Acid–Base Rainbow Fountain

Introduction

Quench your students thirst for knowledge with a fountain of concepts in this guided-inquiry demonstration. Hero’s Fountain has been around for a long time—about 1900 years! Hero of Alexandria described a water fountain that used compressed air to lift water to a point higher than its origin. At first glance it seems the fountain requires no energy to run. Acid–base chemistry add a colorful twist to the Hero’s Fountain in this demonstration.

Concepts

• Air pressure and vacuum
• Acid–base chemistry
• pH and indicators
• Neutralization

Experiment Overview

The fountain connection device demonstrates air pressure and pressure changes. Through its natural mechanism the solutions from the top bottle and the bottom bottle mix. With an indicator, an acid and a base present, the solution changes color as the pH changes.

Materials

Safety Precautions

Sodium hydroxide solutions are corrosive to all body tissues, especially to the eyes. Hydrochloric acid is toxic by ingestion or inhalation and corrosive to all body tissues. Universal indicator solution is a flammable liquid and is toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

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 Excess hydrochloric acid may be neutralized with a base, such as the leftover sodium hydroxide solution, and then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24b. The leftover sodium hydroxide solution may be neutralized with an acid, such as the leftover hydrochloric acid solution, and then flushed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #10. The resulting demonstration solution may be neutralized with either acid or base and then rinsed down the drain according to Flinn Suggested Disposal Methods #10 or #24b.

Prelab Preparation

Assemble the Fountain Connection

1. Assemble the fountain connection by inserting the two clear plastic tubes into the connector as shown in Figure 1. Insert the tubes approximately 1.5 cm. Note: The clear plastic tubes have small holes at the top of each tube. These holes should not be blocked The tubes should not be pushed into the fountain connection so far that the holes end up on the opposite side.
   {12347_Preparation_Figure_1}

Procedure

Procedure

1. Read all the directions before proceeding with this demonstration and practice this demonstration before presenting to the students.
2. Fill one of the two-liter bottles ¾ full with tap water.
3. Using a Beral-type pipet, slowly add 5 mL of universal indicator to the first bottle—do not mix.
4. Using a clean, graduated pipet, add 1 mL of 2.5 M sodium hydroxide solution dropwise to the bottle containing the water and indicator solution.
5. Attach the fountain connector to the bottle containing the solution. One tube will be submerged in the bottle containing the solution and the other clear plastic tube will be exposed to the air (see Figure 2).
   {12347_Procedure_Figure_2}
6. Using a Beral-type pipet, add one more mL of 2.5 M sodium hydroxide solution into the top of the clear plastic tube that is in the bottle containing the solution (see Figure 3).
   {12347_Procedure_Figure_3}
7. Quickly and carefully complete steps 8–12.
8. Using a wash bottle, add enough water to just cover the inside bottom surface of the fountain connector. Note: Be careful not to add water to the tubes (see Figure 4).
   {12347_Procedure_Figure_4}
9. Using a clean, graduated pipet, add two drops of universal indicator to the water in the fountain connector (see Figure 5).
   {12347_Procedure_Figure_5}
10. Using a clean, graduated pipet, add 10 drops of 6 M hydrochloric acid solution in the fountain connector (see Figure 5).
11. Slide the empty bottle over the clear plastic tube and screw the bottle into the fountain connection (see Figure 6).
    {12347_Procedure_Figure_6}
12. Placing one gloved hand around the fountain connector and the other on the bottom bottle, invert the entire apparatus so that the empty bottle is now on the bottom.
13. Hold the top bottle with your hand as a safety measure so that the bottle does not tip over. Once half of the solution has emptied from the top bottle to the bottom bottle, the entire setup will be more stable (see Figure 7).
    {12347_Procedure_Figure_7}

Optional

14. After all the solution has transferred from the top bottle to the bottom bottle unscrew the top (empty) bottle from the fountain connector.
15. Add 10 drops of 6 M hydrochloric acid to the fountain connector without getting any in the clear plastic tubes and reattach the bottle (see Figure 5).
16. Invert the bottle holding the fountain connector with one hand and the entire apparatus securely with the other hand.
17. Steps 14–16 may be repeated until the pH is reduced to 4 and the color no longer changes.
18. The system can be recharged at any time by adding 2.5 M sodium hydroxide solution to the clear plastic tube that is inserted in the bottle containing the solution. Recharging the system can be best performed during step 15 (see Figure 3).

