8.3 Properties of Acids and Bases One of the chemicals used to make soaps is sodium hydroxide....

8.3 Properties of Acids and Bases

One of the chemicals used to make soaps is sodium hydroxide. Sodium hydroxide reacts with animal or vegetable fats to make glycerol and soap.

Sodium hydroxide belongs to a class of compounds known as bases.


What are some general properties of acids?

An acid is a compound that produces hydronium ions (H3O

+) when dissolved in water.

Identifying Acids

Some general properties of acids include sour taste, reactivity with metals, and ability to produce color changes in indicators.


When hydrogen chloride gas dissolves in water, it ionizes and forms hydronium ions and chloride ions.

HCl + H2O H3O+ + Cl–

The solution that results is called hydrochloric acid.

Identifying Acids


Identifying Acids


Sour Taste

Foods that taste sour often contain acids.• Lemons, grapefruits, limes, and oranges

contain citric acid. • Vinegar contains acetic acid.• Dairy products that have spoiled contain butyric

acid.

Identifying Acids


Reactivity With Metals

The reaction between an acid and a metal is an example of a single-replacement reaction.

When zinc is added to a test tube containing hydrochloric acid, bubbles form in the tube.

Zn + 2HCl H2 + ZnCl2

Identifying Acids


Color Changes in Indicators

An indicator is any substance that changes color in the presence of an acid or base.

Litmus paper is made by coating strips of paper with litmus, a kind of dye derived from lichens.

Blue litmus paper turns red in the presence of an acid.

Identifying Acids


Apples contain several acids, including malic acid, ascorbic acid (vitamin C), and citric acid.

Blue litmus paper is an indicator for acids.

Identifying Acids


What are some general properties of bases?

A base is a compound that produces hydroxide ions (OH–) when dissolved in water.

Identifying Bases

Some general properties of bases include bitter taste, slippery feel, and ability to produce color changes in indicators.


Sodium hydroxide, NaOH, is an example of a base.

When sodium hydroxide dissolves in water, it dissociates into sodium ions and hydroxide ions.

NaOH Na+ + OH–

Identifying Bases


The plaster in this boy’s cast contains a base.

Identifying Bases


Bitter Taste

Without sugar, chocolate tastes bitter. Cacao beans contain a base that gives unsweetened chocolate its bitter taste.

Many liquid medicines contain bases. Fruit flavorings are often added to mask the taste of these basic solutions.

Identifying Bases


Slippery Feel

Bases feel slippery. Wet soap and many cleaning products that contain bases are slippery to the touch.

When wet, some rocks feel slippery because the water dissolves compounds trapped in the rocks, producing a basic solution.

Identifying Bases


Color Changes in Indicators• Bases turn red litmus paper blue. • Phenolphthalein is another acid-base

indicator.• In a solution containing a base,

phenolphthalein is red. • In a solution containing an acid,

phenolphthalein is colorless.

Identifying Bases


These hydrangea flowers contain natural indicators. The color of the flowers depends on whether the plant is growing in acidic or basic soil.

When hydrangeas grow in acidic soil, the flowers are bluish-purple. When hydrangeas grow in basic soil, the flowers are pink.

Identifying Bases


What are the products of neutralization?

The reaction between an acid and a base is called neutralization.

Neutralization and Salts

The neutralization reaction between an acid and a base produces a salt and water.


The negative ions in an acid combine with the positive ions in a base to produce an ionic compound called a salt.

The hydronium ions from the acid combine with the hydroxide ions from the base to produce water.



When hydrochloric acid reacts with sodium hydroxide, a neutralization reaction occurs.

If you let the water in the resulting solution evaporate, sodium chloride would crystallize out of solution.



Table salt is the most common example of a salt compound.

• Baking soda, NaHCO3, is produced during the

neutralization reaction between sodium hydroxide and carbonic acid, H2CO3.

• The ocean contains many dissolved salts, including chlorides and sulfates of potassium, calcium, magnesium, and sodium.



The common salts listed in the table can all be made by reacting an acid with a base.



What are proton donors and proton acceptors?

Proton Donors and Acceptors

Acids can be defined as proton donors, and bases can be defined as proton acceptors.


When an acid and a base react in water, a proton from the hydronium ion from the acid combines with the hydroxide ion (OH–) from the base to form water (H2O).

Acids lose, or “donate,” protons. Bases “accept” protons, forming water, a neutral molecule. This definition allows you to classify a wider range of substances as acids or bases.



Based on the definitions of acids and bases that you read earlier in this section, water is neither an acid nor a base.

Using the proton-donor or proton-acceptor definition, water can act as either an acid or a base.



When hydrogen chloride dissolves, water acts as a base. It accepts a proton from hydrogen chloride and becomes a hydronium ion.



When ammonia dissolves, water acts as an acid. It donates a proton to the ammonia, which acts as a base.



Assessment Questions

1. What ion is formed when a base dissolves in water? a. hydroxide ion

b. hydronium ion

c. hydrogen ion

d. sodium ion



1. What ion is formed when a base dissolves in water? a. hydroxide ion

b. hydronium ion

c. hydrogen ion

d. sodium ion

ANS: A



2. What products are formed in a neutralization reaction?a. an acid and a baseb. water and a protonc. a base and a saltd. water and a salt



2. What products are formed in a neutralization reaction?a. an acid and a baseb. water and a protonc. a base and a saltd. water and a salt

ANS: D



3. In the reaction between ammonia and water to form an ammonium ion, water acts as a(n) a. acid because it donates an electron to ammonia.

b. base because it donates an electron to ammonia.

c. acid because it donates a proton to ammonia.

d. base because it donates a proton to ammonia.



3. In the reaction between ammonia and water to form an ammonium ion, water acts as a(n) a. acid because it donates an electron to ammonia.

b. base because it donates an electron to ammonia.

c. acid because it donates a proton to ammonia.

d. base because it donates a proton to ammonia.

ANS: C



1. A food that contains acidic compounds can often be identified by its bitter taste.

TrueFalse



1. A food that contains acidic compounds can often be identified by its bitter taste.

TrueFalse

ANS: F, sour

8.3 Properties of Acids and Bases One of the chemicals used to make soaps is sodium hydroxide....

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