acidic, basic, and neutral solutions fall to earth as acid precipitation—rain, snow, and sleet....

Science ContentStandards

Students know how to determine‘hether a solution is acidic, basic or neutral.D Plan and conduct a scientificivestigation to test a hypothesis.DEvaluate the accuracy and!producibility of data.• Construct appropriate graphs fromata and develop quantitative statementsbout the relationships between variables.

Reading Guide

What You’ll LearnDistinguish acids frombases by their properties.

Understand the pH scale.

Identify different ways ofmeasuring pH.

Why It’s ImportantMany chemical reactionstake place in an aqueoussolution, and these reactionsare affected by the presenceof hydronium and hydroxideions.

Vocabularyacidhydronium ionbasepHindicatorpH meter

Review Vocabularyatom: very small particlethat makes up all matter(p. 174)

Acidic, Basic, andNeutral Solutions(Main Idea The pH scale measures the acidity of a solution.

Real-World Reading Connection If your stomach is upsetbecause of excess acid, you might take an antacid. You can thinkof an antacid as the opposite of an acid. Antacids are bases. Theywork in opposition to the way that acids work.

What are acids and bases?You may have heard that acids eat through clothing and even

destroy things made of metal. You may also know of productscontaining bases that are used for the tough jobs of cleaningclogged drains and ovens. These acids and bases are strongproducts. You should protect your hands and eyes with glovesand goggles when you use them. Many other products that youbuy at the supermarket are also acids or bases. For example,vinegar and lemon juice are acids. Soap and baking soda arebases. Figure 15 shows additional acids and bases that you mightknow about.

Figure 15 These foods contain acids, such ascitric acid and folic acid, that are important in yourdiet. Several different bases, including sodium hydroxide and ammonia, are found in cleaning pioducts.

ff1is,.

Vinegar

I;’

94 Chapter 9 • Acids and Bases in Solution

Science ContentStandards

~Students know how to determinewhether a solution is acidic, basic or neutral.e Plan and conduct a scientificinvestigation to test a hypothesis.0 Evaluate the accuracy andreproducibility of data.0 Construct appropriate graphs fromdata and develop quantitative statementsabout the relationships between variables.

Reading Guide

What You’ll Learn~ Distinguish acids from

bases by their properties.

~. Understand the pH scale.

~ Identify different ways ofmeasuring pH.

Why It’s ImportantMany chemical reactionstake ptace in an aqueoussolution, and these reactionsare affected by the presenceof hydronium and hydroxideions.

Vocabularyacidhydronium ionbasepHindicatorpH meter

Review Vocabularyatom: very small particlethat makes up all matter(p. 174)

Figure 16 When anacid reacts with ametal, such as zinc,hydrogen bubblesform.

Figure 17 When anacid reacts with limestone, which is calciumcarbonate, carbon dioxide gas is formed.State whether the abilityto neutralize is a physical orchemical property.

Acidic, Basic, ndNeutral Solu ions(Main Idea The pH scale measures the acidity of a SOlution

Real-World Reading Connection If your stomach is Upsetbecause of excess acid, you might take an antacid. You can thinj(of an antacid as the opposite of an acid. Antacids are bases. Theywork in opposition to the way that acids work.

What are acids and bases?You may have heard that acids eat through clothing and even

destroy things made of metal. You may also know of productscontaining bases that are used for the tough jobs of cleaningclogged drains and ovens. These acids and bases are strongproducts. You should protect your hands and eyes with glovesand goggles when you use them. Many other products that youbuy at the supermarket are also acids or bases. For example,vinegar and lemon juice are acids. Soap and baking soda arebases. Figure 15 shows additional acids and bases that you mightknow about.

Figure 15 These foods contain acids, such ascitric acid and folic acid, that are important in yourdiet. Several different bases, including sodium hydroxide and ammonia, are found in cleaning products.

