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.
See how temperature stays constant while latent heat is absorbed or released during melting, boiling, condensation and freezing.
Closely packed, vibrating in fixed positions.
Closely packed, slipping and moving throughout the liquid.
Widely separated, moving fast in random directions.
Latent heat is the “hidden” heat absorbed or released when a material changes its phase without changing its temperature.
Latent heat is heat absorbed during melting and boiling or released during condensation and freezing without any change in temperature.
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
where \(l\) = specific latent heat (\(\text{J kg}^{-1}\)), \(Q\) = quantity of heat absorbed or released (\(\text{J}\)), and \(m\) = mass (\(\text{kg}\)).
The quantity of heat, \(Q\), absorbed during melting or released during freezing of \(1\text{ kg}\) of the substance without any change in temperature.
The quantity of heat, \(Q\), absorbed during boiling or released during condensation of \(1\text{ kg}\) of the substance without any change in temperature.
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.
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.
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.
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 \(100^\circ\text{C}\).
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.
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?
Calculate the heat needed to completely melt \(2\text{ kg}\) of ice at \(0^\circ\text{C}\). (Given \(l_f = 3.34 \times 10^5\text{ J kg}^{-1}\))
Why is steam at \(100^\circ\text{C}\) more dangerous and able to cause more severe burns than boiling water at \(100^\circ\text{C}\)?
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.
Objective: Test your understanding of specific latent heat, phase changes and the refrigerator cooling cycle.
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?
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 .
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.
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