Summary

In this lesson, students will test different flavors of Gatorade and other liquids to investigate acids and bases. This will take several days unless students already know about acids and bases.

Grade Level

Middle school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations

Objectives

By the end of this lesson, students should be able to

• Explain, on the molecular level, that pH is a measure of the concentration of the H₃O⁺ ions in water and that adding an acid or a base to water affects the concentration of these ions.
• Design an experiment to classify an unknown liquid either as an acid or a base.
• Identify liquids as either an acid, base, or neutral by its pH or reaction to an indicator.
• Identify variables in an experiment.

Chemistry Topics

This lesson supports students’ understanding of

• Acids & bases
• pH
• Chemical changes
• Solutions
• Conductivity

Time

Teacher Preparation: 1 hour

Lesson: 2 class periods (45 minutes each)

Materials

For each group:

• Depression plates filled with 12 liquids or beakers filled & labeled with liquids (such as rubbing alcohol, dilute sodium hydroxide, vinegar, tap water, distilled water, several flavors of Gatorade, soda pop, lemon juice, liquid soap, dilute sulfuric acid, copper sulfate, table salt solution, baking soda solution, sugar solution, citric acid)
• Conductivity tester
• Indicator such as litmus paper, universal indicator paper and/or pH probes

Safety

• Safety goggles should be worn at all times.
• The conductivity tester should be rinsed between each test. Liquids should not be mixed.
• Citric acid is an eye irritant.
• Liquid Universal indicator is alcohol-based and flammable.
• Read and follow all safety warnings on the label.
• At the end of the lesson, have students pour their used solutions in a waste container. Dispose of this waste down the drain or according to local regulations. Wash hands.

Vocabulary Terms

• Acids
• Bases
• pH
• Electrolytes
• Ions
• Neutral
• Concentration
• Salt

Teacher Notes

Background information:

• When a solution conducts electricity, the charge is carried by ions moving through the solution. Ions are atoms or small groups of atoms that have an electrical charge. Some ions have a negative charge and some have a positive charge. Pure water contains very few ions, so it does not conduct electricity very well. When table salt is dissolved in water, the solution conducts very well, because the solution contains ions. The ions come from the table salt, whose chemical name is sodium chloride. Sodium chloride contains sodium ions, which have a positive charge, and chloride ions, which have a negative charge. Because sodium chloride has ions and lights up the bulb, it is called an electrolyte. Not all substances are made up of ions. Some are made of uncharged particles called molecules. Sugar is a molecule. When sugar is dissolved in water, the solution does not conduct electricity, because there are no ions in the solution.
• Start this lesson with Middle School Chemistry Chapter 6, lesson 8 if students need to build background understanding of acids & bases
• Teachers can use the PhET pH Scale simulation to review the pH scale, as well as the Strength of Acids and Bases simulation.

Lesson Structure:

• Engage: Show students a Gatorade commercial. Have them list the vocabulary used to catch their attention. What words do they know? What words are new to them? (Hint: Find a commercial with the word electrolyte mentioned, or do a demo with a salt solution that lights a light bulb).
• Explore:
  • Pass out the Student Activity Sheet: Why Drink Gatorade?
  • Discuss independent & dependent variables. Have students identify what the variables were in the engage activity with a person beside them.
  • Pair up & gather their supplies.
  • Students predict which liquids are acids, bases or neutral and then taking turns writing or recording on a computer, each pair designs a table to collect information on the different liquids.
  • Use the acid/base indicators to test each liquid. Fill in the color change, pH number & whether the liquid is an acid, base or neutral
  • Test the liquids with the conductivity tester & record the brightness of the bulb (dim, bright, not lit)
  • Ask the following questions:
    • Which liquids are acidic? [vinegar, lemon juice, pop, sulfuric acid, Gatorade]
    • Basic? [sodium hydroxide, tap water, liquid soap, baking soda solution]
    • neutral? [copper sulfate, table salt solution, distilled water]
    • Were you surprised by some results? [students often guess the salts are not neutral]
    • What are the controlled variables in this experiment? [some answers: volume of liquid, bulb in conductivity tester]
    • Which liquids made the light bulb burn bright? [copper sulfate, sulfuric acid, sodium hydroxide, table salt solution]
    • Any patterns? [strong electrolytes are strong bases, strong acids or salts]
    • Did you ever test the reservoir at the end of the experiment? [it will have enough ions from the other liquids to light the bulb]