Acidswhat makes orange juice, dill pickles, and grapefruit juice have

a sour taste? Acids cause the sour taste of these and other foods.An acid is a substance that releases a positively charged hydrogenion, H+, in water. When an acid mixes with water, the acid dissolves, releasing a hydrogen ion.

Properties of AcidsAcids are easily recognized by their sour taste. Citric acid makes

lemons and limes taste sour. Vinegar is sour because it containsacetic acid. However, you should never test any material in the laboratory by tasting it. A way to test for acids is to use litmus paper.Litmus paper contains a compound that changes color in acids andbases. Acids turn blue litmus paper red.

“Reading How does litmus paper change when dipped inCheck an acid?

Acids also react with metals and release hydrogen gas, H2.Figure 16 shows that if you place a piece of zinc in an acidic solution, bubbles of hydrogen gas form. Figure 17 shows that bubblesform when an acid reacts with limestone, or calcium carbonate. Inthis case, the bubbles are carbon dioxide gas. Geologists use thisreaction to test and classify rocks. The ability to neutralize a baseis another important chemical property of an acid. Neutralizationis a chemical reaction between an acid and a base in which a saltand water are formed. A neutral solution or substance is one thatis neither acidic nor basic.

What is a hydronium ion?You have just read that an acid contains a hydrogen atom. When

an acid dissolves in water, the hydrogen ion, H+, separates fromthe rest of the acid molecule and combines with a water molecule.The combination of a water molecule and a hydrogen ion producesa hydronium ion. A hydronium ion is positively charged and hasthe formula H30+. This formula, not H+, accurately representshow the hydrogen ion exists in an aqueous solution.

+ H20 — H3O~++

+

Hydrogen Water Hydroniumion molecule ion

All acidic solutions contain positive hydronium ions. Becausethey contain ions, acid solutions conduct electric currents and arecalled electrolytes.

394 Chapter 9 • Acids and Bases in SolutionLesson 2 • Acidic, Basic, and Neutral Solutions 395

Uses of AcidsYou might be surprised to find out how important acids are in

your life. Your stomach contains an acid that helps break down thefood you eat. Amino acids are the monomers in the protein Polymers that make up your body tissues. Carbonic acid and phosphoric acid maintain a delicate balance in your blood. Acids arealso important in your diet. For example, vitamin C, which isfound in orange juice and other foods, is ascorbic acid. Serioushealth problems can result from a lack of vitamin C.

Acids are also used in making many products. Manufacturersuse sulfuric acid in a wide variety of products, including fertilizers, detergents, plastics, and pesticides. Hydrochloric acid, commonly called muriatic acid, is used as a strong cleaner for bricksand concrete. Like sulfuric acid, hydrochloric acid is used in manufacturing products ranging from rubber to medicine. Figure 18describes how acids can dissolve in water ~‘apor in the atmosphereand fall to Earth as acid precipitation—rain, snow, and sleet.

Explain why acids are important to your body.

BasesNow you know something about acids. You might not be as

familiar with bases as you are with acids. A base is a substancethat produces hydroxide ions when dissolved in water. The formula for a hydroxide ion is OH . Every time you wash yourhands, you are using a base.

Sodium hydroxide, NaOH, and magnesium hydroxide,Mg(OH)2, are common bases. Sodium hydroxide is the mainingredient in some drain cleaners. Magnesium hydroxide is usedin antacids.

Ammonia, NH3, is also a base, but notice that its formula doesnot contain a hydroxide ion. Ammonia acts as a base by acceptinga hydrogen ion from water.

