Content Curator
Latent heat is defined as the amount of heat that is radiated or absorbed at constant temperature and pressure, during the transition phase of a substance. The transition can be from gas to liquid or from liquid to solid and vice versa. Latent Heat is related to the heat property called enthalpy.
| Table of Content |
Key Takeaways: Latent Heat, Heat of Vaporization, Heat of Freezing, Latent Heat of Fusion, Latent Heat of Sublimation, Specific Latent Heat, Enthalpy, Transition Phase
What Is Latent Heat?
[Click Here for Sample Questions]
The temperature of the substance remains constant throughout the process. Latent heat is thus the work that is done to overcome the attractive forces that are responsible for holding atoms and molecules together, thus, making a substance capable of transitioning between phases.
Consider that a solid substance is transitioning to a liquid state. In order to achieve the proper transition, the substance will absorb energy to push the atoms and molecules away from each other. However, when a substance has to change from gas to liquid phase, it needs to release energy to let the molecules and atoms come closer together. The heat that is absorbed in the first case and that is released in the second so as to boil or melt a substance is what is latent heat.
Examples of Latent Heat :
The main two examples of latent heat are the following:
- The heat of vaporization
- The heat of freezing
The heat of vaporization is the amount of heat that is to be added in order to transfer a liquid into a gas and the heat of freezing is the amount of heat that must be released for a liquid to freeze.
Read More: Coefficient of Linear Expansion
Latent Heat Formula
[Click Here for Sample Questions]
The formula for latent heat is given as
Q = m x L
In which,
Q is the amount of heat that is absorbed or released
m is the mass of the substance and
L is the Specific Latent Heat
The standard or the most common units that are used for latent heat are joules per gram (J/g) and kilojoules per kilogram (kJ/kg).
Types of Latent Heat Transfer
[Click Here for Sample Questions]
Latent Heat Transfer can occur mainly in three types. They are:
- Latent Heat of Fusion
- Latent Heat of Vaporization
- Latent Heat of Sublimation
Here is a detailed description of each of the following.
Latent Heat of Fusion
The absorption or discharge of heat at a constant temperature when a matter changes from solid to liquid and vice-versa is defined as the latent heat of fusion. In simpler words, latent heat of fusion is the heat that is required to change a solid to liquid or a liquid to solid, without bringing any variation in temperature.
The inward energy is higher in the fluid state than in the liquid state. This implies that energy has to be provided in order to dissolve a solid and energy must be released to solidify a liquid. When a fluid is cooled, its temperature falls until it drops just underneath the point of solidification. The temperature stays constant at its point, allowing the liquid to solidify. 3.34 x 105 joules per kilograms is the latent heat of fusion of ice.
Latent Heat of Vaporization
The absorption or release of heat at a constant temperature when a matter disintegrates from fluid state to gaseous state is called the Latent heat of Vaporization. Latent heat of vaporization thus simply is the measure of heat that is absorbed or released to change a matter from its fluid state to gaseous state without bringing any variation to the temperature. The heat of vaporization is characterized as the measure of heat that can transform 1g of a fluid into a fume at a constant temperature.
The heat of vaporization of water is known to be the most elevated one. Particles of water vapour at a temperature of 100oC (373K) is found to have more energy than water at the same temperature. This is the result of the presence of extra heat in water vapour in the form of latent heat of vaporization.
Latent Heat of Sublimation
Substances like naphthalene and camphor are capable of getting directly transformed into a gaseous state from solid-state. The measure of heat that is either absorbed or released by a substance to directly transfer from solid to gaseous state or vice versa without bringing any variation in temperature is known as the latent heat of sublimation.
Specific Latent Heat
[Click Here for Sample Questions]
Specific latent heat is defined as the amount of energy that is required per unit mass of a substance (ice) to change its physical state (water) at a constant temperature. The specific latent heat is calculated using the formula,
L = ΔQ/m
In which,
L is the specific latent heat
ΔQ is the heat energy that is added or released and
m is the mass of the substance that changes its physical state.
Since the temperature stays constant during the process, the formula does not have any temperature parameter.