Student Worksheet PDF

12347_Student1.pdf

Teacher Tips

• This kit contains enough chemicals and materials to perform the demonstration ten times: 50 mL of 6 M hydrochloric acid solution, 100 mL of 2.5 M sodium hydroxide solution, 50 mL of universal indicator, 30 disposable pipets, a reusable fountain connection, and two reusable 2-L bottles.
• The tubes of the fountain connection have small holes on one side. The position of these holes is important. These tubes will not work if the holes are blocked or come out the other side of the fountain connection.
• Other indicators such as phenolphthalein can also be used. If using phenolphthalein, add the phenolphthalein to the acid since the solution will remain colorless, Then, when enough base is added to the acidic solution to raise the pH to 8, the solution will turn pink. Phenolphthalein is colorless in acidic solutions (pH <7) and pink in solutions having ph = 8–10.
• Periodically the plastic 2-liter bottles will need to be replaced. traditional 2-liter soda bottles can be used after they have been cleaned. warm air from a hair dryer may aid in label removal for better visibility during demonstrations.

Answers to Questions

1. Draw a picture of the apparatus, naming major components. What solutions were initially present in the bottle(s) and the connector?
   {12347_Answers_Figure_8}
   The upper bottle contained a basic solution of sodium hydroxide and the connector contains an acidic solution of hydrochloric acid.
2. Describe the water flow and color changes when the bottle was inverted.

   After inversion the upper bottle containing purple solution flows into the bottom bottle bringing the acid from the connector along with it. The bottom bottle is more acidic at first (red) and then it will slowly go through the indicator colors (red → orange → yellow → green → teal → blue → purple) as more base flows from the upper bottle.

3. Explain the water flow in terms of the pressure in each bottle. What was the purpose of using an indicator in the Hero’s fountain?

   Gravity feeds water from the upper bottle to the lower bottle. As the water leaves the upper bottle the pressure is reduced creating a partial vacuum. As the water flows into the lower bottle the air is compressed, increasing the pressure. The indicator is added to the fountain so the acid-base reaction can be visually observed in an otherwise colorless reaction.

4. Would a drinking straw work in space where there is no atmospheric pressure? Explain. A drinking straw needs atmospheric pressure to work. The drinking straw relies on the atmospheric pressure to push the liquid up into the straw when a lower pressure is created above the liquid in the straw by the person drinking the liquid. If there was no atmospheric pressure, there would not be any pressure to push the liquid into the straw even if the person drinking from the straw was able to create a lower pressure above the liquid in the straw.

Discussion

Hero of Alexandria (ca 62 AD) described a water fountain that used compressed air to lift water to a point higher than its origin. The result was a fountain that seemed to defy both logic and the laws of nature. A closer study of the fountain will reveal the principles of its operation.

When the bottles are inverted, gravity pulls the water from the upper bottle down through the lower tube and compresses the air in the lower bottle. When water leaves the upper bottle, a decrease in pressure, or partial vacuum, is formed. Air is then forced from the lower bottle, up the fountain tube, and takes the place of the water that is leaving the upper bottle (see Figure 9).

If the clear tubes are examined near where they enter the bottle connector, several small holes will be visible. When the bottle is turned over, water runs into these small holes and is pushed upwards by the air with enough force to form the fountain at the top of the bottle. This is the same type of action that moves water in fish tank filters and drinking straws. When a drinking straw is put inside a beverage, the pressure exerted on both the liquid inside the straw and the liquid inside the cup is the same—atmospheric pressure. A drinking straw works because the pressure above the liquid inside the straw is reduced by a person’s mouth creating a lower pressure, but the pressure on the rest of the liquid in the cup is not changed. The atmospheric pressure continues to push on the rest of the liquid in the cup, forcing the liquid up the straw. The tube in the lower bottle has no effect on the formation of the fountain. The fountain connection can be shown to work when the bottom tube is removed. The fountain is also a unique way to mix solutions and to see beautiful color changes with indicators and acid–base chemistry. Universal indicator is added to the solution so the acid–base reaction can be visually observed in an otherwise colorless reaction.

Universal indicator reveals pH changes by changing colors (see Figure 10). When sodium hydroxide solution (NaOH) is added to the water, the pH increases to greater than 10. When universal indicator is added, the sodium hydroxide and water solution is a violet color. As small amounts of 6 M hydrochloric acid are added, the pH will decrease. A fundamental neutralization reaction is an acid and a base reacting to form a salt and water. The acid and base in this demonstration react as follows:

HCl(aq) + NaOH(aq) → H₂O(l) + NaCl(aq)

The HCl to NaOH ratio is 1:1 but the molarities are not the same. Using 2 mL of 2.5 M NaOH, it will take 0.8 mL of 6 M hydrochloric acid solution to neutralize the solution. When using the starting amount of 2 mL of 2.5 M NaOH, it should take 3 additions of hydrochloric acid solution at 0.5 mL each (10 drops) to cycle through all of the universal indicator colors. When the hydrochloric acid solution is added to the fountain connection and the system is inverted, the solution in the near empty bottom bottle will be highly acidic. The indicator slowly becomes more basic as solution transfers from the top bottle into the bottom bottle.