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Latent Heat

When we boil water to evaporate it then what we observe. We observe that when we boil water to evaporate it at 100 °C, then all the given heat is absorbed by the liquid. This absorbed heat is called latent heat of vaporization and taken by vapour molecules. Similarly, when we melt ice then it is converted into water because ice molecules absorb this heat. This is called latent heat of fusion.

Thus, we can say that the amount of absorbed or released heat is called latent heat. This heat is released or absorbed during physical change. It is named according to the process of physical change. It is expressed as per mole amount of released or absorbed heat taken by substance. It can be done with change in temperature or without change in temperature or phase change process. Let’s discuss the detail description of latent heat and its different types during different stages.

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Latent Heat Definition

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When energy is transferred to a substance, usually, temperature of the substance increases. If the substance is at higher temperature then the surroundings, temperature of the substance decreases, as heat is transferred from the system or substance to the surroundings. From this we can conclude that a substance undergoes change in temperature whenever flow of energy transfer between system and its surroundings takes place.

But there are scenarios during which energy transfer between system and its surroundings does not result in the change in temperature of the system. This occurs whenever system changes from one of its phase to the another or we can say that when the transfer of energy results in the change in the physical properties of the system. This change is termed as Phase Change or Phase Transition. Two main Phase transitions are,
  1. Solid to Liquid
  2. Liquid to Gas
Other phase transition is when change takes place in the crystalline structure of the solid. When phase Transition occurs temperature of the system does not change but change in the internal energy of the system takes place.

At the boiling point of the water when heat is transferred to the water it results in the breaking of molecular bonds in water. As a result, the molecules become apart as their potential and kinetic energies get increased and water gets converted to the gaseous state. Every substance has different molecular and atomic structure. So, for the phase transition to occur every substance requires different amount of energy depending on their internal structure. It also depends on the mass of substance involved for the phase transition. The substance of a particular type with larger quantity and mass requires more heat for phase transition as compared to the other. It takes less energy to boil water in cooking pan than to boil a lake.

If a Q amount of energy needs to be transferred for phase change of a substance having mass m, the ratio symbolizes an important property of that substance. As the energy added or removed is not resulting in the change of temperature, the quantity L is termed as the latent heat. It can also be termed as the hidden heat of the substance.

Latent heat for a substance depends mainly on two factors :
  1. Phase change Nature
  2. Properties (atomic, molecular structure of the substance)

Latent Heat Formula

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From the definition, it is clear that the amount of heat required for a phase transition is latent heat. The mathematical representation of the given statement is,
$Q = mL$
L = $\frac{Q}{m}$

Where,
L = Latent heat of the substance
Q = Amount of energy transferred for the Phase change
m = Mass of the substance

Latent Heat of Vaporization

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Latent heat of Vaporization is the Latent heat (explained above) when phase of the substance changes from Liquid to gaseous state. Latent heat of vaporization is denoted by LV.

Latent Heat of Condensation

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Latent Heat of Condensation is defined as the quantity of heat needs to be transferred to convert a unit mass of a substance in gaseous phase into liquid phase at its condensation point at one atmosphere pressure.

L = $\frac{Q}{m}$ K- Cal/Kg

Latent Heat of Freezing

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Latent Heat of Freezing is defined as the quantity of heat needs to be transferred to convert a unit mass of a substance in liquid phase into solid phase at its freezing point, at one atmosphere pressure.

L = $\frac{Q}{m}$ K- Calorie/Kg

Latent Heat of Fusion

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Latent Heat of Fusion is the latent heat (explained above) when phase of the substance changes from Solid to Liquid phase. Latent heat of Fusion is denoted by LF. Also fuse means melting.

For a particular substance Latent heat of Vaporization is generally greater than its Latent heat of Fusion.

