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2.3: Water, Acids, and Bases

Difficulty Level: At Grade Created by: CK-12

Lesson 2.3: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Water is a chemical.

_____ 2. The hydrogen atoms in a water molecule attract electrons more strongly than the oxygen atom does.

_____ 3. Hydrogen bonds are very strong bonds.

_____ 4. Water is a reactant in photosynthesis.

_____ 5. Enzymes in the small intestine need an acidic environment in order to work.

_____ 6. Pure water has a pH of 7.

_____ 7. Lemon juice is a stronger acid than orange juice.

_____ 8. An ion is an electrically charged atom or molecule.

_____ 9. The stomach is a very acidic environment.

_____ 10. Water is released during cellular respiration.

_____ 11. Soap is very acidic.

_____ 12. Hydrogen bonds cause water to have a relatively high boiling point of 100°F.

_____ 13. Acids have a pH lower than 7.

_____ 14. Bases have a pH lower than 7.

_____ 15. A water molecule has positive and negative parts to it.

Lesson 2.3: Critical Reading

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

Acids and Bases

Water is the main ingredient of many solutions. A solution is a mixture of two or more substances that has the same composition throughout. Some solutions are acids and some are 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 break down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation

2H2O → H3O+ + 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 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.

pH Scale. The pH scale ranges from 0 to 14, with 7 being the point of neutrality. What is the pH of lemon juice? Of milk? (Image courtesy of Edward Stevens, modified by CK-12 Foundation, and under the Creative Commons license CC-BY-SA 3.0.)

Acids and Bases in Organisms

Acids and bases are important in living things because 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 work. For example, 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, and the stomach secretes a strong acid that allows pepsin to work. However, 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 strong base into the small intestine, and this base neutralizes the acid.

Water and Life

The human body is about 70% water (not counting the water in body fat, which varies from person to person). The body needs all this water to function normally. Just why is so much water required by human beings and other organisms? Water can dissolve many substances that organisms need, and it is necessary for many biochemical reactions. The examples below are among the most important biochemical processes that occur in living things, but they are just two of many ways that water is involved in biochemical reactions.

  • Photosynthesis — In this process, cells use the energy in sunlight to change carbon dioxide and water to glucose and oxygen. The reactions of photosynthesis can be represented by the chemical equation

6CO2 + 6H2O + Energy → C6H12O6 + 6O2

  • Cellular respiration — In this process, cells break down glucose in the presence of oxygen and release carbon dioxide, water, and energy. The reactions of cellular respiration can be represented by the chemical equation

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Water is involved in many other biochemical reactions. As a result, just about all life processes depend on water. Clearly, life as we know it could not exist without water.


1. Describe the best environment for the enzyme pepsin.

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2. Why is water so important for life?

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3. Which is a stronger acid: lemon juice or soda pop? Why?

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4. What is a hydronium ion? How does one form?

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5. In terms of water, what is one main difference between photosynthesis and cellular respiration?

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Lesson 2.3: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. Earth is sometimes called the
    1. “water planet,” because almost 75% of its surface is covered with water.
    2. “oxygen planet,” because oxygen is necessary for life.
    3. “carbon planet,” because carbon is the central element in organic compounds.
    4. all of the above.
  2. The oxygen in a water molecule
    1. attracts electrons more strongly than the hydrogen atoms.
    2. has a slight negative charge.
    3. binds to a hydrogen of another water molecule through a hydrogen bond.
    4. all of the above
  3. Which of the following is an example of a solution?
    1. a pepperoni pizza
    2. a box of Lucky Charms cereal
    3. a glass of orange juice
    4. a hot fudge sundae
  4. Which is the strongest acid?
    1. vinegar
    2. soda pop
    3. orange juice
    4. lemon juice
  5. A solution with a lower concentration of hydronium ions than pure water
    1. can have a pH of 6.5.
    2. is a base.
    3. can taste sweet.
    4. all of the above
  6. How do hydrogen bonds affect water’s properties?
    1. Hydrogen bonds explain why water molecules stick together.
    2. Hydrogen bonds cause water to have a relatively high boiling point.
    3. Hydrogen bonds also cause water to expand when it freezes.
    4. all of the above
  7. Where is most of the freshwater found?
    1. as ground water
    2. in icecaps, glaciers and inland seas
    3. in the oceans
    4. in other areas

Lesson 2.3: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.


_____ 1. for water, 212°F or 100°C

_____ 2. a range from 0 to 14

_____ 3. has a pH less than 7

_____ 4. has a pH more than 7

_____ 5. photosynthesis

_____ 6. OH-

_____ 7. a measure of the acidity of a solution

_____ 8. has the same composition throughout

_____ 9. needs an acidic environment to work

_____ 10. an organ that secretes a strong base into the small intestine

_____ 11. a difference in electrical charge within the same molecule

_____ 12. holds water molecules together


a. acid

b. base

c. boiling point

d. hydrogen bond

e. hydroxide ion

f. pancreas

g. pepsin

h. pH

i. pH scale

j. polarity

k. solution

l. 6CO2 + 6H2O + Energy → C6H12O6 + 6O2

Lesson 2.3: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Water’s ____________ gives it unique properties that help explain why it is vital to all living organisms.

2. In water, the ___________ atom attracts electrons more strongly than the ___________ atoms do.

3. Ice floats on water because ice has a ____________ density.

4. A mixture of two or more substances with the same composition throughout is a ___________.

5. pH is a measure of the ____________ of a solution.

6. A(n) ____________ has a pH lower than 7.

7. Water molecules are held together by ____________ bonds.

8. ____________ is a difference in electrical charge between different parts of the same molecule.

9. 100°C is water's ____________ point.

10. Water is essentially neutral, with a pH of ____________.

11. ____________ is slightly basic with a pH just above 7.

12. In a water molecule, the hydrogen atoms have a ____________ charge.

Lesson 2.3: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Using specifics in describing the structure of the water molecule, and discuss why water is referred to as a “polar molecule.”

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