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3.52: Properties of Acids

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Look at the boy in the photo above. You can tell by the expression on his face that lemon juice tastes sour. Lemon juice is an acid, and all acids taste sour. They share certain other properties as well. You will learn more about acids and their properties in this article. For a musical introduction to acids and related concepts, watch the video at this URL: http://www.youtube.com/watch?v=zTLiJE-j1-I

What Are Acids?

Acids are ionic compounds that produce positive hydrogen ions (H + ) when dissolved in water. Ionic compounds are compounds that contain positive metal ions and negative nonmetal ions held together by ionic bonds. (Ions are atoms that have become charged particles by gaining or losing electrons.) An example of an acid is hydrogen chloride (HCl). When it dissolves in water, it separates into positive hydrogen ions and negative chloride ions (Cl - ). This is represented by the chemical equation:

HCl \mathrm{\overset{H_2O}{\rightarrow}} H + + Cl -

Properties of Acids

You already know that a sour taste is one property of acids. ( Warning : 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 when dissolved in water because they consist of charged particles in solution. (Electric current is a flow of charged particles.) Acids can also react with metals, and when they do they produce hydrogen gas. An example of this type of reaction is hydrochloric acid reacting with the metal zinc(Zn). The reaction is pictured in the Figure below . It can be represented by the chemical equation:

Zn + 2HCl → H 2 + ZnCl 2

Hydrochloric acid reacting with the metal zinc

Q: What sign indicates that a gas is being produced in this reaction?

A: The bubbles are hydrogen gas rising through the acid.

Q: Besides hydrogen gas, what else is produced in this reaction?

A: This reaction also produces zinc chloride ZnCl 2 , which is a neutral ionic compound called a salt.

You can see an online demonstration of another acid-metal reaction at this URL: http://www.youtube.com/watch?v=oQz5YEsx7Fo

Detecting Acids

Certain compounds, called indicators, change color when acids come into contact with them, so indicators can be used to detect acids. An example of an indicator is the 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 the Figure below . Litmus isn’t the only indicator for detecting acids. Red cabbage juice also works well, as you can see in this entertaining video: http://www.youtube.com/watch?v=vrOUdoS2BtQ&feature=related

Drawing of blue litmus paper turning red in acid.

Strength of Acids

The strength of acids is measured on a scale called the pH scale. The pH value of a solution represents its concentration of hydrogen ions. A pH value of 7 indicates a neutral solution, and a pH value less than 7 indicates an acidic solution. The lower the pH value is, the greater is the concentration of hydrogen ions and the stronger the acid. The strongest acids, such as battery acid, have pH values close to zero.

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 be seen in the Figure below .

Summary

  • Acids are ionic compounds that produce positive hydrogen ions (H+) when dissolved in water.
  • Acids taste sour, conduct electricity when dissolved in water, and react with metals to produce hydrogen gas.
  • Certain indicator compounds, such as litmus, can be used to detect acids. Acids turn blue litmus paper red.
  • The strength of acids is measured on the pH scale. A pH value less than 7 indicates an acid, and the lower the number is, the stronger the acid.
  • Acids have many important uses, especially in industry.

Vocabulary

  • acid : Ionic compound that produces positive hydrogen ions (H + ) when dissolved in water.

Practice

At the following URLs, watch the videos about hydrochloric acid, and then answer the questions below.

http://www.youtube.com/watch?v=U6viBSkLkSk (Part I)

http://www.youtube.com/watch?v=W7GE56dxS8o (Part II)

  1. What is hydrochloric acid?
  2. Everyone carries around a sample of hydrochloric acid. Where is the hydrochloric acid, and what is its function?
  3. Describe what happens when a cheeseburger and then a goose skull are placed in hydrochloric acid.
  4. When zinc metal is added to hydrochloric acid, how can you tell that hydrogen gas is produced?

Review

  1. What is an acid?
  2. List properties of acids.
  3. How can you tell whether a compound is an acid?
  4. Milk is a very weak acid. What might its pH value be?
  5. Based on your knowledge of the properties of acids, which one of the following substances do you think is an acid?
    1. vinegar
    2. salt
    3. sugar
    4. baking soda

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 14, 2012

Last Modified:

Sep 05, 2014
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