• Explain: Students will share their data as a presentation to the whole class. They choose one
  • Sing a song
  • Create a poster or Power Point
  • Design an artistic concept map
  • Create signs of strong/weak substances and then allow students to move to act out or stand under the sign for the liquid
  • Write a conclusion with given vocabulary = liquids, electrolyte, conductivity, light bulb, ions, solution, acids, bases

• Students should include some of the following information in their explanations:
• Acids are compounds that have one or more hydrogen atoms combined in a certain way with one or more other atoms. When dissolved in water, acids have a sour taste. They react with metals to liberate hydrogen gas, and they also react very readily with the compounds called bases. Acids produce color changes in certain dyes called indicators. One indicator, litmus paper, changes from a blue color to red in the presence of acid.
• Bases are compounds that have one or more groups of oxygen and hydrogen atoms (OH) combined with one or more other atoms. They are characterized by a bitter taste and a slick, soapy texture. Bases produce color changes in indicators. When an acid and a base react with each other, they form salt and water. The reaction of an acid solution on a basic or alkaline solution is called neutralization because both the acidic and basic properties neutralize each other.
• When a solution conducts electricity, the charge is carried by ions moving through the solution. Ions are atoms or small groups of atoms that have an electrical charge. Some ions have a negative charge and some have a positive charge. Pure water contains very few ions, so it does not conduct electricity very well. When table salt is dissolved in water, the solution conducts very well, because the solution contains ions. The ions come from the table salt, whose chemical name is sodium chloride. Sodium chloride contains sodium ions, which have a positive charge, and chloride ions, which have a negative charge. Because sodium chloride has ions and lights up the bulb, it is called an electrolyte.
• Not all substances are made up of ions. Some are made of uncharged particles called molecules. Sugar is such a substance. When sugar is dissolved in water, the solution does not conduct electricity, because there are no ions in the solution.

• Elaborate: Teachers could give 4 unknown liquids to students and ask them to determine whether the liquids are acids or bases and weak or strong electrolytes. Students design their own experiment to solve this problem (some might use litmus paper while others use universal indicator paper).
• Suggestions for 4 unknown liquids (tip: select liquids that do not have an odor nor recognizable color and pH values are different): very dilute acid, liquid soap, very dilute base, solution of baking soda, citric acid. Don’t use liquids that have pH close to 7 unless students have access to pH sensors.

• Evaluate: The teacher can hold up the chemical symbol or the chemical name of a liquid on a card. Students signal whether the item is an acid (1), neutral (2) or a base (3). Students read more about sports drinks. Alternatively teachers can assign the PhET pH Scale simulation to assess understanding of the pH scale, or the Strength of Acids and Bases simulation.

Other tips:

• Use cafeteria trays for students to conduct work & this also an easy way for them to gather their supplies from the front of the room & less likely anything will spill
• A piece of white paper under the depression plate is helpful when identifying the color of the liquids
• Students need to test each liquid by placing the tester gently into each depression or beaker. After each test, place the tester into the reservoir or beaker filled with distilled water. When they are finished testing all the liquids, take a paper towel & soak up ONLY the reservoir or dump the beaker into the sink. The teacher will need to refill the reservoir or beaker with distilled water after each class.
• Students must wear goggles at all times and remain seated while conducting experiment. The lights might need to be turned off so students can see the light bulb brightness on the tester.