The most widely seen changes while the temperature stays constant are state changes like solidifying, liquefying, vaporization or build-up. The heat or energy required to do these physical changes are called ‘latent’ as they are seen to be covered up inside the atoms until the change of state happens.
Things to Remember
- Latent heat is defined as the amount of heat that is radiated or absorbed at constant temperature and pressure, during the transition phase of a substance from one state to another.
- The two examples of Latent heat are Latent heat of vaporization and Latent heat of Freezing.
- The heat of vaporization is the amount of heat that is to be added in order to transfer a liquid into a gas and the heat of freezing is the amount of heat that must be released for a liquid to freeze.
- The absorption or discharge of heat at a constant temperature when a matter changes from solid to liquid and vice-versa is defined as the latent heat of fusion.
- Specific latent heat is defined as the amount of energy that is required per unit mass of a substance (ice) to change its physical state (water) at a constant temperature.
-
The specific latent heat is calculated using the formula, L = ΔQ/m.
Sample Questions
Ques. What is the latent heat of a substance with a mass of 10 kg if the amount of heat needed by it for the transition phase is 300 Kcal? (2 marks)
Ans.
Here,
Q = 300 Kcal
M = 10 kg
The latent heat formula is,
L = Q/m
Applying the values, we get,
L = 300/5
= 60 k.cal kg-1
Hence the latent heat of the substance is 60 k.cal kg-1
Ques. Give a brief account of the applications of Specific Latent Heat. (4 marks)
Ans. The concept of specific latent heat has some applications. They are as follows.
Putting out the fire using water
Water when poured over fire becomes hot making it capable of absorbing a greater amount of latent heat from the fire.
Using salt to melt ice
The specific latent heat of ice is more than that of water. Thus, when salt is placed over a thick layer of ice, it absorbs energy from it, thus making the ice melt at a faster pace.
Using cold water and ice to cool drinks
Adding cold water or ice into a drink can help to make it cold. This happens because the ice that is put on the drink will absorb a large amount of latent heat to melt. Thus, the drink will lose heat making the drink cool.
Steaming of Food
Food while being cooked is usually covered to stop the steam from escaping. The steam is a result of the vaporization of water and the specific latent heat of the vaporization of water is immense. If the steam stays, the food becomes capable of absorbing this heat, thus helping in cooking the food faster.
Ques. What is the energy that is required to change 0.65 kg of ice at 0°C into the water at 0°C ? (3 marks)
Ans. Since ice is to be converted to water, it is the latent heat of fusion.
We know that latent heat of fusion of ice 3.34 x 105 J/kg
The latent heat formula is,
L = Q/m
In which,
Q is the amount of heat that is absorbed or released
m is the mass of the substance and
L is the Specific Latent Heat
So, to find the required energy Q, we have to use the formula,
Q = L × m
Here,
L = 3.34 x 105 J/kg
m = 0.65 kg
Thus,
Q = 3.34 × 105 × 0.65
= 217100 J
Thus, the heat energy required to melt 0.65 kg water is 217100 J
Ques. In order to produce water at 100°C, 5.65 × 106 J of heat energy is released from a mass of steam at 100°C. Calculate the mass of the water that is produced. (3 marks)
Ans. Steam is converted to water here so,
Latent heat of vaporization = 22.6 × 105 J/kg
The latent heat formula is,
L = Q/m
In which,
Q is the amount of heat that is absorbed or released
m is the mass of the substance and
L is the Specific Latent Heat
Here, we have to find the mass of water produced.
So, we can use the formula,
m = Q/L
= 5.65 ×106 × 22.6 × 105
5.65 × 106 × 22.6 × 105
= 2.5 kg
Thus, the mass of water produced is 2.5 kg
Ques. 400 k.cal heat is required to change the state of a 2 kg substance. Calculate its latent heat. (2 marks)
Ans.
From the question, we have,
The required heat, Q = 400 k.cal
Mass of the substance, m = 2 kg
The latent heat formula is,
L = Q/m
In which,
Q is the amount of heat that is absorbed or released
m is the mass of the substance and
L is the Specific Latent Heat
Applying the values in the formula, we get,
L = 400/2
= 200 k.cal/kg
Thus, the latent heat of the substance is 200 k.cal/kg
Read More:
Comments