An acid is an ionic compound that produces positive hydrogen ions when dissolved in water. A base is an ionic compound that produces negative hydroxide ions when dissolved in water. The strength of acids and bases is measured by pH. The reaction of an acid and a base produces a salt and water.
- SCI.CA.8.PS.3.b; SCI.CA.8.PS.5.a, e
- Describe acids and how to detect them.
- Describe bases and how to detect them.
- Explain what determines the strength of acids and bases.
- Outline neutralization reactions and the formation of salts.
acid: ionic compound that produces positive hydrogen ions (H+) when dissolved in water
acidity: concentration of hydrogen ions in a solution
base: ionic compound that produces negative hydroxide ions (OH-) when dissolved in water
neutralization reaction: reaction of an acid and a base that produces water and a salt, both of which are neutral in acidity
pH: measure of the acidity, or hydrogen ion (H+) concentration, of a substance
salt: ionic compound formed when an acid and a base react
Introducing the Lesson
Pique student interest in acids and bases by writing—and then revealing—a mystery message using “invisible ink.” With a cotton swab dipped in lemon juice, write a brief message on white paper. The message will not be visible until you spray it lightly with red cabbage juice. The cabbage juice is an acid/base indicator, and it will turn pink when it contacts the acidic lemon juice. (If you had written your message with a base, the cabbage juice would turn green instead). Tell students they will understand the secret of the “invisible ink” when they learn about acids and bases in this lesson.
Have students do the online activity “Alien Juice Bar” to virtually test different substances with red cabbage juice in order to determine whether they are acids, bases, or neutral substances.
Use the activity at the following URL to demonstrate neutralization reactions using Erlenmeyer-flask “stomachs,” stomach acid (HCl), and a variety of over-the-counter antacid tablets.
This well-done visual presentation on acids, bases, and pH may help visual learners and less proficient readers better understand lesson content. It includes a quiz and test so students can self-check their comprehension.
Have a small group of students who need extra challenges do the acid rain research activity at the URL below. Students will use the Internet to research the causes and effects of acid rain and the pH levels of rainwater for their state. Then they will collect data from the community and investigate solutions for acid rain.
Assign the online challenge game at the URL below. In the game, students will find clues to learn how to test liquids for acidity, and then they will test several common liquids to determine whether they are acidic, basic, or neutral. They will also use the liquids to try to launch a cork rocket. Finally, they will use their results to identify a pattern (acids mixed with a metal carbonate, i.e., baking soda, produce carbon dioxide gas).
Ask students whether the following statements are true or false. (Both are false, as explained in brackets following the statements.)
All substances containing H are acidic, and all substances containing OH are basic.
- [Although all acids contain H and all bases contain OH, not all substances containing H or OH are acids or bases. They are acids or bases only if they form H+ or OH- ions when dissolved in water. Glucose is a good example. It contains both H and OH but forms a neutral solution when dissolved in water because it does not form ions.]
Strength and concentration mean the same thing.
- [Concentration refers to the amount of solute (acid or base) per volume of solution. Strength refers to the percent of solute particles that form ions in solution. Hydrochloric acid is a strong acid because almost all of it ionizes in solution. Acetic acid is a weak acid because only about 1% of it ionizes in solution.]
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.
Lesson Review Questions
Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.
Check students’ mastery of the lesson with Lesson 10.3 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.
Points to Consider
Neutralization reactions, like the other chemical reactions you have read about so far, involve electrons. Electrons are outside the nucleus of an atom. Certain other reactions involve the nucleus of an atom instead. These reactions are called nuclear reactions. You will read about them in the next chapter.
- How do you think nuclear reactions might differ from chemical reactions?
- Elements involved in nuclear reactions are radioactive. How do you think radioactive elements differ from other elements?