Boiling Water at Room Temperature

Introduction

Water boils at 100 degrees Celsius right? Not always! Every liquid boils at the temperature at which its vapor pressure equals the pressure above its surface. Is water boiling in a pressure cooker? No, the pressure is so great that the water reaches temperatures much higher than 100 °C without ever boiling. The vapor pressure of water reaches 760.0 mmHg (normal atmospheric pressure) at a temperature of 100 °C. The vapor pressure of water at 30 degrees is 31.82 mmHg. If we can lower the air pressure over 30° water to 31.82 mmHg or below, the water should boil. Follow the quick procedure below to demonstrate this surprising property to your students.

Materials

Safety Precautions

Be careful when pulling the plunger back. It has a tendency to snap back very quickly if it is not being held tightly.

Procedure

Fill syringe with approximately 5–10 mL of water. Warm water will boil more readily than colder water because its vapor pressure is higher. We encourage you to experiment with different temperatures. Close off the stopcock and pull the plunger out until you see the water boil. You may want every student to take a turn at pulling the plunger.

Discussion

You will notice that if the plunger is held back long enough, the boiling slows and eventually stops. As the water boils, the water vapor produced is pressurizing the area above it. When the plunger is originally pulled the air pressure in the syringe falls below the water’s vapor pressure, causing the water to boil. The water will continue to boil until the pressure equals that of the vapor pressure or until there is no liquid left—whichever comes first.

Water is used in this experiment mainly because when your students think of boiling water, they think that it must be very hot. However, this experiment can be done with other liquids as well, such as ethyl alcohol.

References

Special thanks to Mary Ellen Heus of New Berlin West High School in New Berlin, WI, for giving the author the inspiration for this activity.