Latent Heat & Phase Change

Edwin Ogie Library

Change of State: Latent Heat and Phase Changes

Objectives:
(i) Differentiate between latent heat and specific latent heats of fusion and vaporization;
(ii) Differentiate between melting, evaporation and boiling;
(iii) Examine the effects of pressure and dissolved substances on boiling and melting points;
(iv) Solve numerical problems.

Introduction to Change of State

Change of state refers to the transformation of matter from one phase to another—such as from solid to liquid or liquid to gas. This process requires or releases energy in the form of latent heat.

In this lesson, we discuss latent heat, specific latent heats of fusion and vaporization, and the processes of melting, evaporation, and boiling. We also examine how pressure and dissolved substances affect the melting and boiling points, along with practical applications in appliances.

Latent Heat

Latent Heat is the energy absorbed or released by a substance during a phase change at a constant temperature. This energy does not result in a temperature change but is used to break or form intermolecular bonds.

Specific Latent Heats of Fusion and Vaporization

Specific Latent Heat is the energy required to change the phase of one unit mass of a substance without a change in temperature.

• Specific Latent Heat of Fusion: Energy required to change a unit mass from solid to liquid (or vice versa) at the melting point.
• Specific Latent Heat of Vaporization: Energy required to change a unit mass from liquid to gas (or vice versa) at the boiling point.

Q = mL

where Q is the heat energy, m is the mass, and L is the specific latent heat.

Melting, Evaporation and Boiling

Melting: The process by which a solid turns into a liquid when heated to its melting point.

Evaporation: The gradual process by which molecules at the surface of a liquid gain enough energy to enter the gas phase, occurring at temperatures below the boiling point.

Boiling: The rapid vaporization of a liquid when it is heated to its boiling point, where bubbles form throughout the liquid.

Effects of Pressure and Dissolved Substances

The melting and boiling points of substances are influenced by external conditions:

• Pressure: Increasing pressure generally raises the melting point and can either raise or lower the boiling point depending on the substance.
• Dissolved Substances: The presence of solutes can lower the freezing point (freezing point depression) and raise the boiling point (boiling point elevation) of a solvent.

Applications in Appliances

The principles of latent heat and phase changes are applied in many everyday appliances:

• Refrigerators and Freezers: Use the evaporation and condensation of refrigerants to absorb and release heat, respectively.
• Air Conditioners: Operate on similar principles to refrigerators.
• Heat Pumps: Transfer heat from a cooler to a warmer place using the principles of latent heat.
• Industrial Processes: Involve phase changes in distillation, crystallization, and other processes.

Summary and Key Concepts

• Latent Heat: Energy involved in a phase change at constant temperature.
• Specific Latent Heat: Energy per unit mass required for a phase change.
• Melting, Evaporation, Boiling: Different phase change processes with distinct characteristics.
• Pressure and Dissolved Substances: Factors that influence melting and boiling points.
• Applications: Practical uses in appliances and industrial processes.

Understanding these concepts is crucial for solving numerical problems and for applications in both daily life and industrial settings.

Extended Discussion and Further Applications

Advanced studies in thermodynamics further explore the behavior of latent heat during phase transitions, including non-ideal conditions. The principles discussed here are foundational for understanding more complex phenomena such as supercooling, superheating, and the critical point of substances.

Engineers and scientists use these principles to design efficient systems for energy transfer, environmental control, and material processing.

10 Worked Examples on Change of State (Solutions Hidden)

Example 1: Differentiating Latent Heat and Specific Latent Heat

Question: Explain the difference between latent heat and specific latent heat.

Solution:

Latent heat is the total heat energy required for a phase change, while specific latent heat is the heat required per unit mass for that phase change.

Example 2: Specific Latent Heat of Fusion

Question: Describe how you would determine the specific latent heat of fusion of a solid using a simple experiment.

Solution:

One method is to heat a known mass of the solid until it melts and measure the energy required (using, for example, the electrical method). The specific latent heat of fusion is then calculated using c = Q/(m), where Q is the heat energy and m is the mass.

Example 3: Specific Latent Heat of Vaporization

Question: How can the specific latent heat of vaporization of water be determined using the method of mixtures?

Solution:

Heat a known mass of water to boiling and then mix it with a known mass of cooler water. Measure the final temperature and use energy conservation to calculate the energy used for vaporization, then determine the specific latent heat.

Example 4: Melting, Evaporation, and Boiling

Question: Differentiate between melting, evaporation, and boiling.

Solution:

Melting is the phase change from solid to liquid at the melting point. Evaporation is the gradual transition from liquid to gas at temperatures below the boiling point, while boiling is rapid vaporization throughout the liquid at its boiling point.

Example 5: Pressure and Boiling Point

Question: Explain how an increase in external pressure affects the boiling point of a liquid.

Solution:

An increase in external pressure raises the boiling point because a higher temperature is required for the vapor pressure of the liquid to equal the external pressure.

Example 6: Effect of Dissolved Substances on Melting Point

Question: How do dissolved substances affect the melting point of a solvent?

Solution:

Dissolved substances typically lower the melting point of a solvent (freezing point depression) due to the disruption of the orderly arrangement of molecules.

Example 7: Calculating Heat Energy for Phase Change

Question: If 5000 J of heat is supplied to 200 g of ice at 0°C, explain how you would determine whether the ice completely melts.

Solution:

Compare the supplied energy with the energy required to melt the ice completely using the specific latent heat of fusion for ice. If 5000 J is greater than or equal to the energy required, the ice melts completely.

Example 8: Using the Electrical Method

Question: Describe how the electrical method can be used to determine the heat capacity of a substance.

Solution:

An electrical heater supplies a known amount of energy (calculated as E = IVt) to the substance. By measuring the temperature increase, the heat capacity is calculated using C = E/ΔT.

Example 9: Newton’s Law of Cooling Application

Question: Explain how Newton’s law of cooling can be applied to determine the specific heat capacity of a liquid.

Solution:

By measuring the rate at which the liquid cools and applying Newton’s law (which relates the rate of cooling to the temperature difference between the liquid and its surroundings), one can deduce the specific heat capacity using the cooling curve data.

Example 10: Application in Appliances

Question: Provide an example of an appliance that uses the principles of phase change and latent heat to perform its function.

Solution:

Refrigerators use the evaporation and condensation of refrigerants. The refrigerant absorbs heat when it evaporates (using latent heat of vaporization) and releases heat when it condenses, thereby cooling the interior.

30 CBT JAMB Quiz on Change of State

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