MadSci Network: Chemistry

Re: How does placing a glass of water on salt prevent condensation?

Date: Sun May 10 17:33:12 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 894662123.Ch

Condensation occurs when humid atmosphere finds itself chilled to a 
temperature where it cannot hold all of its water vapour, and gets rid of 
the excess when liquid droplets form on a local surface.

Salt -- pure sodium chloride -- has some affinity for water vapour, and 
will remove it from humid air. That is why salt left in the open air 
usually becomes damp and lumpy. But the affinity of pure sodium chloride 
for air is not so very great.

Table salt, the sort you get in your salt shaker, is not pure sodium 
chloride. If it were, it would not be a very satisfactory product, because 
it would gradually get damp and lumpy and not pour freely. To help it to 
work properly as a dry fine crystalline product that pours freely, other 
materials are added. The formulation of table salt varies from country to 
country, according to the climate and local regulations. In large countries 
like the US or Australia, it probably even varies from region to region. 
But one of the additives is something to remove water vapour from air a bit 
more efficiently than salt itself, and thus stop it from getting damp and 
caking. I believe cornflour (corn starch?) is commonly used for this 
purpose. You might be able to tell by looking at the ingredient list on a 
pack of table salt. In Tennessee I believe you have a very humid climate in 
the warmer months, and so an additive of this sort with a large capacity 
for water vapour would be needed. [Editors note: My container of Morton's 
Iodized Salt contains calcium silicate and dextrose, both of which would have 
the effect Dr Christie describes...]

So if there is a layer of (table) salt under your glass, the salt itself, 
or more likely the additives, will remove enough of the water vapour that 
liquid water will not get onto the table.

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