Wednesday, November 6, 2013

Why icebergs aren’t salty.

Floating around in salty sea water, did you know that ice bergs are not salty? Part of the reason is because ice bergs are created by glacial ice and water, which is sweet water. But how do you think all that icy mass, at the poles, got sweet (un-salty) in the first place?

Hint: Have you noticed how salt gets left behind in salt pans, when sea water evaporates?

The compound H2O can exist in three phases - solid (ice), liquid (water) and gas (water vapour). In each phase, the way H2O molecules interact with each other is different. While in ice, they’d be bound together tightly in a crystalline pattern, they would get more and more lose as they shift to liquid state and then to gaseous state. 

However, the trick happens when H2O is transforming from one phase to another. Lets begin with the water to water vapour transition. 

Heating water increases the kinetic energy of the H2O molecules, helping them break free from the tighter liquid state and jumping out into the air, turning to a lose, gaseous state. When you dissolve salt in water, what happens is that H2O molecules cling onto the salt (NaCl) molecules. That is what makes the salt disappear (dissolve) in water. So now when you heat salt water, H2O finds it really hard to break free from clinging onto salt. That is why you have to heat it more and more, until the water molecules can break free and jump out into the air. But salt doesn’t jump out just yet. It needs a much higher temperature to evaporate and hence, it gets left behind. 

Extrapolating this to freezing water, salt water in the sea follows the same phenomenon. Water molecules are clinging onto salt molecules with dear life. So when the temperature drops to 0 deg. C. (32 deg. F), although water molecules try hard to get close to each other and form that icy crystalline structure, salt gets in the way. So water molecules begin to freeze and melt simultaneously (a phenomenon known as ‘dynamic equilibrium’). As it gets colder, the rate at which water molecules freeze increases while the rate at which they melt gets slower. So every time some of the molecules melt, salt goes away with the liquid part and the water molecules can form bigger ice crystals. That’s is how salt gets pushed into the liquid sea while some pure water molecules freeze into floating ice. 

And that is why ice is not salty. 


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