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Name: Pam
Status: Student
Grade: 9-12
Location: N/A
Country: Australia
Date: November 2008


Question:
If salt melts ice, and the ocean is salt water, would raising the salinity of oceans cause or contribute to the melting of polar ice caps?



Replies:
Pam,

Salt helps to melt ice by lowering the melting temperature of the ice. The egree of lowering of the melting point, dT, depends on the molality of the solution. Molality is calculated by finding the moles of solute (equal to the

mass of solute divided by the molecular weight of the solute) divided by the kilograms of solvent.

From this we see that 1 mole of solute (58 grams of NaCl) would need to be dissolved for every 1 kg of water in order to lower the freezing point of ice by 1 degree (from 0degC to -1degC). Currently, on average, about 25g of salt is dissolved for every kilogram of water. We would need to double that in order to allow for the 1degC change in melting temperature of the ice.

Considering that it is very cold in regions where there are icebergs, we would need a lot more salt to be added to ocean water in order to allow the ice to actually melt at the temperatures of the region.

So, in short, while adding salt does lower the temperature of melting of ice, quite a bit of salt is needed (especially considering the amount of water in the ocean) in order to affect the polar ice caps.

Greg (Roberto Gregorius)


Yes "in principle". However the issue is more complex than just "increasing the salinity". First, it takes MUCH salt (primarily NaCl) to raise the salinity. Second, the ocean is a dynamic system, and increasing the salt content increases the density of the water so that it tends to "sink", but this process introduces turbulence, which further complicates the whole process. Third, related to this are ocean currents that move both deep water and surface water to and away from the polar ice caps. These currents are driven by processes that are not isolated from the rest of the global waters. The Gulf Stream is one example of this global mixing. Fourth, it is not exactly accurate to say that "salt melts ice". What is happening is that salts (and other dissolved substances) lower the freezing point of water. So the solid left is "pure" water. It is only at a much lower temperature that (water + salt) solidifies. This of course does not cause salt water from being dispersed in the ice cap by physically being incorporated by winds and tides. So your question, while pertinent, is a very special case of a number of process that are occurring.

Vince Calder



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