acids and bases

uses of bases

Bases are used for a variety of purposes. For example, soaps contain bases such as potassium hydroxide. Other uses of bases are pictured in Figure 10.9.

textbook_image

acids

An acid is an ionic compound that produces positive hydrogen ions (H+ ) when dissolved in water. An example is hydrogen chloride (HCl). When it dissolves in water, its hydrogen ions and negative chloride ions (Cl ) separate, forming hydrochloric acid. This can be represented by the equation: HCl H2 O + ! H + Cl

properties of acids

You already know that a sour taste is one property of acids. (Never taste an unknown substance to see whether it is an acid!) Acids have certain other properties as well. For example, acids can conduct electricity because they consist of charged particles in solution. Acids also react with metals to produce hydrogen gas. For example, when hydrochloric acid (HCl) reacts with the metal magnesium (Mg), it produces magnesium chloride (MgCl2 ) and hydrogen (H2 ). This is a single replacement reaction, represented by the chemical equation: Mg + 2HCl ! H2 + MgCl2 You can see an online demonstration of a similar reaction at this URL:

reactions of acids and bases

As you read above, an acid produces positive hydrogen ions and a base produces negative hydroxide ions. If an acid and base react together, the hydrogen and hydroxide ions combine to form water. This is represented by the equation: H+ + OH ! H2 O An acid also produces negative ions, and a base also produces positive ions. For example, the acid hydrogen chloride (HCl), when dissolved in water, produces negative chloride ions (Cl ) as well as hydrogen ions. The base sodium hydroxide (NaOH) produces positive sodium ions (Na+ ) in addition to hydroxide ions. These other ions also combine when the acid and base react. They form sodium chloride (NaCl). This is represented by the equation: Na+ + Cl ! NaCl Sodium chloride is called table salt, but salt is a more general term. A salt is any ionic compound that forms when an acid and base react. It consists of a positive ion from the base and a negative ion from the acid. Like pure water, a salt is neutral in pH. Thats why reactions of acids and bases are called neutralization reactions. Another example of a neutralization reaction is described in Figure 10.12. You can learn more about salts and how they form at this URL: (13:21). MEDIA Click image to the left or use the URL below. URL:

textbook_image

textbook_image

textbook_image

why ph matters

Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish also need a pH close to 7. Some air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower (see Figure 10.10). Figure 10.11 shows the effects of acid fog and acid rain on a forest. Acid rain also lowers the pH of surface waters such as streams and lakes. As a result, the water became too acidic for fish and many other water organisms to survive. Even normal (not acid) rain is slightly acidic. Thats because carbon dioxide in the air dissolves in raindrops, producing a weak acid called carbonic acid. When acidic rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms caves, like the one that opened this chapter.

the ph scale

The strength of acids and bases is measured on a scale called the pH scale (see Figure 10.10). The symbol pH represents acidity, or the concentration of hydrogen ions (H+ ) in a solution. Pure water, which is neutral, has a pH of 7. With a higher concentration of hydrogen ions, a solution is more acidic but has a lower pH. Therefore, acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. You can watch a video about the pH scale at this URL: MEDIA Click image to the left or use the URL below. URL:

concentration of ions

The strength of an acid depends on the concentration of hydrogen ions it produces when dissolved in water. A stronger acid produces a greater concentration of ions than a weaker acid. For example, when hydrogen chloride is added to water, all of it breaks down into H+ and Cl ions. Therefore, it is a strong acid. On the other hand, only about 1 percent of acetic acid breaks down into ions, so it is a weak acid. The strength of a base depends on the concentration of hydroxide ions it produces when dissolved in water. For example, sodium hydroxide completely breaks down into ions in water, so it is a strong base. However, only a fraction of ammonia breaks down into ions, so it is a weak base.

strength of acids and bases

The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through skin. The base in antacid tablets is weak enough to take for an upset stomach. The base in drain cleaner is strong enough to cause serious burns. What causes these differences in strength of acids and bases?

detecting acids

Certain compounds, called indicators, change color when acids come into contact with them. They can be used to detect acids. An example of an indicator is a compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of acid on a strip of blue litmus paper, the paper will turn red. You can see this in Figure 10.6. Litmus isnt the only indicator for detecting acids. Red cabbage juice also works well, as you can see in this entertaining video: .

textbook_image

detecting bases

Bases change the color of certain compounds, and this property can be used to detect them. A common indicator of bases is red litmus paper. Bases turn red litmus paper blue. You can see an example in Figure 10.8. Red cabbage juice can detect bases as well as acids, as youll see by reviewing this video: MEDIA Click image to the left or use the URL below. URL:

textbook_image

properties of bases

All bases share certain properties, including a bitter taste. (Never taste an unknown substance to see whether it is a base!) Did you ever taste unsweetened cocoa powder? It tastes bitter because it is a base. Bases also feel slippery. Think about how slippery soap feels. Soap is also a base. Like acids, bases conduct electricity because they consist of charged particles in solution.

