LATENT HEAT | LATENT HEAT OF FUSION | LATENT HEAT OF VAPORISATION | STATE OF CHANGE OF MATTER SOLID TO LIQUID AND LIQUID TO GAS VICE VERSA| Heating curve | Heating Graph|

 When heat is given or released to a matter, its temperature increases or decreases.

                                    or

In other words, it can be said that on heating a matter, the bonds between the molecules present in it get weakened And the matters are in their maximum expansion. now if you heat the matters, it will not change its temperature, but it changes its state of phase  that means  solid to liquid and liquid to gas.The reverse process is achieved when heat is released.

                       In above explanation we found

These state can be changed into one another by heating or cooling. this process of conversion of one stare of a matter into another state , is known as change of state.

 

***LATENT HEAT(L)

The amount of heat required to change the state of  one gram mass of a matter at constant temperatute  pressure , is known as latent heat.

                   or

The amount of heat required or reliased to change the  state of  matter per  unit mass at constant temperature and pressure is called latent heat.

               Δ Q  = L m

Δ Q = Heat  ,  m = mass , L = Latent heat

:- Latent heat is the scalar quantity.

:- S I unit of   L  is joule/ Kg.

:- Dimensional formula of L is [ M⁰L⁻²T⁻² ]

:- The value of L depends on matter.

       THERE ARE TWO TYPES

(1)  Latent heat of  fusion(Lᶠ)

(2) Latent heat of vaporisation(Lᵥ)

***  Latent heat of fusion (Lᶠ)  :-

The amount of heat required to change the state of  one gram mass from solid to liquid or liquid to solid at constant temperatute and  pressure , is known as latent heat of fusion

    Latent heat fusion = 80 cal/g

si unit of latent heat fusion = 3.36×10⁵ J/Kg

***Latent heat of vaporisation( L ᵥ ):-

The amount of heat required to change the state of  one gram mass from  liquid to gas or gas to liquid at constant temperatute and  pressure , is known as latent heat of vaporisation

:-Water starts evaporating at 100 °C

:-Steam starts condesation at 100°C

The value of latent heat vaporisation

       (1) L ᵥ = 540cal/g

        (2) L ᵥ = 2.268× 10⁶ J/kg (in s I unit)

***Heat capacity (C) :- 

The amount of heat required to raise the temperature of substance by 1°C or 1K , is called heat capacity .

          

*** Relation between heat capacity and specific heat capacity.

          Δ Q = m s Δ T ---------(1)

          C = ΔQ/ΔT     -------------(2)

       From eqⁿ  (1) & (2)

                 C = ms

here  C = heat capacity , s= Specific heat

    m = mass

***Molar heat capacity:-

The amount of heat required to raise the temperature of one mole substance by 1°C, is known as molar heat capacity.

***Water Equivalent : - 

 Water equivalent of a body is  the mass of water that  is required to increase the temperature of the body by same amount if same amount of heat is supplied.

               Δ Q₁  =̇ Δ Q₂

         m₁s₁ Δ T = m₂ s₂ Δ T

              m₁s₁   =  m₂s₂

                     Practice Question

Q₁  How much heat is required to convert 15g ice at -30°C to water at 40°C?

Q₂ How much heat is required to convert 30g ice at -10°C to steam at 140°C?

Q₃ 50g ice at -10°C mixed with a 25g steam at 105°C. find final temperature and composition.

Q₄ An iron ball of mass 200g is heated to 10°C  and put into a block of ice at 0°C . if the latent heat of fusion of ice is 80cal/g ,then the specific heat of iron in  cal g⁻¹ °C⁻¹ is

(a) 1             (b)  0.2            (c) 0.8            (d)0.08

Q₅ What is heat capacity of 5g  water?

Q₆  What is molar heat capacity of 50g steam?

**** Heating curve / Heating graph