NH3 + H20 — NH4~ + 0H+

H H HN + N + H

H H H H

Ammonia Water Ammonium ion Hydroxide

C, . What are the products of the reaction betweenOre k ammonia and water?


visualizing Acid Precipitation

‘I—

-

‘I

,•. ~. ~‘ ‘a 4, 0’ •

~‘1•~s

a

~I i~

£ k~

i•

~~•1•

Figure 18When fossil fuels such as coal and oil are burned, a variety ofchemical compounds are produced and released into the air. In theatmosphere, some of these compounds form acids that mix withwater vapor and fall back to Earth as acid precipitation: rain, sleet,snow, or fog. The effects of acid precipitation on the environmentcan be devastating. Winds carry these acids hundreds of milesfrom their sources, damaging forests, corroding statues, andendangering human health.

\

Sulfur dioxide and nitrogen oxidesreact with water vapor in the air toform highly acidic solutions of nitricacid (HNO3) and sulfuric acid (H2504).These solutions eventually return toEarth as acid precipitation.

(Power plants and

cars burn fossil fuelsto generate energyfor human use. In theprocess, sulfur dioxide(502) and nitrogenoxides are releasedinto the atmosphere.

Some acid rain inthe United States has apH as low as 2.3. Thisis close to the acidityof stomach acid.

Co tributed by National Geographic Lesson 2 • Acidic, Basic, and Neutral Solutions 397

Properties of BasesLike acids, bases have a common set of properties. In aqueou5

solutions, they feel slippery on your skin. You experience thiswhen you use soap, which contains a base. If you have ever gottensoap in your mouth, you know how bitter it tastes. A bitter taste isanother physical property of bases. Solutions of bases cause red litmus paper to turn blue. Like solutions of acids, solutions of basescontain ions, so they are electrolytes. Bases neutralize acids byforming salts and water.

What is a hydroxide ion?You may be wondering where the ions in a solution of a base

come from. Most bases are ionic compounds. When a base dissolves in water, it separates into a positive ion and a negativehydroxide ion, as shown in Figure 19. All the properties of basesare a result of the presence of hydroxide ions that form when thebase is dissolved in water.

Figure 19 Sodium hydroxide is a base because it produces hydroxide ionsIii when it dissolves in water.

Identify the two ions produced in a solution of sodium hydroxide.

Water Sodium ionmolecules

Hydroxide ion

P4 H5~I~&


ses 0? basesYou may already know how handy a base such as magnesium

hydroxide can be if you have indigestion. Magnesium hydroxidejs found in milk of magnesia, a medicine used to soothe stomachdistress. Another base, baking soda, is used to make biscuits andother breads. Gardeners use bases to make acidic soil neutral.Strong bases, such as sodium hydroxide, are used for cleaningbecause they are able to eliminate grease. Bases are also used toproduce new products. For example, sodium hydroxide is usedto manufacture soap, rayon, and paper. Calcium hydroxide isused to make plaster and mortar.

Not all bases are equally strong. The same is true of acids. Scientists have developed a way of measuring the acidity or basicity ofsolutions. Why is this important?

What is pH?Biologists at the Monterey Bay Aquarium in California must ORD 0mG N

monitor the acidity of water to ensure the survival of the animalssheltered there. Jellies, shown in Figure 20, are sensitive to changes from P (German Potenz;

means potency, power) and Hin acidity. Aquarium biologists must be able to measure and con- (the symbol for hydrogen);

trol the acidity of the water. How do they measure acidity using coined by S. P. L. Sorensen

numbers?

The pH ScaleH is a numerical scale used to indicate how acidic or basic a

solution is. The scale runs from below 0 to above 14. Acidic solutions have a pH below 7. Highly acidic solutions have pH valuesnear 0. Basic solutions have a pH above 7. Highly basic solutionshave pH values near 14. Neutral solutions have a pH of 7.

- If a solution has a pH of 10,15 the solution acidicCheck or basic?

Figure 20 The water in thetank that is home to these jelliesat the Monterey Bay Aquariummust be kept at a pH between8.1 and 8.4.

