Lesson Basically Acids

Quick Look

Grade Level: 10 (9-11)

Time Required: 15 minutes

Lesson Dependency: None

Subject Areas: Chemistry, Life Science, Physical Science

Photo shows three cups of cabbage juice viewed from above. Acid was added to the left one (pink) and bases to the other two (blue, teal), showing the indicator color range.
Using red cabbage juice as a pH indicator.
Copyright © 2010 Rachel Howser, GK-12 Program, University of Houston


Students use the associated activity to learn the basics of acid/base chemistry in a fun, interactive way by studying instances of acid/base chemistry found in popular films such as Harry Potter and the Prisoner of Azkaban and National Treasure. Students learn what acids, bases and indicators are and how they can be used, including invisible ink. They also learn how engineers use acids and bases every day to better our quality of life. Students' interest is piqued by the use of popular culture in the classroom.

Engineering Connection

Acid/base chemistry is a pervasive scientific concept used across many engineering disciplines. Engineers use their knowledge of acids and bases to design non-corrosive material combinations, car batteries, chemical fertilizers and food preservation techniques. Writing with invisible ink that can be read using an acid/base indicator was commonly employed by engineers, inventors and military leaders during the American Revolutionary War to communicate secret messages.

Learning Objectives

After this lesson, students should be able to:

  • Hypothesize whether a liquid is an acid or a base.
  • Identify whether a liquid is an acid or base based on its color interaction with cabbage juice.
  • Explain how acids and bases were used during the American Revolution to send spy letters.
  • Describe how engineers use acids and bases.

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Worksheets and Attachments

Visit [www.teachengineering.org/lessons/view/uoh_hp_lesson_acids] to print or download.

Pre-Req Knowledge

A basic understanding of the periodic table of elements.


(If conducting this lesson AND its associated activity, Basically Acidic Ink, follow this suggested order for the most efficient timing: First present the activity Introduction Part 1 > then conduct activity Procedure Part 1 > while the activity "invisible ink" index cards are drying, present activity Introduction Part 2 and the lesson content > then conduct activity Procedure Parts 2 and 3 to conclude. Be ready to show students video clips from two movies.)

(Begin by showing a video clip from Harry Potter and the Prisoner of Azkaban. In the clip, Harry is given the Marauder's Map, a magical map that shows all of Hogwarts, the wizard school, and the locations of people in it, in real time. The map is a blank piece of parchment until magical words are spoken over it. When the words are spoken, ink appears.)

What happened in that video clip? (Expect students to say that Harry was given a map with appearing ink.) Does appearing or disappearing ink exist in the real world, or is it purely magical? (Some students may have had experience playing with disappearing or invisible ink. They may say that invisible and disappearing ink are real.) If invisible ink is real what is it made of? Do you think we can make it ourselves using common household materials? What are some uses for disappearing ink?

Invisible ink played a large part in the American Revolution. Spies wrote letters to military leaders using invisible ink. To do this, the spy would write a regular letter in regular ink, but between the lines of writing, s/he would write a secret message using invisible ink. A symbol would often be put in the corner of the letter to tell the receiver how to read the secret message. What are some ways that you think the reader could have read the secret message? Can you think of a movie that was released in which this concept was used? (Show a video clip from National Treasure, in which three main characters are applying acid and heat to the back of the Declaration of Independence to reveal a message written in invisible ink.)

What were the two methods used in the video to reveal the invisible ink? (Answer: Heat and lemons/acid.) What is an acid? What is an example of an acid? How might acids be important to engineers?

This is just one way of applying what we know about acids and bases to solve a problem. Enginees use their understanding of acids and bases - the pH measuring and rating system - to design non-corrosive material combinations, batteries, chemical fertilizers and ways to preserve food. What role do you think engineers had in designing invisible ink?

Lesson Background and Concepts for Teachers

Acids and Bases

Acid/base chemistry is an important concept used across the broad range of engineering disciplines. All substances are categorized as acids, bases or neutral substances using the pH nomenclature and are rated with different strengths as indicated by the pH scale. A common acid/base theory, the Arrhenius theory, states that acids ionize in water to produce H+ ions and bases ionize to produce OH- ions. Strong acids and bases completely dissociate in water (no acid/base left in original solution), leaving a solution of water and salt. Weak acids partially dissociate into water so that some acid is left.

