Cincinnati Form Franklin Flask

Introduction

The ultimate in discrepant events—show your students that you can boil water by cooling it!

Concepts

• Boiling point
• Vapor pressure

Materials

Safety Precautions

Do not attempt this demonstration with a Florence flask. Any stressed areas on the flask could break and cause a dangerous implosion. The flask and the hot water can cause burns. Trying to seat the stopper too firmly may cause the neck of the flask to shatter. Wear chemical splash goggles, thermal gloves and a chemical-resistant apron.

Prelab Preparation

1. Carefully insert a thermometer half-way into the 1-hole stopper.
2. Add 500 mL of water to the Franklin flask. Add the stopper with the thermometer into the neck of the flask. Invert the flask and ensure that the thermometer tip is still in the water.
3. Attach the ring to the ring stand. Make sure the inverted flask with the thermometer clears the surface of the stand (see Figure 1).
   {13933_Preparation_Figure_1}
4. Set the flask down on the benchtop and remove the stopper and thermometer.

Procedure

1. Fill the Franklin flask with approximately 500 mL of distilled or deionized water.
2. Set the flask on a hot plate and bring the water to a vigorous boil.
3. Using the thermal gloves, remove the flask from the hot plate and place it on a ceramic fiber pad.
4. When steam stops coming out of the neck of the flask, insert the stopper with the thermometer firmly into the flask.
5. With the insulated gloves, invert the flask, over the sink, to test for leaks.
6. If no leaks occur, place the inverted flask in the ring.
7. Place crushed ice into the concave bottom of the flask. Water will boil until the ice melts. Note the temperature.
8. Continue placing crushed ice in the concave bottom until the water temperature approaches within 10–15 °C of room temperature.

Student Worksheet PDF

13933_Student1.pdf

Answers to Questions

1. Draw a diagram of the set-up for this demonstration.

   See Figure 1 in the Preparation section for general diagram.

2. Describe what happened after the ice was placed in the concave bottom of the flask.

   After the ice was placed in the bottom of the flask, water vapor started condensing on the cold surface created inside the flask. Soon, the ice began to melt, and simultaneously the water in the flask began to boil.

3. The pressure inside the stoppered flask is initially equal to the vapor pressure of the water. When ice is placed in the bottom of the flask, some of the water vapor condenses on the cold surface, which lowers the pressure inside the flask. Explain why this causes the water to start boiling again.

   A liquid boils when its vapor pressure is equal to the surrounding or external pressure. As water vapor condenses on the cold surface, the pressure inside the flask decreases. This upsets the equilibrium established between the vapor pressure and the surrounding gas pressure inside the flask. The water begins to boil in order to reestablish this equilibrium.

Discussion

The boiling point of a liquid is defined as the temperature at which its vapor pressure is equal to the surrounding or external pressure. Since the water in the flask was boiling before the flask was stoppered, most of the air in the flask was replaced with water vapor. The pressure inside the closed flask will equal the vapor pressure of the water at any given temperature (see Figure 2).

When ice is added to the concave bottom, it creates a cold surface. Some of the water vapor inside the flask condenses on the cold surface and reduces the pressure inside the flask below the equilibrium vapor pressure of water (see Figure 3). The water then begins to boil to reestablish equilibrium (see Figure 4). The water molecules with the highest kinetic energy enter the vapor phase, which in turn lowers the average kinetic energy and the temperature of the water via evaporative cooling.