Lesson 2 • Acidic, Basic, and Neutral Solutions 399

Figure 22 The pH scale is related to the concentration of hydronium ions. As the numberson the scale increase, the concentration ofH30+ decreases.

pH14 1 M NaOH (14.0)

13 Lye (13.0)

12 Household ammonia (11.9)

Milk of magnesia (10.5)10 Detergent solution (—10)

9

8 ~ Sea water (7.0—8.3)Blood (7.4)NEUTRALMilk (6.4)

6 Urine (4.8—7.5)

5 —~ Coffee (5.0)

4

3 Vinegar (2.4—3.4)Lemon juice (2.2—2.4)

2

1Stomach add (1.0— 3.0)

0 1MHCI(0.0)

pH and Hydronium IonConcentration

pH is a measure of the concentration ofhydronium ions, H30+, in a solution. Thehigher the concentration of hydronium ions,the more acidic the solution is. The lower theconcentration of hydronium ions, the morebasic the solution is. It’s helpful to notice thatthe hydronium ion concentrations and pH values go in opposite directions. At low pH values, the concentration of hydronium ions ishigh. As the pH values increase, the hydronium ion concentration gradually decreases.At high pH values, the concentration of hydro.nium ions is low.

Hydronium Ions, HydroxideIons, and pH

All acid and base solutions contain bothhydronium ions and hydroxide ions. What distinguishes an acid from a base is which of thetwo ions is present in the greater concentration. Figure 21 shows that in an acid solution(pH less than 7), hydronium ions are presentin greater concentration than hydroxide ions,0H. In basic solutions (pH greater than 7),the concentration of hydroxide ions is greaterthan the concentration of hydronium ions. Inneutral solutions (pH = 7), the hydronium ionand hydroxide ion concentrations are equal.

f’ Which ion_hydroxide or hydroCheck nium—is present in the greateramount in an add solution?

Milk has a pH of 6.4. What does this pHvalue mean? Figure 22 shows that pH 6.4 isclose to pH 7, which is neutral. At pH 7, thehydronium and hydroxide ion concentrationsare equal. A pH of 6.4 means that milk is aslightly acidic mixture. The concentration ofhydronium ions in milk must be slightlyhigher than in a neutral solution. SimilarlY,blood, with a pH of 7.4, is a slightly basic solution. The hydroxide ion concentration in bloodmust be slightly greater than it is at pH 7.

Acids and Bases in SolutionNeutral

Acidic

Hydroxide ionincreases

Hydronium ionincreases

0 1 2 3 4 5 6 7 8 91011121314pH

Figure 21 Hydronium ions and hydroxideions are present in any water solution. As oneion increases, the other decreases.Infer At what pH are the two concentrations equal?

11I.)U,‘C.0

0

Z0‘I(C

C

400 chapter 9 • Acids and Bases in Solution

comparing pH ValuesMaybe you’re wondering what the numbers on the pH scale

niean. How is the concentration of hydronium ions different atpH 1 from what it is at pH 2? A change in one pH unit representsa tenfold change in the acidity or basicity of a solution. For example, if one solution has a pH of 1 and a second solution has a pH of2, the first solution is not twice as acidic as the first. It is ten timesmore acidic. To determine the difference in acidity or basicity, usethe following calculations. Each pH unit represents a power of 10.pH 1 is represented by 10’, and pH 2 is represented by 102. Thedifference in acidity or basicity between two solutions is represented by lOt’, where n is the difference between two pH values.For example, how much more acidic is a solution with a pH of1 than a solution with a pH of2? pH 2 — pH 1 = 1; ion — 101 =

10 times more acidic.

“Reading How much more acidic is a solution with a pH of 1- - Check than a solution with a pH of 3?

Figure 23 compares the pH values of orange juice and vinegar.Orange juice has a pH of 5, and vinegar has a pH of about 3. Thedifference between the two values is two pH units. How manytimes more acidic is vinegar compared to orange juice? iO~~ = i0~= 100. Vinegar is 100 times more acidic than orange juice.Figure 23 shows that NaOH and detergent differ by four pHunits. Sodium hydroxide is i04, or 10,000, times more basic thandetergent.

