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Temperature moderation is closely related to the hydrogen bond, as you guys all know. And this temperature moderation happens everyday to human beings through perspiration.

As the water in you body evaporates, which is perspiration, the hydrogen bonds are broken and they leave your body. The energy needed to break the hydrogen bonds come from your body. As the temperature of your body drops, the hydrogen bonds reform, which releases the energy.

The part I do not understand is how the energy created by your body would be taken away as the hydrogen bonds are destroyed and the water evaporates. Doesn't the energy only break the hydrogen bond? Or are they still accumulated in the water molecules somehow? Also, where does the energy formed as the water cooled down and formed hydrogen goes? Do they simply exit out the human body?

Thank you.

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  • $\begingroup$ No hydrogen is formed when hydrogen bonds are broken. Those are weak (when compared to intramolecular bonds) intermolecular bonds. $\endgroup$ Oct 18, 2015 at 23:55

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You're making this more complicated than it really is.

When the body's sweat glands secrete perspiration (mostly water) the heat of the body's skin causes that water to evaporate slowly. To evaporate water costs Enthalpy, known as the Latent Heat of Vaporisation. This Enthalpy is drawn from the skin, thereby cooling it down somewhat. The evaporated water becomes part of the moisture in the air surrounding you.

You are are right that the comparatively high value of Latent Heat of Vaporisation of water is in part due to the formation of hydrogen bonds, which have to be broken to allow water molecules to leave the liquid phase and enter the vapour phase.

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  • $\begingroup$ Thank you! So according to what you have explained, the Enthalpy is the energy I described, and they are taken away by the water molecules that evaporate, therefore cooling down the skin and body temperature? Your answer was quiet clear, but I just have to be sure :) $\endgroup$
    – sooeithdk
    Oct 18, 2015 at 23:09
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    $\begingroup$ @sooeithdk: Yes. But breaking hydrogen bonds is only a part of the Enthalpy of Vaporisation: the water molecules also have to acquire kinetic energy to go from the liquid phase to the vapour phase. $\endgroup$
    – Gert
    Oct 18, 2015 at 23:49

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