The pH Scale

The pH scale is used to measure the "strength" of an acid or base. Technically, it measures the hydrogen-ion (H+) concentration of an aqueous solution. The pH scale ranges between 0 and 14, with 7 being neutral. Water has a neutral pH of 7. A solution with a pH below 7 is acidic, and a solution with a pH greater than 7 is basic. An acid or base with a pH near 0 or 14, respectively, is considered strong, while an acid or base with a pH near 7 is considered weak.


Photo from above shows three cups of cabbage juice: an acid was added to the left cup (pink) and a base to the right cups (blue and teal)  to show the indicator's color range.
Using red cabbage juice as a pH indicator.
Copyright © 2010 Rachel Howser, GK-12 Program, University of Houston

Indicators are tools used to indicate the pH of substances. Often, the indicator color corresponds to a pH level. Two common indicators are litmus paper and red cabbage juice. Litmus paper turns red when dipped in an acidic solution and blue with a basic solution. Red cabbage juice is naturally purple, but becomes pink when an acid is added to it and blue, green and yellow as it becomes more basic.

Examples of Acids and Bases in Engineering

What looks like a greenish, orangish glob of metal, roughly shaped like the front prow of a boat, with curved railing on top.
The bow of the Titanic showing corrosion.
Copyright © 2004 Courtesy of NOAA/Institute for Exploration/University of Rhode Island http://en.wikipedia.org/wiki/File:Titanic_wreck_bow.jpg

Removing Corrosion: Engineers use strong acids to remove rust and other corrosive materials from metals through a process called pickling.

Batteries: Engineers design all kinds of batteries, such as car batteries, that include acids. A typical car battery is made of plates of lead and lead dioxide submerged in an acidic solution. A chemical reaction releases electrons to create the electricity used to start a car.

Fertilizers: Chemical engineers combine acids and bases to create useful products, such as chemical fertilizers for agriculture. To make a fertilizer, nitric acid, an acid, is combined with ammonia, a base, to produce ammonium nitrate.

Cleaners: Common household cleaners, such as ammonia, baking soda and bleach, are all bases.

Food Preservation: Acids are common food preservatives. On food labels, ingredients such as phosphoric acid, citric acid, tartaric acid, malic acid, famaric acid, lactic acid and vinegar serve as preservatives.

Associated Activities

Lesson Closure

  • What are some places and purposes we see acids and bases used every day? (Brainstorm with students and create a list.)
  • Are acids and bases a good thing? When might acids and bases be bad?
  • In what ways do engineers make use of acids and bases to make products and services that help people?


acid: A compound that usually dissolves in water, has a sour taste, reacts with a base to form a salt, and turns litmus paper red.

base: A compound that reacts with an acid to form a salt, has a bitter taste, and turns litmus paper blue.

indicator: A substance used to show visually (as by change of color) the presence of acid or base in a solution.


Post-Introduction Assessment

Questions/Answers: Ask the students: Are invisible inks real? What might they be made of? After watching the National Treasure film clip, ask: What are acids and bases? What are they used for?

Lesson Summary Assessment

Worksheet(s): At lesson end, have students completed either this lesson's Basically Acids Worksheet or the lab sheet for the associated Basically Acidic Ink activity to demonstrate their knowledge of the topic. The lab sheet and worksheet both include some higher thinking application questions, such as "How do engineers use acids and bases?"

Additional Multimedia Support

Borrow from your school or public library DVDs of the Harry Potter and the Prisoner of Azkaban and National Treasure movies, so you can show students the invisible ink clips. If not available, describe the scenes, since most students are familiar with the movies and will be able to recall the scenes and describe them more fully to other classmates.


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Foster, Kate, et al. Spy Letters of the American Revolution: Secret Methods and Techniques: Invisible Ink. 13 May 1999. Accessed 24 March 2010. http://www2.si/umich.edu//spies/index-main2.html


© 2013 by Regents of the University of Colorado; original © 2010 University of Houston


Rachel Howser; Christine Hawthorne

Supporting Program

University of Houston, National Science Foundation GK-12 and Research Experience for Teachers (RET) Programs


This digital library content was developed by the University of Houston's College of Engineering under National Science Foundation GK-12 grant number DGE-0840889. However, these contents do not necessarily represent the policies of the NSF and you should not assume endorsement by the federal government.

Last modified: May 21, 2019

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