Figure 23 Vinegar and orange juice differ by two units, so vinegaris 100 times more acidic than orange juice. Sodium hydroxide is10,000 times more basic than detergent.

S ‘2.

SODIUHHYDROX ThE

LaundryDetergw’t

(A c~. Or. I: Q~ I II a”

rence = 2


Neutralization and pHHow do antacids, such as milk of magnesia, relieve indigestion?

Indigestion is caused by excess stomach acid, which is hydroch1o~~~acid, HCI. Hydrochloric acid helps break down the food you eat,but too much of it can irritate your stomach or digestive tract.Milk of magnesia contains the base magnesium hydroxide,Mg(OH)2. Stomach acid reacts with magnesium hydroxideaccording to the following equation: -

2HCI + Mg(OH)2 - MgCI2 + 2H20

Figure 24 The hydroxideion in lime, Ca (OH)2, neutralizes excess hydrogen ion inacidic or “sour” soil.

This is a neutralization reaction. Recall that neutralization is achemical reaction between an acid and a base that produces waterand a salt. In the equation above, the salt is magnesium chloride,MgCI2. Neither of the products, magnesium chloride nor water, isacidic or basic. The resulting solution has a pH of 7, and so it isneutral.

2 “ReadingCheck

What are the products of the reaction betweenmagnesium hydroxide and hydrochloric acid?

Sometimes you need to know the amount of acids or bases in asolution. For example, you may want to determine the amount ofacetic acid in vinegar. This can be done by using a process calledtitration (ti TRAY shun), in which a solution of known concentration is used to determine the concentration of another solution.Neutralization is used to find out how much acid is contained inan unknown solution. Slowly add a base with a known concentration to the unknown acid while measuring the mixture’s pH.When the pH equals 7, the acid is neutralized. The amount of basethat was added is equal to the amount of acid originally present.Read on and find out how to measure pH.

How is pH measured?Providing a suitable environment for jellies is only one reason

why knowing the pH of a solution is important. The pH of shampoo is balanced so that it does not damage hair. Acid levels in foodare controlled for people with sensitive stomachs. Farmers andgardeners must measure and adjust the pH of the soil to grow certain plants. In Figure 24 a gardener is adding lime, Ca(OH)2 toraise the pH of acidic soil. The pH of swimming pools must becarefully controlled to prevent the growth of bacteria and algae. Inall of these instances, indicators, pH strips, or pH meters are usedto keep track of pH.

~4’Reading Explain why knowing the pH of solutions is impor‘s Check tant to humans and animals.


Figure 25 Using indicators is one way tomonitor the pH of this swimming pool. If thepH is too high or too low, the water coulddamage your skin or bacteria could grow.

indicators ACADEMIC VOCABULARYYou may have seen someone use indicators to test the pH of approximate

swimming pooi water, as shown in Figure 25. An indicator is a (adjective) nearly correct or

compound that changes from one color to another within a partic- She did a quick mental calcula

ular pH range. Figure 26 shows the color changes for some corn- tion and came up with an

mon indicators. Notice that the pH ranges for the color change are approximate answer.

different for each indicator. If you choose the correct indicator,you can determine the approximate pH of any solution. The universal indicator at the bottom of Figure 26 is a mixture of a number of indicators. The universal indicator changes color acrossmost of the pH range. When the indicator is added to an acidic ora basic solution, the solution changes color. The color can bematched to standard colors. Figure 26 shows that a yellow colormeans that the solution has a pH of about 6.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14crystal violet

cresol red

Thymol blue

Bromphenol blue

Methyl orange

Bromcresol green

Methyl red

Bromcresol purple

AlizarinIgure 26 A blue color indicates a

Bromthymol blueof about 5 for bromcresol green.Phenol redIttefy the p11 of a solution if methyl red

es a yellow color. PhenolphthaleinThymolphthalein

Alizarin yellow GG

Universal indicator


,___—.__ __.

pH 054

j~b——

1 1 ~ 4 S S 7•0 ••

•••• ••~ II II II CT S I S

Figure 27 pH testing strips are a quickway of determining the approximate pH ofa solution.