Latent Heat of Melting

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Latent Heat of Melting is defined as the quantity of heat needs to be transferred to convert a unit mass of a substance in solid phase into liquid phase at its melting point, at one Atmosphere pressure. It can be expressed as,

L = $\frac{Q}{m}$ Calorie/gm or K-Calorie/Kg

Latent Heat of Sublimation

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Latent Heat of Sublimation is defined as the quantity of heat needs to be transferred to convert a unit mass of a substance in solid phase into gas phase without passing the substance through the intermediate state of Liquid, and directly to the gaseous phase at its sublimation point and at one atmosphere pressure.

Latent Heat Flux

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Latent Heat Flux is denoted by Qe when water evaporates, it removes significant amounts of heat. The energy that is added to break the hydrogen bonds to allow evaporation is removed by the water vapor. It can be calculated with the help of Bulk Formula,

$Q_{e}$ = $\rho _{a}L_{E}C_{L}U_{10}(q_{a}-q_{s})$
Where,
$\rho _{a}$ = Density Of air (∼ 1.2 kg/m3)
$L_{E}$ = Latent Heat of vaporization (∼2260 kJ/kg)
$C_{L}$ = Bulk transfer coefficient (Stanton Number)
$U_{10}$ = Wind speed at 10 m
$q_{a}$ = specific Humidity at saturation
$q_{s}$ = specific Humidity of overlying air

Latent Heat vs Sensible Heat

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Latent Heat is the heat which is required to change the phase of substance from one state to other. And in the other hand, Sensible Heat is the heat which is added or the heat which is removed and it is the heat which changes the temperature.
Sensible Heat

When energy or heat is transferred to the substance, it results in increase in its temperature. The increase in the heat of the substance is called sensible heat. Similarly, when heat is transferred from the substance to its surroundings, heat is removed from the substance and results in decrease in the temperature of the system. The heat removed from the substance is also sensible heat. Heat, because of which change in temperature of the substance takes place, is called sensible heat.

Latent Heat
In contrast to Sensible Heat, Latent heat does not change the temperature of the system but it results in transition of the phase of the substance from one phase into other.

Latent Heats of Substances

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Latent Heat of Water, Latent Heat of Ice and Latent Heat of Steam are given in the tables below. Latent Heat of Fusion and Latent Heat of Vaporization for different substances are given in the below tables.

Table 1:

Substance Melting Point (°C)
Latent Heat Of Fusion (J/kg)
Boiling Point (°C) Latent heat Of Vaporization (J\kg)
Helium -269.65 5.23$\times 10^{3}$
-268.93 2.09$\times10^{4}$
Nitrogen -209.97 2.55 $\times 10^{4}$ -195.81 2.01$\times 10^{5}$
Oxygen -218.79 1.38 $\times 10^{4}$ -182.97 2.13$\times 10^{5}$
Ethyl Alcohol -114 1.04 $\times 10^{5}$ 78 8.54 $\times10^{5}$
Water0.003.33 $\times 10^{5}$100.002.26$\times 10^{6}$
Sulphur 119 3.81 $\times 10^{4}$ 444.60 3.26$\times 10^{5}$
Lead 327.3 2.45 $\times 10^{4}$ 1750 8.70$\times 10^{5}$
Aluminum
660 3.97 $\times 10^{5}$ 2450 1.14$\times 10^{7}$
Silver 960.80 8.82 $\times 10^{4}$ 2193 2.33$\times 10^{6}$
Gold 1063.00 6.44 $\times 10^{4}$ 2660 1.58$\times 10^{6}$
Copper 1083 1.34 $\times 10^{5}$ 1187 5.06$\times 10^{6}$


Table 2:


Substance
Latent Heat of
Fusion (kJ/kg)

Melting Point
(°C)
Latent heat Of
Vaporization
(J\kg)
Boiling Point (°C)
Alcohol,Ethyl 108 -114 855 78.3
Ammonia 339 -75 1369 - 33.34
Carbondioxide 184 -78 574 - 57
Helium 21 - 268.93
Hydrogen(2) 58 -259 455 - 253
Lead[6] 24.5 327.5 871 1750
Nitrogen 25.7 -210 200 - 196
Oxygen 13.9 -219 213 - 183
R134a -101 215.9 - 26.6
Toluene -93 351 110.6
Turpentine 293
Water 334 0 2260 100
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