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KEY IDEAS
As you read this section, keep these questions in mind:
•
What properties do acids have?•
What properties do bases have?•
How can pH tell you if something is an acid or a base?What Are the Properties of Acids?
Does the thought of eating a lemon cause your mouth to pucker? You expect a lemon to taste sour. All the foods in the figure below taste sour because they contain acids.
When acids dissolve in water, they ionize, or form ions. When acids ionize, they form hydrogen ions, H+. The hydrogen ions bond to water molecules, H
2O, to form hydronium ions, H
3O+. An acid is a compound that increases the number of hydronium ions when it dissolves in water. Hydronium ions give acids their properties.
Some properties of acids are listed below.
• sour taste
• conduct electricity
• turn blue litmus paper red
• corrosive (able to destroy or eat away certain materials)
READING TOOLBOX
Outline Make an outline of the material in this sec- tion. Use the headers in the section to help you organize your outline.
READING CHECK 1. Define What is an acid?
KEY IDEAS
1
Citrus fruits, such as lemons, grapefruits, limes, and oranges, contain citric acid.
Apples contain malic acid. Grapes contain tartaric acid.
What Is the Difference Between Strong and Weak Acids?
All acids ionize when they are dissolved in water.
However, some acids ionize completely, while others do not. The relative numbers of acid molecules that ionize in water determine if the acid is strong or weak.
STRONG ACIDS
The figure below shows what happens to nitric acid when it is dissolved in water. The arrow pointing to the right shows that all of the nitric acid ionizes.
Nitric acid is a strong acid. When strong acids are dis- solved in water, they ionize completely. This means that all the dissolved acid molecules break apart into ions.
When nitric acid ionizes, it forms hydronium ions and nitrate ions. The ions move around freely in the water and can conduct electricity. A substance that conducts electricity when it is dissolved in water is called an electrolyte. Strong acids are strong electrolytes. This is because solutions of these acids contain as many hydronium ions as the acid can possibly form.
WEAK ACIDS
Weak acids, such as acetic acid, do not completely ionize. Only some of the molecules of acid ionize in water. Examine the figure at the top of the next page.
The arrow pointing to the right shows that dissolved acetic acid molecules break apart into ions. The arrow pointing to the left shows that ions are also recombining to form molecules. Therefore, a solution of a weak acid contains fewer hydronium ions than a solution of a strong acid with the same concentration.
READING CHECK
2. Identify What determines whether an acid is strong or weak?
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3. Identify What happens to the total charge of the solution when HNO3 dissolves?
READING CHECK 4. Define What is an electrolyte?
+
+ +
_
HNO3 + H2O H3O+ + NO3 –
Nitric acid Water Hydronium ion Nitrate ion
Because there are few ions in a solution of a weak acid, the solution does not conduct electricity very well.
Thus, weak acids are weak electrolytes.
ACID DANGERS
Although many products you use every day contain acids, strong and concentrated acids can be very danger- ous. For example, your stomach makes a very strong acid called hydrochloric acid. You need this acid to help you digest food. Normally, your stomach has a thick lin- ing that protects it from the strong acid. However, if your body makes too much acid, the acid can destroy your stomach.
Even weak acids are not always safe to handle. Acids can damage your skin. The vapors are also harmful to your eyes, lungs, and mouth. This is why you should always wear goggles, gloves, and a laboratory apron when you work with acids. Never taste a chemical to determine if it is an acid.
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5. Infer Which of the two beakers contains more ions?
How do you know?
6. Identify Which beaker contains a strong acid?
Explain your answer.
READING CHECK 7. List Identify three pieces of safety equipment you should use when you work with an acid.
Both of these beakers contain acids. One contains a weak acid and one contains a strong acid.
+
+ +
_
CH3COOH + H2O H3O+ + CH3COO–
Acetic acid Water Hydronium ion Acetate ion
Some Common Acids
Acid Formula Strength Uses
Hydrochloric acid HCl strong Cleaning
masonry;
treating metals before plating or painting;
adjusting pH of swimming pools Sulfuric acid H2SO4 strong making fertilizers;
the acid in car batteries
Nitric acid HNO3 strong making fertilizers
and explosives
Acetic acid CH3COOH weak making
chemicals, plastics, and medicines; the acid in vinegar
Formic acid HCOOH weak dying cloth; the
acid used by stinging ants Citric acid H3C6H5O7 weak making flavorings
and soft drinks;
the acid in citrus fruits
What Are the Properties of Bases?
