH of some common chemicals.

It turns out that many of the foods we consume or use for cooking are acidic. Onthe other hand, many of our household cleaning products are basic.

A pH indicator In the Investigation, you will be testing the pH of common chemicals usinganother item that you may find in your kitchen. You will measure pH using a pHindicator—a chemical that changes color at different pH values.

Common foods can also be used as pH indicators. In the Investigation, theindicator you will use is made from the juice of a red cabbage.

Household chemical Acid or base pHlemon juice acid 2

vinegar acid 3soda water acid 4baking soda base 8.5

bar soap base 10ammonia base 11

Acids, bases, and taste

Our taste buds are sensitive to acids and bases. We taste acids as sour and basesas bitter. Lemon juice is strongly acidic, and soap is strongly basic. Acids thatare stronger than lemon juice and bases that are stronger than ammonia are soreactive that they can harm your skin and damage clothing. Figure 25.5: The pH scale

showing common substances.

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25.1 Acids, Bases, and pH

Figure 25.6: Some causes of low blood pH (acidosis) include holding your breath, excessive lactic acid produced during heavy exercise, and production of ketone acids because of fasting or having low levels of insulin (diabetes). High blood pH (alkalosis) can be caused by hyperventilating. Under normal conditions, your blood pH ranges between 7.35 and 7.45.

Health Perspective: pH and your blood

It is very important for your blood pH to stay within the normal range. At higher or lower pHvalues, your body does not function properly. Fortunately, you can regulate the pH of your bloodsimply by breathing!Blood is a watery solution that contains many solutes including the dissolved gases carbondioxide and oxygen. Carbon dioxide appears in your blood because it is produced by respiration.Recall that respiration is the combustion of sugar by your body. You breathe in oxygen to getthis process going. The end products of this reaction are energy, water, and carbon dioxide.C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

The rate at which you breathe controls the concentration of carbon dioxide in your blood. Forexample, if you hold your breath, more carbon dioxide enters your blood. If you hyperventilate,you blow off carbon dioxide, so that significantly less is in your blood. These two processesinfluence blood pH. The equation below illustrates how carbon dioxide dissolves in an aqueous(watery) solution like blood:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

carbon dioxide + water ↔ carbonic acid ↔ hydrogen ion + bicarbonate ion

When CO2 dissolves, H+ ions are produced in solution. Therefore, the more CO2 in your blood,the more acidic your blood will become. If you breathe slowly, the added CO2 makes your bloodmore acidic. However, if you breathe too often and too quickly (hyperventilating), the loss ofCO2 makes your blood more basic. You can offset this effect by breathing into a paper bag. Thisforces you to re-breathe carbon dioxide.When you breathe normally, your blood pH ranges between 7.35 and 7.45.

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Figure 25.7: If a body of water has a pH of 4.5 or lower, almost nothing can grow in it. Other animals that live near the water, such as frogs and insects, often die as well.

Figure 25.8: Ideally, fish require water within the 6 - 8 pH range.

25.2 Acid RainYou have probably heard about acid rain. You might know that acid rain harms the environment. Butwhat is acid rain?

Life occurs within certain ranges of pH. Most plants and animals function best when pH is nearlyneutral. At the extreme ends of the pH scale, where you find strong acids and strong bases, many lifeprocesses will not occur. For example, fish have trouble reproducing when the pH of their wateryenvironment is too acidic.

Also, plants have trouble growing if soil is acidified below 5.1, and aquatic animals have difficultysurviving in water below pH 5.5. It was recently discovered that lakes and forests in certain parts of theworld were not as healthy as they once had been. The cause was higher acidity in the lakes, resultingfrom rainfall, snowfall, or fog that was highly acidic. How can rain or snow be harmful to theenvironment?

The definition of acid rain

Rain and snow arenaturally acidic

Rainfall, snowfall, or any precipitation happens to be naturally acidic. This isbecause the rain, snow, or fog mixes with carbon dioxide, a gas present in air, andforms small amounts of carbonic acid. The pH of precipitation before theIndustrial Revolution (which started in the mid-1700’s) was probably around 5.6.Therefore, the normal pH of rain and other forms of precipitation is considered tobe 5.6. Any rain, snow, or fog that has a pH lower than 5.6 is called acid rain oracid precipitation.

Acid rain resultsfrom gases

produced duringcombustion

reactions

The chief indicators of acid rain in the atmosphere are the gases sulfur dioxide(SO2), sulfur trioxide (SO3), nitrogen oxide (NO), and nitrogen dioxide (NO2). Alarge part of SO2 in the atmosphere is created by the burning of coal and oil thatcontain sulfur, and by industrial processes such as metal purification. Nitrogenoxide is a leading by-product of fuel combustion from traffic and power plants.

Sulfur trioxide and nitrogen dioxide are created from the reactions of sulfurdioxide and nitrogen oxide, respectively, with oxygen-containing compounds inthe atmosphere. Both of these reactions speed up due to chemicals present inparticulate matter, a common pollutant released by traffic and industrial processes.

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25.2 Acid Rain

Sulfur andnitrogen gases

mix with water toform sulfuric and

nitric acids

Sulfur oxides in the air mix with water, ozone, or hydrogen peroxide to formsulfuric acid (H2SO4). This strong acid is the number one cause of acid rain. Evenbefore acid rain is formed, the four gases that cause acid rain (SO2, SO3, NO, andNO2) create health problems in cities. They are irritants to the respiratory systemand may increase the incidence of asthma and other respiratory ailments.

Nitrogen oxides react with components in the atmosphere to form nitric acid(HNO3), the second greatest cause of acid rain.

Nitric and sulfuric oxides are produced by cars and industry. Theseairborne chemical react with water to form nitric and sulfuric acids.Weather patterns (i.e., wind) can transport these acids to other areas wherethey will fall as precipitation. Acid precipitation can affect the health ofpeople, trees, and aquatic life and erodes buildings and statues.

Figure 25.9: You can help reduce the emission of acid rain gases by using these alternative means of transportation.

Chapter 25 Review

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Chapter 25 Review

Vocabulary review

Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms.

Set One Set Two1. acid a. A chemical that contributes H+ ions to a

solution1. acid rain a. The naturally occurring rock that protects some

areas of the country from acid rain

2. base b. A solution that has no H+ or OH- ions 2. acid precipitation b. Rain with a pH lower than 5.6

3. pH c. A solution that has equal numbers of H+ and OH- ions

3. smog c. The process of turning an acidic or basic solution into a neutral (pH 7) solution

4. neutral d. A chemical that conducts current when dissolved in a solution

4. acid shock d. A sudden increase of acid precipitation into a lake or other ecosystem

5. electrolyte e. The measurement of the amount of H+ and OH- ions in a solution

5. neutralization e. A combination of gases produced by industry and automobiles; a combination of smoke and fog

f. A chemical that contributes OH- ions to a solution

f. Rain, snow, or fog with a pH lower than 5.6