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20.1: Properties of Acids and Bases

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Lesson Objectives

The student will:

  • list the properties of acids.
  • list the properties of bases.
  • name an acid or base when given the formula.
  • write the formula for an acid or base given the name.

Vocabulary

  • indicators

Introduction

We interact with acids on a daily basis without even realizing it. For example, the chemical names for aspirin and vitamin C are acetylsalicylic acid and ascorbic acid; both will produce \mathrm{H}^+ ions when dissolved in water. Acetic acid (\mathrm{HC}_2\mathrm{H}_3\mathrm{O}_2) is the primary component in vinegar, and formic acid (\mathrm{HCO}_2\mathrm{H}) is what causes ant bites to sting. Hydrochloric acid (HCl) is stomach acid, phosphoric acid (\mathrm{H}_3\mathrm{PO}_4) is commonly found in dark soft drinks, and sulfuric acid (\mathrm{H}_2\mathrm{SO}_4) is used in car batteries. As you work your way through this chapter, try to notice how the properties of acids and bases manifest themselves in everyday situations.

Properties of Acids

One property that is common to all acids is a sour taste. You are probably most familiar with this in relation to citric acid, which is what makes lemons and other citrus fruits taste sour. In fact, sour taste buds are essentially just complicated \mathrm{H}^+ sensors. The fact that one of our primary tastes is concerned solely with determining the acidity of what goes in our mouths further underscores the importance of acids in our lives.

However, testing whether something is acidic by taste is generally not a good idea. Another way to test for acidity is to use an indicator. Indicators are substances that can be used to determine the relative acidity or basicity of a solution, generally through a very distinct color change.

One common type of indicator is litmus paper. If a piece of blue litmus paper turns red when dipped into a solution, it means that the solution is acidic.

Another property common to many acids is that they can react with certain metals to form hydrogen gas. Examples of this type of reaction are shown below. Note that these are all single replacement reactions where a pure element reacts with a compound

\mathrm{Zn}_{(s)} + 2 \ \mathrm{HCl}_{(aq)} \rightarrow \mathrm{ZnCl}_{2(aq)} + \mathrm{H}_{2(g)}
\mathrm{Mg}_{(s)} + 2 \ \mathrm{HCl}_{(aq)} \rightarrow \mathrm{MgCl}_{2(aq)} + \mathrm{H}_{2(g)}
\mathrm{Ba}_{(s)} + 2 \ \mathrm{HCl}_{(aq)} \rightarrow \mathrm{BaCl}_{2(aq)} + \mathrm{H}_{2(g)}

Names and Formulas of Acids

The chemical formula for an acid typically begins with one or more hydrogen atoms. This means that hydrogen is the cation. The name of an acid depends on the anion. When the anion ends in “–ide,” such as chloride or sulfide, the prefix “hydro-” is used before the name of the nonmetal, and its “-ide” ending is replaced by “-ic acid.” For example, the acid \mathrm{HCl} is named hydrochloric acid. The anion is chloride, so the prefix “hydro-” is used, and the “-ide” ending is replaced by “-ic acid.”

When the anion ends in “–ate,” such as sulfate and phosphate, the “–ate” is replaced by “-ic acid.” For example, the acid \mathrm{HNO}_3 is named nitric acid. The anion is nitrate, so the “–ate” ending is replaced by “–ic” acid.

When the anion ends in “–ite,” such as nitrite or sulfite, the “–ite” is replaced by “–ous acid.” For example, the acid \mathrm{HClO}_2 is named chlorous acid. The anion is chlorite, so the “–ite” is replaced by “–ous acid.”

Example:

Write the names of each of the following acids:

  1. \mathrm{HF}
  2. \mathrm{HNO}_2
  3. \mathrm{H}_2\mathrm{SO}_4

Solution:

  1. hydrofluoric acid
  2. nitrous acid
  3. sulfuric acid

Properties of Bases

Bases also have a number of characteristic properties. Most bases are slippery and quite bitter (though not all bitter compounds are basic). Caffeine and milk of magnesia (chemical formula \mathrm{Mg(OH)}_2) are two bases that you may have had the opportunity to taste, although the bitterness is generally masked by other flavors when these compounds are consumed. Other common bases are found in a number of cleaning products, including Drano (\mathrm{NaOH}) and Windex (\mathrm{NH}_4\mathrm{OH}).

Like acids, bases can be identified by the use of an indicator. For example, if red litmus paper is dipped into a basic solution, it will turn blue.

This video discusses the properties of acids and bases (5a): http://www.youtube.com/watch?v=mm7Hcff5b6g (8:02).

A discussion of the difference between strong and weak acids is available at (5c) http://www.youtube.com/watch?v=XTdkWGImtSc (9:47).

Names and Formulas of Bases

The chemical formula for a strong base often ends with one or more hydroxide (\mathrm{OH}^-) ions. This means that hydroxide is the anion. This type of base is named in the same way as ay other ionic compound.

Example:

Write the names of each of the following bases.

  1. \mathrm{NaOH}
  2. \mathrm{KOH}
  3. \mathrm{Ca(OH)}_2

Solution:

  1. sodium hydroxide
  2. potassium hydroxide
  3. calcium hydroxide

Lesson Summary

  • Acids turn blue litmus paper red, taste sour, and react with metals to produce hydrogen gases.
  • Common acids include vinegar (\mathrm{HC}_2\mathrm{H}_3\mathrm{O}_2) and stomach acid (\mathrm{HCl}).
  • Bases turn red litmus paper blue, have a bitter taste, and are slippery to the touch.
  • Common bases include Drano (\mathrm{NaOH}), soft soap (\mathrm{KOH}), milk of magnesia (\mathrm{Mg(OH)}_2), and Windex (\mathrm{NH}_4\mathrm{OH}).

Further Reading / Supplemental Links

Visit this website to learn more about examples and properties of acids and bases.

Review Questions

  1. What are the properties of acids? Give a common example.
  2. Which statement best describes a characteristic of acid solutions?
    1. They react with some metals to form hydrogen gas.
    2. They turn red litmus paper blue.
    3. They taste bitter.
    4. They are made from nonmetal oxides.
  3. Write the reaction between:
    1. magnesium and sulfuric acid.
    2. calcium and acetic acid.
  4. Which of the following will react with acids and produce hydrogen gas?
    1. chlorine
    2. ammonia
    3. carbon
    4. magnesium

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