Why Rock Salt bites for Melting Ice

Sorry for the strong language in the title, but it’s true: the rock salt you go to sprinkle over the icy sidewalk every year is actually a pretty mediocre substance.  Sure, it melts ice, but it’s not as efficient as other substances.  You’d be better off saving that salt for your next soup or gumbo, or whatever it is you’d like to cook.  Why is salt not that great at melting ice? We’ll delve into the very simple structure of salt to find out the answer, and then we’ll see what you should be using instead to get a much better, less slippery sidewalk!

You might already know the chemical formula for table salt is NaCl. It just means for every sodium atom, there is a chloride atom to go with it.  It’s a very simple substance, but it’s not a molecule — a term which is reserved for discrete groups of atoms (sugar molecules or water molecules, for example, are always the same and don’t associate with each other so much that we can’t separate them).  Salt is really an ionic substance, which just means that instead of having a discrete, separate “NaCl” molecule, it builds on itself, and each sodium bonds to a couple of chlorides, and each chloride finds a couple of sodiums.  The end result is a crystal, just like diamond, that’s highly ordered because all the atoms bond in the exact same way with all the others.  This is why coarse salt comes in those very geometrically ordered crystals — it’s the natural way that salt comes, all sharp lines and angles.

Pretty much any ionic compound like salt will dissolve in water because of this strange bonding behavior.  Water grabs each atom and pulls them apart from each other, and when all the sodiums and chlorides have stopped associating with each other, it’s dissolved and invisible!!  In other words, table salt in water separates into its constituent atoms (in this case, called ions).  As with all of science, it’s more complicated than that, but we’ll save that for another time.

The other key thing you might not know is that ice is not slippery.  Yes, ice is not slippery, but water is.  Ice on the sidewalk has a very thin layer of water on top of it, which makes it hard to walk on.  Any amount of pressure or heat creates this layer, so there’s literally no way for you to walk only on ice — you’re always walking on the buffer of water above it.  Now imagine dissolving some table salt into that top layer of water.  Just like in a glass of water, the atoms of salt will come apart and associate with water instead.  Since it’s probably below freezing at this point, the water would really, truly like to become frozen water, which some people call ice.  Thanks to salt, it no longer can.

The reasoning is pretty simple — imagine a little spherical sodium ion (just an atom, remember now) settling into some liquid water, swishing around with all the other moving cells.  When things become solid their movement is largely restricted, and water in particular tends to freeze into its own crystal structure.  Take a look at the picture to the right to see how water molecules arrange themselves when they become ice — there’s plenty of free space for ions to get in the way and prevent this structure from coming together.  Imagine a big ol’ sodium or chloride atom just forcing its way into those nice little bonds there — it’d be a little harder for the ice to form.  Because of the way water freezes, it’s less dense as ice than it is as liquid water, so ions have an easier time jumping into the fray and ruining ice’s day.

The only way water is going to freeze now is if it loses even more heat — it has to lose a little bit more heat, which means molecules will move around a little less, and at some point their energy is sufficiently low to allow them to bond to each other, albeit imperfectly.  Ice will do this to account for the disruption in bonding from the salt.  At 32 degrees, water wouldn’t freeze with salt in it.  So Mother Nature drops the temperature a few more degrees, and water is able to freeze again.  Yay!

Each mole of CaCl2 is more densely packed with ions, making it the better choice for melting ice.

So why is rock salt weaksauce for melting ice? It’s only made of two ions – sodium and chloride ions.  An equivalent amount (chemists use moles to compare “amounts” of things) of CaCl2, called calcium chloride, breaks apart into three ions — it’s 50% better at disrupting the bonds needed to make ice.  In fact, any ionic compound with more than just two atoms per formula unit.  A formula unit is just the smallest discrete “molecule” of a substance, except that ions make giant crystals instead of staying as separate molecules, like water or sugar.  Imagine if 100 water atoms could all associate to form a single giant water crystal — the formula unit would be H2O because that’s the formula unit that makes up the crystal as a whole.

There are many more ionic compounds that are more effective than rock salt, including ammonium sulfate and magnesium chloride.  In other words, when we buy rock salt because it’s the cheapest de-icer, we’re shortchanging ourselves.  We could be using a lot less “stuff” on our sidewalks while getting the same effect, or using the same amount compared to rock salt and get an even more effective result.

