The Boiling Point of Water

Oct 13, 2025 by wbrum.org 10 views

Introduction

Imagine the following sequence of events:

I. Some ice cubes are left in a pan, then closed with a glass lid with a small opening (exhaust) on a stove.

II. After some time in the pan, the water is in a state of equilibrium, with the temperature of the water being equal to the temperature of the surroundings.

In this thought experiment, we will explore the concept of boiling point and how it relates to the behavior of water in a closed system. We will delve into the world of chemical thermodynamics and examine the factors that influence the boiling point of water.

The Boiling Point of Water

The boiling point of water is the temperature at which the vapor pressure of water equals the surrounding pressure. At this temperature, the water molecules have enough energy to escape the surface tension of the liquid and turn into vapor. The boiling point of water is 100°C (212°F) at standard atmospheric pressure.

The Role of Pressure

The boiling point of water is influenced by the surrounding pressure. As the pressure increases, the boiling point of water also increases. This is because the higher pressure forces the water molecules to have more energy to escape the surface tension of the liquid. Conversely, as the pressure decreases, the boiling point of water decreases.

The Effect of a Small Opening

In our thought experiment, the pan is closed with a glass lid that has a small opening (exhaust). This small opening allows for the escape of vapor, but it also allows for the entry of air. As the water in the pan heats up, the vapor pressure increases, and some of the vapor escapes through the small opening. However, the air that enters the pan through the small opening also cools the water, which slows down the rate of evaporation.

The Equilibrium State

After some time in the pan, the water is in a state of equilibrium. The temperature of the water is equal to the temperature of the surroundings, and the rate of evaporation is equal to the rate of condensation. At this point, the system is in a state of dynamic equilibrium, where the forward and reverse processes are occurring at the same rate.

The Chemical Perspective

From a chemical perspective, the boiling point of water is a manifestation of the intermolecular forces between the water molecules. The hydrogen bonds between the water molecules are responsible for the high boiling point of water. As the temperature increases, the kinetic energy of the water molecules also increases, which allows them to break free from the hydrogen bonds and turn into vapor.

The Thermodynamic Perspective

From a thermodynamic perspective, the boiling point of water is a manifestation of the second law of thermodynamics. The second law states that the total entropy of a closed system will always increase over time. In the case of the boiling point of water, the entropy of the system increases as the water molecules turn into vapor and escape the surface tension of the liquid.

Conclusion

In conclusion, the boiling point of water is a complex phenomenon that is influenced by a variety of factors, including pressure, temperature, and intermolecular forces. The thought experiment presented in this article highlights the importance of understanding the chemical and thermodynamic principles that govern the behavior of water in a closed system.

References

Atkins, P. W. (1998). Physical Chemistry. Oxford University Press.
Chang, R. (2008). Physical Chemistry for the Biosciences. University Science Books.
Levine, I. N. (2009). Physical Chemistry. McGraw-Hill.

Glossary

Boiling Point: The temperature at which the vapor pressure of a liquid equals the surrounding pressure.
Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid phase.
Entropy: A measure of the disorder or randomness of a system.
Hydrogen Bond: A type of intermolecular force that occurs between molecules with a hydrogen atom bonded to a highly electronegative atom.
Boiling Point: A Q&A Guide =============================

Introduction

In our previous article, we explored the concept of boiling point and how it relates to the behavior of water in a closed system. In this article, we will answer some of the most frequently asked questions about boiling point and provide a deeper understanding of this complex phenomenon.

Q: What is the boiling point of water?

A: The boiling point of water is 100°C (212°F) at standard atmospheric pressure.

Q: Why does the boiling point of water increase with pressure?

A: The boiling point of water increases with pressure because the higher pressure forces the water molecules to have more energy to escape the surface tension of the liquid.

Q: What is the effect of a small opening on the boiling point of water?

A: A small opening in a closed system allows for the escape of vapor, but it also allows for the entry of air. This can slow down the rate of evaporation and affect the boiling point of water.

Q: What is the role of intermolecular forces in boiling point?

A: Intermolecular forces, such as hydrogen bonds, play a crucial role in determining the boiling point of a substance. The strength of these forces determines how easily the molecules can escape the surface tension of the liquid.

Q: Can the boiling point of water be affected by other factors?

A: Yes, the boiling point of water can be affected by other factors, such as temperature, pressure, and the presence of impurities. For example, the boiling point of water can decrease with increasing temperature and increase with increasing pressure.

Q: What is the difference between boiling point and freezing point?

A: The boiling point and freezing point are two distinct temperatures that are related to the behavior of a substance. The boiling point is the temperature at which a substance changes state from liquid to gas, while the freezing point is the temperature at which a substance changes state from liquid to solid.

Q: Can the boiling point of water be used to determine the purity of a substance?

A: Yes, the boiling point of water can be used to determine the purity of a substance. Impurities in a substance can affect the boiling point, so by measuring the boiling point, you can determine the level of impurities present.

Q: What are some common applications of boiling point?

A: Boiling point has many practical applications, including:

Distillation: Boiling point is used to separate mixtures of liquids based on their boiling points.
Purification: Boiling point is used to remove impurities from a substance.
Cooking: Boiling point is used to determine the cooking time and temperature for various foods.
Chemical reactions: Boiling point is used to control the rate of chemical reactions.

Q: Can the boiling point of water be affected by altitude?

A: Yes, the boiling point of water can be affected by altitude. At higher altitudes, the atmospheric pressure is lower, which can cause the boiling point of water to decrease.

Q: What is the boiling point of water at high pressure?

A: The boiling point of water at high pressure is higher than at standard atmospheric pressure. For example, at a pressure of 1000 psi, the boiling point of water is around 374°C (675°F).

Conclusion

In conclusion, boiling point is a complex phenomenon that is influenced by a variety of factors, including pressure, temperature, and intermolecular forces. By understanding the principles of boiling point, we can better appreciate the behavior of substances in various situations.

References

Atkins, P. W. (1998). Physical Chemistry. Oxford University Press.
Chang, R. (2008). Physical Chemistry for the Biosciences. University Science Books.
Levine, I. N. (2009). Physical Chemistry. McGraw-Hill.

Glossary

Boiling Point: The temperature at which the vapor pressure of a liquid equals the surrounding pressure.
Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid phase.
Entropy: A measure of the disorder or randomness of a system.
Hydrogen Bond: A type of intermolecular force that occurs between molecules with a hydrogen atom bonded to a highly electronegative atom.