Baking soda is an example of a common base. It is used to make cakes and cookies and may be found in toothpastes and cleaning supplies. A base is a compound that forms hydroxide ions, OH−, when it is dissolved in water. Hydroxide ions give bases their properties. Some of these properties are listed below.
•
bitter taste•
feel slippery•
conduct electricity•
turn red litmus paper blue•
corrosiveEHHDBG@<EHL>K
8. Identify Which common acid is found in car batteries?
9. Identify Is the acid in vinegar strong or weak?
READING CHECK 10. Identify What kind of ions do bases form when they are dissolved in water?
All these products contain acids.
STRONG BASES
When strong bases are dissolved in water, they ionize completely. Most strong bases are made up of metal ions and hydroxide ions. These kinds of bases are called metal hydroxides.
Sodium hydroxide is an example of a metal hydroxide.
Some drain cleaners contain sodium hydroxide. Sodium hydroxide completely ionizes when it is put into water.
Sodium hydroxide and other strong bases are strong electrolytes. That is, they conduct electricity well when they are dissolved in water.
WEAK BASES
Ammonia is an example of a weak base. Ammonia, NH3, does not contain hydroxide ions. Where do the oxygen and hydrogen that make up the hydroxide ion come from? Ammonia reacts with water to form OH−. This reaction is shown in the figure below.
The double arrow tells you that ammonia does not ionize completely when it is dissolved in water.
Therefore, ammonia is a weak base and a weak electrolyte.
DANGERS OF BASES
The table on the next page gives examples of common bases. Many bases are used in soaps, dyes, and fertilizer.
However, concentrated bases can be very dangerous.
They may dissolve many materials, including skin. When you work with bases, you should wear safety goggles, gloves, and a laboratory apron to protect yourself.
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11. Compare In terms of ionization, how are strong acids and strong bases similar?
READING CHECK 12. Explain How do bases that do not contain hydroxide ions form hydroxide ions?
+
+ +
_
NH3 + H2O NH 4+ + OH–
Ammonia Water Ammonium ion Hydroxide ion
Some Common Bases
Base Formula Strength Uses
Potassium hydroxide
KOH strong making soap; dyeing
products Sodium
hydroxide
NaOH strong making soap; refining
petroleum; cleaning drains;
making synthetic fibers Calcium
hydroxide
Ca(OH)2 strong treating acid soil; treating lakes polluted by acid rain;
making mortar, plaster, and cement
Ammonia NH3 weak fertilizing soil; making
fertilizers; making nitric acid; making cleaning solutions
Methylamine CH3NH2 weak making dyes and
medicines; tanning leather Aniline C6H5NH2 weak making dyes and varnishes;
used as a solvent
What Is the Difference Between Strong and Concentrated?
Many people think the words concentrated and strong mean the same thing. Similarly, many people think that weak and dilute mean the same thing. In chemistry, how- ever, these words have different meanings.
Recall that the concentration of a solution refers to the number of solute particles in a particular amount of solution. A solution with a large number of solute particles is concentrated. A solution with a small number of solute particles is dilute.
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13. Identify Identify two common bases used in making soap.
All these products contain bases.
STRONG AND DILUTE
Suppose a scientist added a very small amount of hydrochloric acid to a liter of water. Because the solution contains only a small amount of solute, you could describe it as dilute. However, when any amount of hydrochloric acid is dissolved in water, it ionizes completely. At any concentration, hydrochloric acid is a strong acid. Thus, a strong acid may form a dilute solution.
What Is pH?
How can you tell if a solution is acidic or basic? One way is to use an indicator. An indicator is a substance that changes color if a solution is acidic or basic. Litmus paper and pH paper are examples of indicators.
An indicator can tell you something is an acid or a base. However, in many cases, you need to know how acidic or basic a solution is. The pH of a solution is a value that expresses how acidic or basic a solution is.
The pH scale ranges from 0 to 14. Solutions with a pH less than 7 are acids. Solutions with a pH greater than 7 are bases. Solutions with a pH of 7 are neutral. A neutral solution is not acidic or basic.
Acidic: pH < 7 Basic: pH > 7 Neutral: pH = 7
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14. Infer Can a weak base form a concentrated solution? Explain your answer.
READING CHECK 15. Identify Is a solution with a pH greater than 7 basic or acidic?
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16. Identify Is a soft drink more acidic or less acidic than milk?
