Question

As a liquid is heated at constant pressure, its temperature rises. This trend continues until the boiling point of the liquid is reached. No further rise in temperature of the liquid can be induced by heating. Explain.

Short answer

Heating only increases temperature until the boiling point; beyond that, it facilitates phase change at constant temperature.

Step-by-step solution

Understanding Heating at Constant Pressure

When liquid is heated at a constant pressure, energy is added in the form of heat. This energy typically increases the kinetic energy of the molecules, resulting in a temperature rise. Temperature, which is a measure of average kinetic energy of molecules, therefore rises as heat continues to be added to the liquid.

Reaching the Boiling Point

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. At this point, the liquid can transition to the gas phase. Initially, added heat raises the temperature of the liquid until this point is reached.

Heat Addition at the Boiling Point

Once the boiling point is reached, additional heat is used to overcome intermolecular forces not to increase kinetic energy. This energy facilitates the phase transition from liquid to gas, known as vaporization. Therefore, temperature remains constant during this process.

Energy Usage During Phase Change

During the phase change, the heat (known as latent heat of vaporization) is used entirely to convert the liquid into vapor without temperature change. Consequently, the temperature does not rise further even as heating continues.

Key concepts

Constant Pressure Heating

When a liquid is heated at a constant pressure, heat energy is continuously added to it. This heat energy influences the temperature of the liquid by increasing the kinetic energy of its molecules. Under these conditions,

• the added energy leads directly to a rise in temperature.
• each unit of heat energy increases the movement of molecules, resulting in higher kinetic energy.

However, the situation changes once the liquid reaches its boiling point. At this stage, the temperature stops increasing even though energy is still being added.

Kinetic Energy

Kinetic energy is a crucial concept when discussing the heating of liquids. In physics, kinetic energy describes the energy of motion. For liquids:

• a higher kinetic energy means the molecules are moving faster.
• temperature is a measure of this average kinetic energy.

As heat energy is added to a liquid, the molecules move more rapidly, raising the liquid's temperature. This direct relationship continues until the boiling point is achieved. At the boiling point, kinetic energy stops increasing because energy is directed towards the phase change.

Vaporization

Vaporization is the process where the molecules in a liquid gain enough energy to enter the gaseous phase. At constant pressure, when a liquid reaches its boiling point:

• any additional energy is used to break intermolecular forces that hold the liquid together.
• this energy is known as the latent heat of vaporization.
• it allows molecules to move apart and transition into the gas phase.

Due to this energy redirection, the temperature remains steady during vaporization, even though heating continues.

Phase Change

A phase change refers to the transition between different states of matter, such as from liquid to gas. During this transformation:

• the energy added doesn't increase the temperature, but facilitates the change of state.
• this process is driven by the latent heat of vaporization.
• the temperature plateau, despite ongoing heating, evidences this energy focus shift.

It's important to note that phase changes like vaporization require significant energy input to occur, even as the temperature remains unaltered until the transition completes.