Eli5: Why is Salt Effective on Everything?

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Salt, it seems like it’s the answer to everything.

– Sore Throat? Salt Water
– Icy Roads? Rock Salt
– Food Bland? Table Salt
– Dehydrated? Gatorade
– Tough Stains? Salt
– Chill Beverages Faster? Salt Water
– Spirits? Shot Gun (Salt Rounds)

There’s probably more uses.

In: 246

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Salt is an ionic compound composed of a negative half (Cl-) and a positive charged half (NA+). Water is polar (water molecules also have negative and positive charged sides) so salt dissolves very well in water, and because it is an ionic compound, the Cl and Na halves dissociate and float around freely. This gives salty water some interesting properties.

For one thing, adding salt alters the freezing point of water slightly. The Na and Cl ions ‘get between’ the water molecules, so to speak, and prevent them from so readily forming a solid. This is why we spread rock salt on icy roads – the salt dissolves in the thin film of liquid water that is always present on ice, dropping its freezing point, which will in turn begin to melt the ice. This is also the property of salt water that you can take advantage of to chill a bottle of wine very quickly – it causes the ice to melt at a lower temperature, and lower-temperature icewater is better for chilling your drinks.

Some of the other useful properties of saltwater relate to the fact that saltwater has, uh, more salt in it than water that doesn’t. Let me explain. Solutions want to reach equilibrium with each other, even across very thin membranes. When a saltwater solution is brought into contact with a pure water solution, water molecules will move to the salty side. This exchange of molecules across such a membrane is called osmosis. Biology takes advantage of osmotic effects everywhere. So salt is a useful molecule to have in biology for water regulation, because salt concentrations can be used to draw water from one tissue to another. This is why salt is key for water regulation, and also why a saltwater gargle will soothe your throat – breaking up mucus (by drawing out water) and even drawing some water out of your throat tissues.

Finally, as alluded to above, Na+ ions have a positive electric charge, which all animal (and even some plants) life takes advantage of to create action potentials – essentially, electric charge imbalances in cell tissues. This is vital to the functioning of all muscle and nerve cells. So, the sodium component of NaCl is extremely vital for life. This probably explains why we find salt very tasty – in modern times, we can just put salt in everything, but for animals in the wild, it can be harder to come by salt. So we probably evolved to crave it a lil bit to induce us to take it in when it is available.

[Salt as a chemistry term](https://en.wikipedia.org/wiki/Salt_(chemistry)) is fairly broad (and distinct from [table salt](https://en.wikipedia.org/wiki/Sodium_chloride), but still useful), but the general description reveals some things. It is a bunch of positive ions and negative ions that are relatively stable together but not too tightly bound to each other. This means that if there’s a chemical interaction that needs any ions, salt has them right there.

Things that work better with ions:

* Dissolving into water because water likes holding ions, which is why icy roads and colder ice use salt
* Moving electricity through water because it provides the electrons that are less attached to molecules, which is a minor reason why it helps with dehydration (nervous system is electrical)
* Occupying water molecules because the ions also like to hold the water, which is why it is used in food preservation (and thus culturally means purity, which is why it works on ghosts)

Taste wise, it is probably just a side effect of all the things our bodies use it for making our brains crave it (ETA: its use in kidneys is the biggest one, not the nerves as I implied above. Kidneys use salt to keep enough water in our blood, using a combination of my first and third bullet points).

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Long story short, most of this can be reduced to the ways that salt and water interact, which is actually an important part of how our bodies work.

Interaction one: Salt is an ionic compound, and changing the concentration of ions inside (or outside) of something can cause it to either absorb or expel water, which is called osmosis. ([Here’s](https://youtu.be/SrON0nEEWmo) a science experiment showing this mechanism in action, making eggs swell or shrivel up based on the concentrations of the liquids they’re put in.) All living things use osmosis to move water around across their cell membranes, and therefore depend on both ingesting the right ions and being in an environment of the correct concentration. NaCl, or salt, is by FAR the most plentiful and important of these ionic compounds, comprising a significant portion of the earth’s crust and being abundant both in the waters of the sea and the soils and rocks of the land. So a surprisingly high amount of the process of being alive boils down to “make sure you’ve got the right salt concentrations in the right places”. We crave salt, and enjoy adding it to our food and drinks, because it’s important for us to have the right amount of it to stay healthy. And by using the principles of osmosis to our advantage, we can also use salt to dry stuff out, preserving food by making it too salty to be hospitable to microorganisms, or reducing fluid-based swelling and discomfort of our throats and other body parts. (These preservative, purifying associations are also why ghosts are supposedly repelled by it.)

Interaction two: Once all those ions are floating around in water, it gets harder for the water molecules to form the tight grid alignment that makes up ice. So salty water can get colder than pure water without freezing, potentially as low as -6°Celsius. This is why salty water is better for cooling your drinks, and also why salt on the road can help melt ice and snow.