Winter’s grasp has finally let go of us, but keep these concepts in mind for next year – if you have to make a choice between rock salt and calcium chloride at the Get-rid-of-ice store, give it a little more thought.  You just might stretch your hard-earned dollars a little farther.

 

Salt Substitutes: Potassium is the new sodium

I love my salt substitute.  I love chemistry.  What do you get when these two collide?  With any luck, a really interesting discussion about potassium chloride.

Unless you’ve gone out of your way to try it, you’ve probably never tasted it (or noticed its taste).  It’s sold commercially to hypertensive (high blood pressure) customers, who don’t want the salt in their food to wreak havoc on their bodies (hint: it might not be).

In fact, there’s very little about KCl (the chemical formula for potassium chloride) that’s merely a substitute – the atoms it’s made of are equally important for our body as those in regular salt. Let’s see how KCl earns its place in your diet, and maybe you’ll show it a little love next time you run across each other.

First, let’s talk about properties.  KCl is white and crystalline just like its cousin, regular old table salt.  Take a look at the picture on the right — yeah, it’s the same periodic table you’ve seen a thousand times, but notice that Na and K are right next to each other.  This means their properties (melting point, solubility, etc.) are similar, which means that potassium will also want to bind with chloride just like sodium.  KCl doesn’t taste nearly as “salty” as table salt does, but it’s a similar compound in many ways.  KCl will also dissolve just like salt in water, so you can use it for soups or anything wet.

 

Ever studied the brain?  Your brain uses cells called neurons to send signals to your body.  It works by letting in sodium ions to your brain cells, which change the electrical forces around them, resulting in impulses that travel to your muscles as movement.  This seems like a good argument for eating salt, since the sodium we get from it is vital to our very existence, but there’s more to it: not only do sodium ions affect nerve impulses, but both chloride and potassium ions are vital to the mix as well.  Using KCl instead of NaCl is just as beneficial for your health in terms of the essential electrolytes (anything that dissolves to make ions, in our case, table salt or KCl) your body doesn’t just make on its own.

Have you ever gone out for a meal, dumped a ton of salt on it, and then checked the scale the next morning? To your horror, you may have gained a few pounds.  It’s not fat, but rather, sodium that attracts water and holds it.  Each sodium ion you ingest associates with water molecules, and since it takes some time for sodium to be excreted from your body, it contributes to water weight.  The effects disappear after a day or so.  KCl has no affinity for grabbing hold of water, so using a ton of KCl wouldn’t result in you gaining any water weight — a good idea if you’re headed to the beach pretty soon and you’re worried about your figure (I’m certainly not, but hey, I’m not judging anyone).

By extension, the fact that sodium absorbs  water means the water balance in your body is upset and your cells have lost some fluid — which is exactly why eating salt makes you really thirsty.  KCl doesn’t have this problem.

Let’s consider the lethal dose for sodium, which is 3 grams/kg of body weight.  If you weight 150 pounds, that’s about 68 kg, and you’d need to eat 204 grams, or nearly 7 ounces of pure salt for it to kill you.  It’s safe to say you can’t kill yourself from overdosing on salt – I’d be honored to know you if you could stomach more than an ounce.  KCl has a slightly lower lethal dose of 2.5 grams/kg body weight.  Without even doing that math, we know you’d have to eat at least 2 of those 3 ounce containers of Nu Salt in the photo above to kill yourself only 50% of the time.  So it’s pretty safe.  For reference, 10 grams or a third of an ounce of caffeine will put you six feet under — you could fit that much onto a spoon.

So we’ve established a great many things: KCl is as safe as salt, performs better for water retention, hypertension, and is entirely composed of two ions that you need to survive your day-to-day.  What’s the downside here?

It depends whether you like the taste of KCl.  I find it just fine — it has a slight cooling effect, a little metallic-y, bitter-y taste which is not unpleasant, and a slight salty finish.  It’s hard to describe unless you’ve tried it, but you can find any person on the street to describe what table salt tastes like.  So why not try something that’s off most radars, venture into some new territory, and give a little love to the cousin of table salt?  You just may find it suits your taste buds.