Figure 28 A pH meter measurespH to the hundredths of a pH unit.State the pH of vinegar and the pH ofmilk of magnesia.

1~.IWflLLID 1T-

H -TUE La

~05cZA V000

e,.

pH StripsYou have probably used litmus paper to test for

acids and bases. Litmus is one of the simplestindicator test papers, but provides the least information. Recall that blue litmus paper turns redin acid solutions. Red litmus paper turns blue inbasic solutions. No change in color occurs if thesolution is neutral. Other pH test strips providethe approximate pH of the solution. These stripscontain a universal indicator that can change to avariety of colors across the pH range. When youdip the strip into a solution, the strip changescolor. You then match the new color of the stripto one of the standard colors. Notice in Figure 27that each standard color corresponds to a different pH value.

pH MeterspH strips are quick and convenient to use.

However, if you want an accurate measure ofpH, you need a pH meter. A pH meter is anelectronic instrument with an electrode that issensitive to the hydronium ions in a solution.Figure 28 shows that you can measure the exactpH of a solution quickly and accurately.

Because it is an electronic instrument, a pHmeter requires a power supply. Handheld pHmeters have been developed with portable powersupplies such as batteries. These can be takenoutside the laboratory for measuring pH in theenvironment. People can monitor changes in theenvironment by using a handheld meter to studythe pH of the water in lakes, streams, and theocean.

Milk ofMagnesia


what do you know about acidsand bases?

Acids and bases are identified by the ions that form in theiraqueous solutions. Acids form hydronium ions, H30+, and basesproduce hydroxide ions, 0H. The reaction that occurs betweenany acid and any base is called neutralization. In neutralizationreactions, an acid and a base react to form a neutral solution ofsalt and water. The pH of a solution is a measure of the hydroniumion concentration. pH 7 is at the center of the pH scale, neitheracidic nor basic. If a solution has a pH less than 7, the solution isacidic. Any solution with a pH greater than 7 is basic.

ESSUN2 Review

SummarizeCreate your own lessonsummary as you design astudy web.

1. Write the lesson title,number, and page numbers at the top of a sheetof paper.

2. Scan the lesson to findthe main headings.

3. Organize these headingsclockwise on branchesaround the lesson title.

4. Review the informationunder each headingto design a branch foreach blue subheading.

5. List 2—3 details, key terms,and definitions from eachblue subheading onbranches extendingfrom the main headingbranches.

ELA8: R 23

Using Vocabulary

1. How does a hydronium iondiffer from a hydroxideion? 0

2. Write a definition of pH usingyour own words.

Understanding Main Ideas

3. Identify the pH ranges ofacidic solutions and basic solutions.

4. Describe how an indicatorworks.

5. Compare the concentrationsof hydronium ions and hydroxide ions in a neutral solution.0

6. Summarize InformationCopy the graphic organizerbelow and fill in the propertiesof acids and bases. 0

7. Draw a diagram that showshow the concentration ofhydronium ions in a solutionchanges as pH increases fromOto 14.

Applying Science

8. Hypothe&ze how the pHof white vinegar would changeif an ammonia solution wasadded to it a drop at a time.White vinegar has a pH of3.1.

9. Identify which is a propertyof an acid.

A. turns litmus redB. has a bitter tasteC. hasapH of 10D. reacts with acids

SciencFor more practice, visit StandardsCheck at ca8 msscience.com.

Acids and Bases ca8.msscience.com

Standards Check

Lesson 2 • Acidic, Basic, and Neutral SoLutions 405

acidic, basic, and neutral solutions fall to earth as acid precipitation—rain, snow, and sleet....

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