CBSE Class 11-science Answered

Boiling process : When a liquid is heated, it starts boiling at a particular temperature called its boiling point.  At its boiling point (pressure is kept constant), the unit mass of the liquid requires a definite amount of energy to change from the liquid to the vapour state.  The amount of heat supplied is known as the latent heat of vaporisation of the liquid.

Consider a liquid of mass m at its boiling point.  Let P be the external pressure and L be the latent heat of vaporisation.

Therefore, Amount of heat required to convert the whole of the liquid into vapour, dQ =mL.  From the first law of thermodynamics,

dQ = dU + dW

dU = U_f - U_i and dW = PdV = P(V_f - V_i)

where the subscripts i and f denote the initial and final values respectively.

Therefore, mL = (U_f - U_i) + P(V_f -V_i)

or (U_f - U_i) = mL - P(V_f - V_i)

Thus, knowing m, L, V_f, V_i, the increase in the internal energy can be calculated.