Why is a white cloud is formed in the hot water from the pot when salt (NaCl) is added?

  • $\begingroup$ Could you add a picture(preferably gif) or a video so we know what cloud you are talking about? $\endgroup$ – Ali Sep 10 '14 at 15:38
  • $\begingroup$ just let me some time... $\endgroup$ – rnrneverdies Sep 10 '14 at 15:41
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    $\begingroup$ To me the cloud appears similar to tiny bubbles. Possibly the salt is giving nucleation sites for gases that are dissolved. One test might be to boil the water first, then cool, then heat again before adding. There would be fewer gases at that point. If that's what it is, adding salt should have a much reduced effect. $\endgroup$ – BowlOfRed Sep 10 '14 at 16:10
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    $\begingroup$ I can verify this happens all the time when I cook. They do look like small bubbles. $\endgroup$ – BMS Sep 10 '14 at 17:06
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    $\begingroup$ Here is a Youtube video of the effect: How to Salt Boiling Water. He adds the salt around 0:45. $\endgroup$ – Chris Mueller Sep 10 '14 at 17:19

The white cloud you see in the water is steam bubbles. The grains of salt provide nucleation sites that allow the water to vaporize as they fall through the superheated liquid (so BowlOfRed had it right--although it's steam that is forming, not dissolved gasses coming out of solution). If you raise a pot of water to near boiling and toss in a handful of salt, the water can explode out of the pot due to this effect.

Counter to what you might think, the addition of salt to water actually raises the boiling point by about one half degree Celsius for every 58 grams of salt dissolved per kilogram of water. So the steam is not caused by salty water around the dissolving salt crystals boiling at a lower temperature. The nucleation effect diminishes as the salt diffuses throughout the water.

Note that the effect is not limited to salt. If you toss in something that doesn't dissolve--like sand--you will see the same nucleation effect.

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    $\begingroup$ Are you sure it's necessary for the water to be superheated to see this effect? This implies it happens all the time on the stove top, which seems odd to me. $\endgroup$ – BMS Sep 10 '14 at 18:40
  • $\begingroup$ Also I know that in some commercial salts there are additives,like rice flour, to keep it flowing smoothly and be less affected by humidity. It could well be this rice flour creating the cloud. $\endgroup$ – anna v Sep 10 '14 at 18:57

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