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Biology LibreTexts

3.3: Acids and Bases

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  • Disconnect_negative_battery_cable_(4144787843).jpg

    Figure 1

    Danger! Battery Acid!

    You probably know that car batteries like this one contain dangerous chemicals such as strong acids. Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids. There are many acids inside your body, and some of them are as strong as battery acid. Acids are needed for digestion and some forms of energy production. Genes are made of nucleic acids, proteins of amino acids, and lipids of fatty acids.

    Water and Solutions

    Acids such as battery acid are solutions. A solution is a mixture of two or more substances that has the same composition throughout. Many solutions are a mixture of water and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.

    In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation:

    \[\mathrm{2H_2O \rightarrow H_3O^+ + OH^-} \]

    The products of this reaction are a hydronium ion (H3O+) and a hydroxide ion (OH-). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H+). The hydronium ion, which has a positive charge, forms when another water molecule accepts the hydrogen ion.

    Acidity and pH

    The concentration of hydronium ions in a solution is known as acidity. In pure water, the concentration of hydronium ions is very low; only about 1 in 10 million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH, as shown in the figure below. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.


    Figure 2

    This pH scale shows the acidity of many common substances. The lower the pH value, the more acidic a substance is.


    If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is. Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. For example, stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.


    If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the color from clothing.

    Acids, Bases, and Enzymes

    Many acids and bases in living things provide the pH that enzymes need. Enzymes are biological catalysts that must work effectively for biochemical reactions to occur. Most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to do their work.

    Every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job. The stomach secretes the strong acid called hydrochloric acid that allows pepsin to work. When stomach contents enter the small intestine, the acid must be neutralized. This is because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a base named bicarbonate into the small intestine, and this base neutralizes the acid.

    Feature: My Human Body

    Do you ever have heartburn? The answer is probably "yes." More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it's time to seek medical treatment.

    Heartburn doesn't have anything to do with the heart, but it does cause a burning sensation in the vicinity of that organ. Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from back flowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.

    GERD — as well as occasional heartburn — often can be improved by dietary and other lifestyle  changes that decrease the amount and acidity of reflux from the stomach into the esophagus.

    • Some foods and beverages seem to contribute to GERD, so these should be avoided. They include chocolate, fatty foods, peppermint, coffee, and alcoholic beverages.
    • Decreasing portion size and eating the last meal of the day at least a couple of hours before bedtime may reduce the risk of reflux occurring.
    • Smoking tends to weaken the lower esophageal sphincter, so quitting the habit may help control reflux.
    • GERD is often associated with being overweight, and losing weight often brings improvement.
    • Some people are helped by sleeping with the head of the bed elevated. This allows gravity to help control the backflow of acids into the esophagus from the stomach.


    • A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
    • Acidity is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the pH scale.
    • Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
    • Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly. Enzymes are biological catalysts, such as pepsin, which is needed to digest protein in the stomach and requires an acidic environment.


    1. What is a solution?
    2. Define acidity.
    3. Explain how acidity is measured.
    4. Compare and contrast acids and bases.
    5. Hydrochloric acid is secreted by the stomach to provide an acidic environment for the enzyme pepsin. What is the pH of this acid? How strong of an acid is it compared with other acids?
    6. True or False. Strong bases are gentle and cannot hurt you, unlike strong acids.
    7. True or False. The lower the pH, the higher the concentration of hydronium ions.
    8. a. Define an ion.

      b. Identify the ions in the following equation and explain why they are ions:
      \[\mathrm{2H_2O \rightarrow H_3O^+ + OH^-} \]

    9. Explain why the pancreas secretes bicarbonate into the small intestine.

    10. Do you think pepsin would work in the small intestine? Why or why not?

    11. The pH of the stomach is _________ the small intestine.

      A. the same as

      B. not as important as the pH of

      C. higher than

      D. lower than

    12. a. You may have mixed vinegar and baking soda and noticed that they bubble and react with each other. Explain why this happens.

      b. Explain what happens to the pH of this solution after you mix the vinegar and baking soda.

    13. Pregnancy hormones can cause the lower esophageal sphincter to relax. What effect do you think this has on pregnant women? Explain your answer.

    Explore More

    Watch the video below to learn more about acids, bases, and pH.


     The video below discusses the importance of maintaining a stable pH in the body.


    Image Attributions

    [Figure 1] 
    Credit: By dave_7 from Lethbridge, Canada (Disconnect negative battery cable) [CC BY 2.0 (], via Wikimedia Commons; 
    License: CC BY-NC 3.0

    [Figure 2] 
    Credit: By OpenStax College [CC BY 3.0 (], via Wikimedia Commons; 
    License: CC BY-NC 3.0