Basically Acids

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.

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.

A basic understanding of the periodic table of elements.

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.

Indicators

Using red cabbage juice as a pH indicator.CopyrightCopyright 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

The bow of the Titanic showing corrosion.CopyrightCopyright 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.

(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?

• 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?

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 (PDF) 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?"

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