17. Analyze Relationships What happens to acidity as pH increases? What happens to basicity as pH increases?
Increasing acidity Increasing basicity
Lemon juice
Soft
drink Human
saliva
Tap water
Acid rain Clean rain
Human stomach contents Sea
water
Detergents Household ammonia Milk
1 2 3 4 5 6 7 8 9 10 11 12 13
RELATIVE CONCENTRATIONS OF IONS
The pH of a solution indicates the concentration of hydronium ions. An acidic solution has a greater concen- tration of hydronium ions than hydroxide ions. A basic solution has a greater concentration of hydroxide ions than hydronium ions. A neutral solution has equal con- centrations of hydronium ions and hydroxide ions.
CONCENTRATION AND pH
Recall that you can use molarity (M) to describe the concentration of a substance in a solution. You can use the concentration of hydronium ions in a solution to determine the solution’s pH.
In a solution, strong acids like HCl and HNO3 produce one hydronium ion for each dissolved acid particle. The concentration of a strong acid indicates how many parti- cles are in the solution. Because every particle of a strong acid that dissolves will ionize, the hydronium ion concen- tration equals the acid concentration.
Writing the H
3O+ concentration in scientific notation can help you determine pH. When the concentration of hydronium ions is one times ten to a power, the pH is the negative of the power of 10. For example, pure water has a hydronium ion concentration of 0.0000001 mol/L, or 1 × 10–7 M. Thus, pure water has a pH of 7.
CALCULATING pH
A hydrochloric acid solution has a H3O+ concentration of 0.0001 M. What is the pH of the solution?
Step 1: List the given and unknown values.
Given:
concentration of HCl = 0.0001 M
Unknown:
pH
Step 2: Write the molar
concentration of hydroxide ions in scientific notation.
concentration of H3O+
= 0.0001 M = 1 × 10-4 M Step 3: The pH is the negative
of the power of 10 in the H3O+ concentration.
concentration of H3O+ ions
= 1 × 10-4 M pH = -(-4) = 4
The solution has a pH of 4.
READING CHECK 18. Describe Describe the relative concentrations of hydronium and hydroxide ions in an acidic solution.
Skills Math
19. Calculate HNO3 is a strong acid. What is the pH of a 0.01 M solution of HNO3?
INTERPRETING pH DIFFERENCES
As you have seen, pH is based on powers of 10.
Because of this, small differences in pH reflect big differ- ences in hydronium concentrations. For example, the pH of apple juice is about 3. The pH of coffee is about 5. The difference in pH of the two liquids seems small. However, the difference of two pH units is actually quite large.
Because each pH unit represents a power of 10, a pH difference of two represents a difference in acidity of 102, or 100. Thus, apple juice is 100 times more acidic than coffee. Antacid tablets are basic. When antacid tablets are dissolved in water, the solution they form has a pH of about 8. Thus, coffee is 103, or 1,000 times, as acidic as an antacid tablet solution.
MEASURING pH
Indicators such as litmus paper do not measure pH very precisely. To measure pH precisely, you can use a pH meter. Because ions in a solution have electric charge, they can conduct electric current. A pH meter measures pH by determining the electric current created by the movement of ions.
Skills Math
20. Calculate Solution A has a pH of 9. Solution B has a pH of 3. How much more acidic is solution B than solution A?
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21. Apply Concepts Is the tomato acidic or basic?
Explain your answer.
22. Infer How is the pH value shown on the pH meter more precise than pH values you have seen so far?
A pH meter can measure the H3O+ concentration precisely.
SECTION VOCABULARY
acid any compound that increases the number of hydronium ions when dissolved in water base any compound that increases the number
of hydroxide ions when dissolved in water electrolyte a substance that dissolves in water
to give a solution that conducts an electric current
indicator a compound that can reversibly change color depending on conditions such as pH
pH a value that is used to express the acidity or basicity of a system; each whole number on the scale
1. Write Equations Write an equation showing the ionization of hydrochloric acid in water.
2. Explain How do a strong acid and a weak acid behave differently when each is dissolved in water?
3. Identify Relationships What is the relationship between the strength of an acid or base and the strength of the electrolyte that it forms?
4. Classify Suppose you have a solution that has 4 times as many hydronium ions as hydroxide ions. Is this solution acidic, basic, or neutral?
5. Arrange Arrange the following substances in order of increasing acidity: vinegar (pH = 2.8), stomach acid (pH = 2.0), and a soft drink (pH = 3.4).
6. Graph Trends Create two graphs to describe how pH changes as acidity and basicity increase. Be sure to label the axes of your graphs.
