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Latent Heat of Vaporisation of Water The amount of heat energy required to change $1\mathrm{kg}$ of water into vapour at atmospheric pressure, at the boiling point of water is called the latent heat of vaporisation of water. The value of latent heat of vaporisation of water is $22.6\times 10^5\mathrm{J}{\mathrm{kg}}^{-1}$. Thus, the energy of $1\mathrm{kg}$ of steam at $100^{\circ }\mathrm{C}$ is $22.6\times 10^5\mathrm{J}$ more than that of water at the same temperature (Fig. 1.4). This means that steam at $100{}^{\circ }\mathrm{C}$ is a more effective heating agent than water at the same temperature. When $1\mathrm{kg}$ of steam at $100{}^{\circ }\mathrm{C}$ is condensed to $1\mathrm{kg}$ of water at $100{}^{\circ }\mathrm{C}$, heat energy equal to $22.6\times 10^{\circ }J$ is given out. The latent heat of vaporisation of ethanol, diethyl ether and mercury are $8.5\times 10^5{\mathrm{Jkg}}^{-1}$, $3.9\times 10^5\mathrm{J}{\mathrm{kg}}^{-1}$ and $2.8\times 10^5\mathrm{J}{\mathrm{kg}}^{-1}$, respectively.

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Updated on: Sep 17, 2022