Why Boiling Water Temperature Remains Constant Despite Increased Pressure

Understanding Boiling Points and Latent Heat

The temperature of water remains constant at a specific pressure when it is boiling. This is primarily due to the phase change process involved in boiling. When water reaches its boiling point under a certain pressure, it begins to convert from the liquid phase to the gaseous phase (steam). However, this temperature remains constant during the phase change process.

Boiling Point and Pressure Relationship

The boiling point of water is highly dependent on the pressure exerted on it. At standard atmospheric pressure (1 atm), water boils at 100°C (212°F). However, if the pressure is increased, the boiling point also increases. A common example of this is a pressure cooker, where the boiling point of water can exceed 100°C, allowing for faster and more efficient cooking.

Energy Input and Latent Heat of Vaporization

When heat is applied to boiling water, the energy input is primarily used to break the intermolecular forces between the water molecules, rather than to increase the temperature. This energy is known as the latent heat of vaporization. This phenomenon explains why the temperature of boiling water remains constant despite continuous heating, as long as there is still liquid water present.

The Phase Change Process

During the boiling process, water molecules gain enough energy to transform from the liquid phase to the gas phase. The energy required for this phase change is called the latent heat of vaporization. The temperature of the boiling water remains constant during this process because all the energy input is used for the phase change rather than increasing the temperature. Only once all the liquid has been completely boiled away will the temperature rise further.

Further Considerations for Ideal Gas Law

It is important to note that for an ideal gas, the equation PVnRT is applicable. However, when dealing with both liquid and gas phases (as in the case of boiling water), the behavior deviates from the ideal gas law, especially with respect to the constant 'R'. Vastly greater pressure would be required to increase the temperature of the water, particularly if a large volume of liquid water is present.

Conclusion

In summary, while increasing the pressure raises the boiling point, the temperature of boiling water remains constant at that new boiling point until the entire volume of water has been converted into gas. This is a fundamental concept in thermodynamics and helps explain why pressure cookers can achieve faster cooking times without increasing the temperature beyond the boiling point of the water.