Thursday, June 19, 2008

...of the Earth

In Casino Royale, the dastardly Le Chiffre tries to poison James Bond with digoxin, a substance which disrupts heart rhythms if taken in enough quantity. Our valiant hero, no doubt feeling the sense of impending doom associated with heart conditions, rushes to the bathroom in the coolest most Bondish manner possible. Along the way, he picks up a salt shaker. He empties the shaker into a shot glass and mixes it with some water, then gulps it down. He soon begins vomiting, and he gets support from MI6 to help him deal with the toxin that has already made it into his system. Salt saved James Bond's life, and it's indispensable to human life on a day to day basis.

However the practice of using salt as an emetic is not entirely safe. Salt solution have been used for suicide in some parts of the world. Excessive salt levels affect the electrolyte balance of the body, and even if you try to raise the level of water to counter-act the salt, doing it too fast will cause the brain to swell, since it adapts to the saline environment. Bond does not do his best work when brain-dead.

Still salt remains essential to human life. Aside from it's use as an electrolyte, the sodium and chlorine ions are used in the essential machinery of cells. Sodium is vital to muscle movement. Without refrigeration, salt is necessary to preserve meat. It's also extremely important to desert dwellers who to this day travel over long distances with limited water. Salt allows the body to retain more water, and was vital to the economies of nations. The Assyrians were known to salt the earth of agricultural communities to punish them, since most crops do not grow in salt rich soil.

Today salt finds mild favor as a seasoning. Most chefs seem to eschew table salt because it contains anti-caking agents that apparently affect flavor and texture. I do cook for myself often enough, but I'm no chef. Still, from what I understand, salt enhances the flavor of foods. This is why fast food chains like McDonald's supposedly use so much. Apparently we have dedicated ion channel on our tongues to pick up the taste of salt.

However beyond biology, salt finds domestic use in ice cream making (rock salt is simply a mineral form of the stuff,) driveway deicing, and pickling. The reason for the first two is the ability of salt to lower the melting point of water in solution. Salt does this by disassociation of its two ions, sodium and chlorine. They affect what chemists call the colligative properties of the water.

Colligative properties have to do with the number of particles in solution. It doesn't matter how big these particles are, and in fact bigger particles allow less room for other particles, making them less effective. This is why the relatively massive sugar molecule isn't used to melt ice (plus it attracts ants). So how do salt ions lower the melting point of water? The short answer is, I don't know.

It's something I want to know, but don't. This is something I'll have to look into. I have a hypothesis though. The salt ions may interfere with the cohesion of water by binding the polar ends of many of the molecules, weakening the usually strong coulomb force attracting the water molecules together. I'll have to find that out.

As for pickling, the effect is much easier to explain. Salt simply draws moisture out of vegetable (or fruits or animals for that matter) and replaces some of that moisture with salt, which provides a hostile environment for bacteria. This is done through diffusion, technically osmosis since it happens through a membrane. Actually diffusion is very interesting, and I think there is some confusion as to how diffusion actually works. I think I am going to save that explanation for another post. However we see how salt preserves food. While there are some halophilic (that's salt loving) organisms out there, they tend to have evolved in very specialized environments.

Salt is common enough that we tend to take it for granted. I personally pour it out into my hand to sprinkle on food, and find myself throwing the rest of the handful away. While it isn't likely we will encounter major salt shortages in the current global economy, it's important to keep in mind that we can't live without it. So despite the fact that I don't know everything about salt, it's interesting to look at how a simple and common ionic compound can have so much to it.

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