• Question: why is the boiling point of alcohol 80'C and waters boiling point is 100'C

    Asked by 258xygg37 to Gavin, Karen, Mark, Michel, Roisin on 11 Nov 2016.
    • Photo: Michel Destrade

      Michel Destrade answered on 11 Nov 2016:

      Every thing is different from another in many ways. In particular, the temperatures of boiling and freezing are different from one object to another. That’s all to do with what atoms the object is made of, how the atoms are arranged in the object, and how easily they can be made to move with heat. Alcohol boils before water, and freezes at a lower temperature too. Vodka freezes at -25C, so it can be kept in the freezer while staying a liquid!

    • Photo: Roisin Jones

      Roisin Jones answered on 13 Nov 2016:

      Interesting question! In general, differences in boiling points correspond to molecular weight: the heavier a molecule of the substance is, the more likely it is to have a high boiling point. However, in this case it actually has more to do with the different forces holding the molecules of the liquid together: ethanol has a molecular weight of 45 g/mol and water has a molecular weight of 18 g/mol, so you would expect ethanol to have a higher boiling point than water!

      The difference in boiling points here is due to an extra type of bonding in water which is called ‘hydrogen bonding’ (there is some hydrogen bonding in ethanol, but it is much weaker than in water). Hydrogen bonding happens between an oxygen in one molecule and a hydrogen in another molecule: because water is made up purely of oxygen and hydrogen it experiences very strong hydrogen bonding, and you have to put a lot of heat energy into it to break up these hydrogen bonds and push the molecules apart into a gas (which is what happens when you boil a liquid). This is why water has a much higher boiling point than you might expect!

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