• Question: why does salt reduce the boiling tepature of water ?

    Asked by 358bera49 to Colin, John, Kevin, Shikha, Triona on 11 Nov 2014.
    • Photo: Kevin Motherway

      Kevin Motherway answered on 11 Nov 2014:

      It doesn’t!!!

      Now by boil I mean reach the point it turns to steam. You can get a temporary effect where it’ll bubble for a few seconds but not reach full boiling point where it continuously bubbles and generates steam.

      Water contains gas in solution but the warmer water is the less gas it can hold in solution and it comes out of solution and forms a gas bubble that floats to the surface. The hotter the water gets the less gas it can hold in solution the more bubbles it forms and that is why boiling water bubbles. What can also assist the formation of bubbles is when you have a condensation nucleus, basically a spec of dust, grain of sand etc. so when you drop salt into water that is nearly at boiling point you have loads of gas just aching to get out of solution and form a bubble and hey preseto you’ve just chucked in a handful of condensation nuclei and the gas bubbles form instantaneously in a big gurgle of bubbles that quickly dies out. If you chuck in sand it’ll do the same thing. Ironically putting salt in actually raises the boiling point putting in about 100 grams would raise it about 1 degree C.

    • Photo: John Wenger

      John Wenger answered on 12 Nov 2014:

      Good answer Kevin!

      The increase in boiling point of water is an effect that is not just due to added salt…In fact the same thing would happen if you added sugar or some other material. The increase in boiling point does not depend on what chemical you add, but how much! It is a physical effect rather a chemical one.

      John

    • Photo: Shikha Sharma

      Shikha Sharma answered on 12 Nov 2014:

      Hi 358bera49,
      No, adding salt does not lower the boiling point of water. When you add a salt to water, the freezing point will go down and the boiling point will increase. That’s because the vapor pressure of the pure water is lowered. You remember the definition of boiling point is its the temp at which the solution’s vapor pressure becomes equal to the external pressure. By lowering the vapor pressure…boiling point moves to the right in phase diagram (higher),freezing point moves to the left (lower).The change in both boiling point and freezing point is related to how much salt is added to the solvent. So,the boiling point of water is increased slightly, but not enough that you would notice the temperature difference with small amount of salt.

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