There are generally considered to be two major retailers for potassium chloride – Nu Salt, pictured above, is pure KCl — no salt, so use it liberally.  The other contender, Morton’s Lite Salt, is a 50/50 mix of regular table salt and KCl, presumably to mask the perceived bitterness of the KCl.  Even cutting your table salt intake in half might be considered beneficial, so this is a great option if you can’t stomach potassium chloride all by itself.  They’re both sitting in the spice aisle with the coarse kosher salt, organic whole-grain free-range sea salt crystals, picking salt, and of course, regular old table salt.  Both are a great way to get potassium into your diet, which is an easy nutrient to become deficient in.  Bananas and coffee have some potassium, and cream of tartar is chock-full of the stuff, but unlike table salt, there’s no go-to kitchen item that can refill your potassium supplies — except, of course, for the two products we just mentioned.  Try tracking your potassium intake for a few days, and you may just fine you’re not getting as much as you should — low potassium can definitely kill you, because it’s required for your body to send all of its complex electrical signals.

Ah, knowledge is truly delicious (and low sodium to boot)…

 

Radioactivity: All roads point to lead

If you take a look at the periodic table, you’ll notice that lead is element number 82.  Not coincidentally, every element with an atomic number greater than 82 is radioactive, meaning they’re unstable and prone to ejecting particles to seek stability.  It has a lot to do with the number of neutrons and protons in the nucleus; to keep each other in check, their numbers should be roughly equal.  If an element has too many neutrons, it’s at risk of shooting a particle away from itself to form a more stable substance.

Take a look on the flowchart at the right.  We see here that Uranium, at the top of the list, has an atomic number of 92, meaning it has 92 protons.  The number “238” refers to the sum of uranium’s protons and neutrons.  With some quick subtraction, we can deduce uranium has (238 – 92) = 146 neutrons.  Wowzers! It has about one and a half times as many neutrons as protons, so what could we say about the stability of uranium?  Well, if you’ve any inkling for radioactivity, you probably already know that uranium tends to decay and is therefore highly radioactive.  It’s the mystique of such heavy atoms that has placed these radioactive species at the forefront of science fiction, appearing in all sorts of fantastic gadgets like time machines (what was the element they needed in Back to the Future? Ah, yes, it’s plutonium, atomic number 94, just two doors down from uranium. No surprise there!)

You might be asking yourself, “so what kind of particle might uranium want to eject to become stable?” The answer is that it’s complicated.  Radioactive elements generally have 5 ways of ejecting particles — sometimes they shoot off an electron, sometimes they shoot off a proton, or they might try their luck at adjusting their neutron number.  You can see on this flowchart that uranium ejects an alpha particle, which is nothing more than a helium nucleus (helium has 2 protons and 2 neutrons, and since this is the nucleus, there are no electrons emitted).  If uranium shoots out two protons, it loses its identity as it drops its proton count down to 90.  Now it’s thorium!  Notice the huge amount of time listed there: that’s the half-life of uranium decay, meaning that in 4,510,000,000 years from now, a one pound sample of uranium will still be half uranium by weight. It takes quite a long time, but check out what’s next in the line-up.

Thorium ends up emitting an electron from its nucleus.  Wait, what?  There aren’t any electrons in the nucleus, so how can this occur?  It might blow your mind: one of its neutrons splits into a proton and an electron.  The electron is emitted while the proton remains behind.  Since we’ve added one more proton, our element becomes protactinium, and it happens in just a few weeks!

This series of events will continue for quite some time until it reaches the end of the line, which you’ll notice is Pb, or lead.  I don’t think it’s a coincidence that lead is the last non-radioactive element in the periodic table — by definition, radioactive elements will always spit out some particles until they become less and less unbalanced.  It just so happens that uranium will always, always, always eventually decay into lead.

So in a few trillion years from now when we’re mining the far reaches of outer space and living in Jetsons-esque societies on a hundred different planets, nearly all the uranium that exists today will be on its way to become lead.  If we find some elements that are even more radioactive than the ones we’ve already discovered, at least we have some consolation: all that lead converted from uranium will do nicely as a shield from x-rays and other radioactive energy.  Kind of ironic, huh?