bases

A base is an ionic compound that produces negative hydroxide ions (OH ) when dissolved in water. For example, when the compound sodium hydroxide (NaOH) dissolves in water, it produces hydroxide ions and positive sodium ions (Na+ ). This can be represented by the equation: NaOH H2 O ! OH + Na+

uses of acids

Acids have many important uses, especially in industry. For example, sulfuric acid is used to manufacture a variety of different products, including paper, paint, and detergent. Some other uses of acids are illustrated in Figure 10.7.

textbook_image

instructional diagrams

No diagram descriptions associated with this lesson

questions

Acids turn blue litmus paper red.

-->  a. true

b. false

Which of the following substances has a pH greater than 7?

a. normal (clean) rain

b. orange juice

c. bananas

-->  d. soap

Sodium chloride is an example of a base.

a. true

-->  b. false

Acids produce hydrogen gas when they react with

a. salts.

b. bases.

c. water.

-->  d. metals.

Bases cannot conduct electricity.

a. true

-->  b. false

Properties of bases include

a. a sour taste.

b. a slippery feel.

c. the ability to conduct electricity.

-->  d. two of the above.

Which of the following acids is strongest?

-->  a. sulfuric acid

b. lemon juice

c. acid rain

d. vinegar

All acids are harmful.

a. true

-->  b. false

A strong acid has a high concentration of hydrogen ions.

-->  a. true

b. false

Products of a neutralization reaction include

a. hydroxide ions.

b. hydrogen ions.

-->  c. water.

d. two of the above

The symbol pH represents acidity.

-->  a. true

b. false

Ammonia is a stronger base than is bleach.

a. true

-->  b. false

Acids are used to make fertilizer.

-->  a. true

b. false

Acid rain promotes rapid growth of plants.

a. true

-->  b. false

Normal (clean) rainwater has a pH of 7.

a. true

-->  b. false

Litmus is the only indicator for detecting acids and bases.

a. true

-->  b. false

type of chemical reaction in which an acid reacts with a base

a. acid

b. acidity

c. base

d. pH

e. salt

f. litmus

-->  g. neutralization

Acids have many important uses in industry.

-->  a. true

b. false

ionic compound that produces hydroxide ions when dissolved in water

a. acid

b. acidity

-->  c. base

d. pH

e. salt

f. litmus

g. neutralization

ionic compound formed when an acid and a base react

a. acid

b. acidity

c. base

d. pH

-->  e. salt

f. litmus

g. neutralization

When sodium hydroxide dissolves in water it forms hydrogen ions.

a. true

-->  b. false

concentration of hydrogen ions in a solution

a. acid

-->  b. acidity

c. base

d. pH

e. salt

f. litmus

g. neutralization

A neutral substance has a pH of 7.

-->  a. true

b. false

A salt forms when an acid and base react.

-->  a. true

b. false

ionic compound that produces hydrogen ions when dissolved in water

-->  a. acid

b. acidity

c. base

d. pH

e. salt

f. litmus

g. neutralization

measure of the acidity of a solution

a. acid

b. acidity

c. base

-->  d. pH

e. salt

f. litmus

g. neutralization

compound used to detect acids and bases

a. acid

b. acidity

c. base

d. pH

e. salt

-->  f. litmus

g. neutralization

Which of the following substances is an acid?

-->  a. milk.

b. bleach.

c. ammonia.

d. seawater.

A very strong base has a pH close to

a. 1

b. 5

c. 7

-->  d. 14

Properties of acids include

a. a bitter taste.

b. a slippery feel.

-->  c. the ability to conduct electricity.

d. two of the above

Which of the following ionic compounds forms an acid when dissolved in water?

a. KCl

-->  b. HCl

c. NaCl

d. MgCl2

Bases are used to make all of the following products except

a. soap.

b. concrete.

c. deodorant.

-->  d. car batteries.

If a solution has a greater concentration of hydrogen ions than does pure water, then the solution

a. has a pH greater than 7.

b. is a base.

-->  c. is an acid.

d. two of the above

Which of the following chemical equations represents a neutralization reaction?

a. NaOH + H2 O  OH + Na+

b. Na+ + Cl  NaCl

-->  c. HCl + NaOH  NaCl + H2 O

d. Mg + 2HCl  H2 + MgCl2

diagram questions

No diagram questions associated with this lesson