The Designated Driver

Introduction

A solution is poured from a large beaker into four beakers resulting in “strawberry soda,” “blueberry soda,” “red wine” and “water.” Mixing the solutions from the first three beakers produces more “red wine.” The three beakers are refilled with the “red wine,” then all four beakers are combined to produce “water.”

Concepts

• Acid–base indicators
• Complex ions

Materials

Safety Precautions

Sodium hydroxide solution is irritating to skin and eyes and moderately toxic by ingestion. Hydrochloric acid solution is toxic by ingestion and corrosive to skin and eyes. Sodium fluoride solution is highly toxic by ingestion and inhalation and is a strong skin irritant. Phenolphthalein and thymolphthalein indicator solutions are alcohol-based solutions and are therefore flammable liquids that may pose a fire risk. They are both toxic by ingestion. Potassium thiocyanate solution is moderately toxic by ingestion. If potassium thiocyanate is strongly heated in the presence of concentrated acids, toxic fumes of cyanide may evolve. Iron(III) chloride is a corrosive skin and tissue irritant and is slightly toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety information. 

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Check the pH of the final solution and neutralize as needed according to Flinn Suggested Disposal Method #24b.

Prelab Preparation

1. Place four 250-mL beakers in a row across the demonstration table. Place a 1-L beaker behind the four smaller ones.
2. Add 30 drops of 0.5% phenolphthalein solution to the first 250-mL beaker.
3. Add 30 drops of 0.04% thymolphthalein solution to the second 250-mL beaker. Swirl to mix the contents.
4. Add 0.2 g of iron(III) chloride hexahydrate plus 4 mL of 2 M hydrochloric acid to the third 250-mL beaker. Swirl to mix the contents.
5. Add 10 mL of 1 M sodium fluoride solution plus 8 mL of 2 M hydrochloric acid to the fourth 250-mL beaker. Swirl to mix the contents.
6. Add 500 mL of distilled or deionized water, 1 mL of 2 M sodium hydroxide, and 20 drops of 1 M potassium thiocyanate solution to the large 1-L beaker. Carefully swirl to mix the contents.

Procedure

1. Pour approximately 125 mL (one-fourth of the total volume) of the solution from the large 1-L beaker into each of the four 250-mL beakers to produce “strawberry soda,” “blueberry soda,” “red wine,” and “water,” respectively.
2. Pour the contents of the first three 250-mL beakers back into the large 1-L beaker to produce more “red wine.”
3. Refill the first three 250-mL beakers with this “red wine” from the large 1-L beaker.
4. Pour the contents of all four 250-mL beakers into the large 1-L beaker, adding the contents of the fourth 250-mL beaker last, to produce “water.”

Student Worksheet PDF

14090_Student1.pdf

Teacher Tips

• If too much iron(III) chloride is initially placed in the third beaker, the final solution produced in step 4 of the Procedure will be yellow. If this occurs, simply reduce the amount of iron(III) chloride initially added to the third beaker.
• Use distilled or deionized water for best results.
• Be creative when presenting this demonstration to your students by telling a story to go along with the demonstration. An example might be a beleaguered waiter or waitress trying to satisfy many customers at a table, each of whom wants something different to drink, and then changes his or her mind after the others in the group order their drinks.
• Demonstrate the colors of various indicators in acid and base solutions to help students better understand indicators.

Answers to Questions

A mixture of sodium hydroxide and potassium thiocyanate was originally present in the large 1-L beaker that was used to fill the other four, smaller beakers. The contents of those four beakers before the mixture was added are:

1. Describe the color change observed in each beaker. Explain each color change.
   1. Beaker 1

      The solution changed from colorless to pink because of the phenolphthalein, which is pink in basic solutions such as sodium hydroxide.

   2. Beaker 2

      The solution changed from colorless to blue because of the thymolphthalein, which is blue in basic solutions such as sodium hydroxide.

   3. Beaker 3

      The solution changed to a deep red. This is because the iron(III) chloride reacted with the potassium thiocyanate from the large 1-L beaker to produce iron(III) thiocyanate, which is a red-colored complex ion.

   4. Beaker 4

      The solution remained colorless because there is no indicator present and sodium fluoride does not react with sodium hydroxide or the potassium thiocyanate.

2. Why did the combination of the first three beakers turn red?

   The iron(III) thiocyanate was still present and maintained its deep red color. The indicators in beakers 1 and 2, phenolphthalein and thymolphthalein, are both colorless in an acidic solution such as hydrochloric acid, which was present in beaker 3.

3. Why did the combination of all four beakers turn colorless?

   The sodium fluoride reacted with the iron(III) thiocyanate, causing the fluoride ion to displace the thiocyanate ion and form the iron(III) fluoride complex ion (FeF²⁺), which is colorless. The indicators were still colorless due to excess hydrochloric acid.

Discussion

The reactions involved in this demonstration are as follows:

Beaker 1
The indicator used in the first 250-mL beaker is phenolphthalein, which is colorless in acid solutions and pink in basic solutions (pH >8). The solution in the large 1-L beaker contains sodium hydroxide, and is therefore a basic solution. When the basic solution from the large 1-L beaker is added to the phenolphthalein in the first beaker, the indicator turns the solution pink.

NaOH(aq) + phenolphthalein → pink-colored solution

Beaker 2
Thymolphthalein indicator is used in the second 250-mL beaker. It is colorless in acid solutions and blue in basic solutions (pH >10). The solution in the large 1-L beaker contains sodium hydroxide, and is therefore a basic solution. When the basic solution from the large 1-L beaker is added to the thymolphthalein in the second beaker, the indicator turns the solution blue.

NaOH(aq) + thymolphthalein → blue-colored solution

Beaker 3
The iron(III) chloride present in the third 250-mL beaker reacts with the potassium thiocyanate in the large 1-L beaker to produce the complex ion, iron(III) thiocyanate, FeSCN²⁺, producing a deep red–colored solution.

Fe³⁺(aq) + SCN^–(aq) → FeSCN²⁺(aq)

Beaker 4
Some of the acid in the fourth 250-mL beaker neutralizes the sodium hydroxide solution in the large beaker, but the mixture contains excess acid. No indicators are present and the solution therefore remains colorless.

Mixing Beakers 1–3
When the contents of the first three 250-mL beakers are combined, FeSCN²⁺ is still present in solution, maintaining the deep-red color of the mixture. Both of the indicators turn colorless due to the presence of excess hydrochloric acid from beaker 3, which makes the solution acidic.

HCl(aq) + phenolphthalein → colorless solution
HCl(aq) + thymolphthalein → colorless solution
FeSCN²⁺(aq) → deep-red solution

Mixing Beakers 1–4
Once the solution in the fourth 250-mL beaker is mixed with the first three 250-mL beakers, the solution turns colorless. This is due to the reaction between sodium fluoride and iron(III) thiocyanate. The fluoride ion displaces the thiocyanate ion resulting in the formation of the iron(III) fluoride complex ion, which is colorless. Both indicators are also colorless due to the presence of excess hydrochloric acid, which makes the solution acidic.

HCl(aq) + phenolphthalein → colorless solution
HCl(aq) + thymolphthalein → colorless solution
FeSCN²⁺(aq) + F^–(aq) → FeF²⁺(aq) + SCN^–(aq) → colorless solution