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 = Uf - Ui and dW = PdV = P(Vf - Vi)
where the subscripts i and f denote the initial and final values respectively.
Therefore, mL = (Uf - Ui) + P(Vf -Vi)
or (Uf - Ui) = mL - P(Vf - Vi)
Thus, knowing m, L, Vf, Vi, the increase in the internal energy can be calculated.
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