University of Virginia Physics Department Soda Science A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2
• PS.5 (Extensions Section)

Objectives

Students will

• Plan and conduct investigations in which valid conclusions are made after analyzing data.
• Present experimental results in appropriate written form.
• Use graduated cylinders to gather data.
• Investigate and understand chemical properties such as acidity and basicity.
• Show that a neutralization reaction can be used to find the level of acidity of a solution.

Motivation for Learning

Driving Question

Most carbonated drinks conatin dilute solutions of carbonic and phosphoric acids. Do all such beverages contain the same amount of these acids?

Materials

• Overhead projector
• 50 drops of 0.1 M HCl
• 50 drops of 0.1 M NaOH
• 1 drop of universal indicator
• 250 mL beaker
• Dropper
• Universal indicator chart

Procedure

1. Place the beaker on the overhead projector.
2. Place 50 drops of 0.1 M HCl and 1 drop of universal indicator into the beaker. Swirl to mix.
3. Slowly add, with swirling, 50 drops of 0.1 M NaOH.
4. Show the students that equal volumes of strong acids and strong bases of the same concentration produce a neutral solution.
5. Match the final color with the universal indicator chart.

Background Information

An acid is a substance that produces hydronium ions (H+) in a solution. It is the presence of the H+ ions that gives acids their properties. They taste sour, are electrolytes, are corrosive, and they react with certain compounds or indicators to produce a predictable change in color. An indicator is an organic compound that changes color in an acid or a base.

A base is a substance that produces hydroxide ions (OH-) in solution. Bases feel slippery and have a bitter taste, are corrosive, are electrolytes, and also react with indicators to produce a predictable change in color.

Neutralization is a chemical reaction between an acid and a base. During a neutralization reaction, hydronium ions from the acid combine with the hydroxide ions from the base to form water.

H₃ O + (aq) + OH- (aq) --> H₂O (l)

The hydroxide ions present in all basic solutions are negative ions, whereas the hydronium ions in acidic solutions are positive ions. As the reactive hyrdonium and hydroxide ions are removed from the solution, the acidic and basic properties of the reactants are cancelled, or neutralized.

In the student activity, be sure to mix the contents well after each drop of NaOH is added. If not mixed, the acid may be neutralized at the top, giving a false result. If small Erlenmyer flasks are available they will work better than test tubes. The pink color can best be seen if the flasks are sitting on a piece of white paper. For most carbonated beverages, approximately 5 mL of soda requires 15 - 25 drops of NaOH solution. Be sure to try this activity ahead of time to make sure that the two beverages chosen have a measurable difference in acidity. There are many clear, carbonated soft drinks that will work. You can also do this activity with more than 2 types of soda.

Student Activity

To print out the Student Copy only, click here.

Materials

• 2 different colorless, carbonated soft drinks
• 2 large test tubes or small Erlenmeyer flasks
• Test tube rack (if using test tubes)
• Graduated cylinder
• 2 droppers
• 1% phenolphthalein indicator
• Dilute NaOH (sodium hydroxide)
• A sheet of white paper

Procedure

1. Measure 20 mL of colorless, carbonated beverages into test tubes or flasks. Set flasks on sheet of white paper.
2. Add 2 drops of 1% phenolphthalein indicator to each of the beverage samples. Swirl to mix.
3. Add the NaOH solution drop by drop, until a permanent faint pink color indicates that the acid is neutralized. Carefully swirl the test tube after each drop is added.
4. Record in the data table the number of drops of NaOH used.
5. Repeat steps 3 and 4 with the second sample.

Data Table

To print out Data Table only, click here.

Extensions

Write out the balanced chemical equation for the reaction between NaOH and carbonic acid, H ₂ CO ₃ . If additional water were added to the NaOH solution, how would it affect the number of drops required for neutralization?

Also see graphing calculator-based activity for determining the pH of a substance.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

To print out the Data Table only, click here.

1. Did your samples require different amounts of NaOH for neutralization?
   
   
   
   
   
   
   
   
2. Which sample is more acidic?
   
   
   
   
   
   
   
   
3. Phenolphthalein changes from colorless to pink at a pH of 8. Explain why the indicator was colorless in the soft drink and became pink as the base was added.
   
   
   
   
   
   
   

Answers to Assessment

1. Depends on specific samples, but two very different types and brands should require different amounts of NaOH for neutralization because of a difference of acidity. For example, a citrus soda might be more acidic (thus requiring more NaOH) than a cola.
2. This is dependent on specific samples. The one with more base (NaOH) added for neutralization is the one that is more acidic.
3. The phenolphthalein in the soft drink was colorless prior to base addition because the soft drink was of a lower pH than 8.0 meaning that it was either neutral at a pH of 7.0 or most likely acidic with a pH much less than 7.0.