• Salt Not Only Melts Icy Roads – It Raises the Boiling Point of Water!

    February 9, 2012

    Adam Gilbert, Associate Research Fellow

    I got a very good question as a comment to a recent blog I wrote on how sodium chloride (table salt) melts ice: does salt raise the boiling point of water? Sounds crazy – an innocuous substance being able to lower the freezing point and raise the boiling point of the same liquid! Can’t be true….but it is.

    Waters is actually an amazing substance. Given its small size and molecular weight, it should have a really low boiling point –but it doesn’t because of hydrogen bonding. Hydrogen bonding is not like a covalent bond like the bonds from oxygen to hydrogen in water. Hydrogen bonding is a strong attraction between the hydrogens of water which are slightly positively charged and the neighboring oxygen atoms of other water molecules which are slightly negatively charged. It’s kind of like a sticky interaction. A simple explanation for water boiling is when molecules of water escape the H-bonding web and escape into the air. This happens when water is 100 °C and more heat is applied to it.

    When one adds sodium chloride to water, you create a different molecular structure than what you have with liquid water. In water containing sodium chloride, water oxygen atoms surround the positively charge sodium ions and water hydrogen atoms surround the negatively charged chloride ions. As stable as the molecular structure is for pure liquid water, the structure for liquid water containing sodium chloride is even more stable. Thus it takes an even higher temperature for a salt water solution to boil – hence salt raises the boiling point of water. It’s not a large effect, about 2 oz of salt raises the boiling point of water by 1-2 °C, but it’s real.

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  1. iftikhar said:

    Both Actions can occur when melting point raised than boiling decline and when boiling point raise then melting point became in decline .means its biosimilar activity among the boiling and melting phenomenon of salt/sodium chloride

    on February 24, 2012 at 2:30 am

  2. Suresh Kota said:

    Precisely; it is true of any electrolyte; salt is one of them. It is called as the “colligative property” of electrolytes in physical chemistry. What is spread out by machines in preparation for snow day: I think is not real salt NaCl, but some other electrolyte.

    on February 23, 2012 at 8:20 am

  3. Jack Watters said:

    Altitude lowers the boiling point of water, about 1 degree Celcius for every thousand feet. So on an average flight at 30,000 feet water boils at around 70 degrees which is why airline coffee is so bad! If we could just encourage the airlines to make the cofee before take off!

    on February 22, 2012 at 8:14 pm

  4. Dagmar said:

    Fun fact. Boiling point of water and other substances is also influenced by atmospheric pressure… water about 100 C at 1atm… but on top of Mount Everest that would drop to about 72C

    on February 22, 2012 at 7:52 pm

  5. Adrian Roitberg said:

    Nice explanation, but not the whole story. In fact, ANY water soluble , non-volatile substance (and some volatiles ones also) will raise the boiling point of water and decrease it melting point. This is not just about salt only.

    on February 11, 2012 at 6:12 pm