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I've just diluted a 95% rubbing alcohol bottle with water to bring it down to 70%. However, the bottle got noticeably warmer after I did so. I've searched online and I couldn't find any exothermic reaction that happens when alcohol is mixed with water. I did use mineral water by mistake to do so, so my best guess is that alcohol reacted with... one of the salts in the water... maybe?! (I have no idea)

Why did this happen?

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  • $\begingroup$ Why the downvote? It is a good question $\endgroup$ – Micelle Apr 20 '20 at 18:15
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Ethanol does the same. It is the consequence of deviation from the Raoult law.

The other consequences are existence of azeotropes ( affects distillation ) and mixing volume deviations. If you mix 1 L of ethanol and 1 L of water, it warms up. When it cools down back, its total volume is not 2 L, but about 1.9 L.

It is closely related to total energy of inter-molecular bonds in pure solvents and in their mixture.

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