4.3 Specific Latent Heat
Matter exists in three states: solid, liquid, and gas. When a substance undergoes a phase change, such as melting, boiling, condensation, or freezing, heat energy is absorbed or released without any change in temperature. This thermal energy involved in phase transitions is called latent heat.
Learning Objectives
- State the meaning of latent heat and specific latent heat (\(l\)).
- Distinguish between specific latent heat of fusion (\(l_f\)) and specific latent heat of vaporisation (\(l_v\)).
- Explain changes in state of matter based on kinetic theory during heating and cooling curves.
- Solve problems involving specific latent heat using \(Q = ml\) and \(Pt = ml\).
- Describe experimental methods to determine the specific latent heat of fusion of ice and vaporisation of water.
- Explain daily life applications of specific latent heat including cooling by evaporation, refrigeration systems, and food steaming.
Heating and Cooling Curves with Phase Changes
See how temperature stays constant while latent heat is absorbed or released during melting, boiling, condensation and freezing.
Heating Curve: Solid → Liquid → Gas
Cooling Curve: Gas → Liquid → Solid
Solid
Closely packed, vibrating in fixed positions.
Liquid
Closely packed, slipping and moving throughout the liquid.
Gas
Widely separated, moving fast in random directions.
Short Explanation
Latent heat is the “hidden” heat absorbed or released when a material changes its phase without changing its temperature.
What is Latent Heat?
Latent heat is heat absorbed during melting and boiling or released during condensation and freezing without any change in temperature.
What is Specific Latent Heat?
Specific latent heat (\(l\)) is the quantity of heat, \(Q\), that is absorbed or released during a change of phase of \(1\text{ kg}\) of the substance without any change in its temperature.
Specific latent heat formula
\(l = \dfrac{Q}{m}\)where \(l\) = specific latent heat (\(\text{J kg}^{-1}\)), \(Q\) = quantity of heat absorbed or released (\(\text{J}\)), and \(m\) = mass (\(\text{kg}\)).
Specific Latent Heat of Fusion (\(l_f\))
The quantity of heat, \(Q\), absorbed during melting or released during freezing of \(1\text{ kg}\) of the substance without any change in temperature.
Specific Latent Heat of Vaporisation (\(l_v\))
The quantity of heat, \(Q\), absorbed during boiling or released during condensation of \(1\text{ kg}\) of the substance without any change in temperature.
Why is \(l_v\) larger than \(l_f\)?
For any specific substance, \(l_v\) is significantly larger than \(l_f\) because vaporisation requires completely breaking intermolecular bonds and separating molecules far apart against atmospheric pressure, whereas fusion only requires weakening the bonds so molecules can move out of their fixed positions.
Kinetic Theory During Phase Changes
Temperature is determined by the average kinetic energy of molecules. During melting and boiling, heat absorbed is used to overcome or weaken intermolecular bonds, not to increase molecular speed. Therefore, average kinetic energy does not increase and temperature remains constant.
| Stage | Process | Heat | Temperature | Kinetic energy |
|---|---|---|---|---|
| PQ | Solid heated | Absorbed | Increases | Increases |
| QR | Melting at melting point | Latent heat of fusion absorbed | Constant | Constant |
| RS | Liquid heated | Absorbed | Increases | Increases |
| ST | Boiling at boiling point | Latent heat of vaporisation absorbed | Constant | Constant |
| TU | Gas heated | Absorbed | Increases | Increases |
Experiment: Specific Latent Heat of Fusion of Ice
An immersion heater (\(P\)) melts crushed ice in a filter funnel. An identical control set without a heater accounts for ice melted by surrounding heat. Formula: \(l_f = \dfrac{Pt}{m_1 - m_2}\), where \(m_1\) is water collected in the experiment set and \(m_2\) is water collected in the control set.
Experiment: Specific Latent Heat of Vaporisation of Water
A high-power immersion heater (\(500\text{ W}\)) boils water on an electronic balance. Initial reading \(m_1\) is recorded when water boils; after time \(t\), final reading \(m_2\) is recorded. Mass of steam produced = \(m_1 - m_2\). Formula: \(l_v = \dfrac{Pt}{m_1 - m_2}\). A control set is not needed because room temperature is much lower than \(100^\circ\text{C}\).
Applications of Specific Latent Heat
Evaporation of sweat absorbs latent heat of vaporisation from the skin, cooling the body. In a refrigerator, the compressor raises gas pressure and temperature, the condenser releases latent heat to the surroundings, the expansion valve drops pressure, and the evaporator absorbs latent heat from inside the refrigerator. Steam releases a very large quantity of latent heat when it condenses onto cold food, cooking it rapidly. Ice cubes cool drinks by absorbing latent heat of fusion.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why does the temperature of boiling water remain constant at \(100^\circ\text{C}\) even though it is being heated continuously?
Check Answer
Calculate the heat needed to completely melt \(2\text{ kg}\) of ice at \(0^\circ\text{C}\). (Given \(l_f = 3.34 \times 10^5\text{ J kg}^{-1}\))
Check Answer
Why is steam at \(100^\circ\text{C}\) more dangerous and able to cause more severe burns than boiling water at \(100^\circ\text{C}\)?
Check Answer
Common Mistakes
- Using \(Q = mc\Delta\theta\) during phase transitions when there is no temperature change (\(\Delta\theta = 0\)). The correct formula during a phase change is \(Q = ml\).
- Forgetting to convert units, such as time from minutes to seconds (\(t \times 60\)) when using \(E = Pt\).
- Forgetting to subtract the control set mass (\(m_2\)) in the ice fusion experiment, resulting in an inaccurate value for mass melted purely by the immersion heater (\(m_1 - m_2\)).
- Confusing specific latent heat of fusion (\(l_f\)) with specific latent heat of vaporisation (\(l_v\)).
Concept Misunderstandings
Because heat is continuously supplied to boiling water, its temperature must still be rising.
Temperature is a measure of the average kinetic energy of molecules. During boiling, the heat supplied is absorbed as latent heat of vaporisation to break the bonds between molecules and separate them, so the average kinetic energy of the molecules does not increase and the temperature stays constant.
Sweating cools the body merely by the sweat being cold.
Sweat cools the body because it absorbs the specific latent heat of vaporisation directly from skin tissues in order to evaporate into the air.
Summary
- Latent heat is heat absorbed or released during a phase change at constant temperature.
- Specific latent heat: \(l = \dfrac{Q}{m}\) with S.I. unit \(\text{J kg}^{-1}\).
- Specific latent heat of fusion (\(l_f\)) applies to solid \(\rightleftharpoons\) liquid; specific latent heat of vaporisation (\(l_v\)) applies to liquid \(\rightleftharpoons\) gas.
- Values of \(l_v\) are greater than \(l_f\) for the same substance because gas molecules must be completely separated against attractive forces and external pressure.
- In electrical experiments: \(Pt = ml\), or with a control set: \(l = \dfrac{Pt}{m_1 - m_2}\).
Short Activity
Objective: Test your understanding of specific latent heat, phase changes and the refrigerator cooling cycle.
A. Objective Quiz
1 Which of the following occurs to the molecules of a substance while it is boiling at its boiling point?
2 What is the primary purpose of setting up a control set in the experiment to determine the specific latent heat of fusion of ice?
B. Fill in the Blanks
3 The S.I. unit for specific latent heat is .
4 In the refrigerator cooling cycle, the cooling agent absorbs latent heat and evaporates inside the .
C. Matching / Drag and Drop
Match each refrigerator component or process to its correct role or function. If using a phone, tap the answer first, then tap the matching box.
