Salty Water (Chemistry)

EDIT: I've now written a post about buffers, as well!  LINK

                 A few days ago, I was scrounging around my closet looking for a very comfortable, black, V-neck cotton shirt.  Very standard, very normal, very lab friendly.

                “Have you seen it?” I asked my fiancé frantically.  He seemed dumbfounded by my eagerness.

                After searching for a bit longer, I finally remembered that I had to throw out the silly shirt last winter.  The area just above the belt of my jeans had been riddled with tiny holes.  In fact, many of my shirts had met the same fate and the zipper of my favorite fleece vest had finally detached from the fabric last week and ceased to be useful.

Labs destroy clothes.  More exactly: the acids in my lab destroy my clothes.  (Yes, I’m fully aware that this is why lab coats were invented, but I’m too … lazy? … to ever put one on.  Why not just go buy new clothes?)

The most common acid we use in lab is hydrochloric acid (HCl), which is pretty nasty.  It’s what chemists call a “strong acid.”  Figure 3.1 shows a molecule of hydrochloric acid: one hydrogen atom bound to one chlorine atom.  A hydrogen atom carries one electron, while a single chlorine atom has seventeen.  The hydrogen kindly shares its one electron with chlorine and chlorine begrudgingly shares one electron with hydrogen, thus creating a bond between them.  This “sharing” is wildly unequal, however; the electrons play at chlorine’s house far more than hydrogen’s. (I’ll discuss bonding, electrons, and everyone’s general happiness in a different post.  For now, trust me – that chlorine really wants hydrogen’s electron.)  The chlorine is actually so delighted with the hydrogen’s electron that, for all it cares, the hydrogen can leave its electron and fall off the molecule.  Often, hydrogen does.  A hydrogen atom that has lost its electron is now an ion and known as H⁺.  

Three sets of definitions exist for acids*.  Arrhenius claims that acids are molecules that increase the concentration of H⁺ in water.  HCl certainly does this.  Johannes Bronsted and Thomas Lowry say an acid is a molecule that can donate H⁺ to bases.  HCl certainly does this as well.

So what are bases?  Arrhenius says bases are molecules that increase the concentration of hydroxide ions (OH^-) in water.  The strong base, sodium hydroxide (NaOH) fits this definition quite well.  (Incidentally, NaOH is also highly available in my lab and is used quite regularly.)  Much like the chlorine atom, the hydroxide ion only wants one of sodium’s electrons and not much else (Figure 3.2).  Very often, the sodium leaves one electron and falls off the molecule, creating Na⁺ and OH^-.

Let’s pause for a moment and recap (Figure 3.3): HCl will fall apart into H⁺ and Cl^-.  NaOH will fall apart into Na⁺ and OH^-.  If we have these four things hanging out together, will anything else happen?

It turns out, H⁺ and OH^- will readily come together to form H₂O = water!  Water is great stuff.  It’s not acidic (like HCl), it’s not basic (like NaOH); it’s neutral and perfectly safe compared with NaOH and HCl.

If you add one HCl molecule to one NaOH molecule, they will create one H₂O molecule, one Na⁺ ion and one Cl^- ion** (Figure 3.4).  This is why acids (low pH) and bases (high pH) are said to neutralize each other – mix a high pH (NaOH) with a low pH (HCl) and get a neutral pH (water).  

Common table salt (NaCl) is actually an organized array of Na⁺ ions and Cl^- ions.  Everyone knows that if you drop salt into water, it will dissolve.  The “dissolving” is actually the complete breakdown of the organized array into individual Na⁺ and Cl^- ions in water (Figure 3.5).

Look at the right sides of Figures 3.4 and 3.5.  Both show water with Na⁺ and Cl^- floating around.  This means that if you mix equal amounts of the nasty acid HCl with the strong base of NaOH, you get nothing more harmful than if you had dissolved common table salt into water.

 

Unfortunately, without the base around to neutralize the acid spilled on our benches, this chemical will continue to eat holes in my shirts.  I should really wear a lab coat.

P.S. - The topic of acids and bases can be endless!  So many more topics are here to discuss, such as the Henderson-Hasselbach equation, what in the world a buffer is and why it resists a change in pH, what is pH and how is it measured, what do you mean by “strong” acid, are there “weak” acids? (yes) and does H⁺ even exist? Things to cover at another point…  EDIT: I have added a post on this!  LINK

* The complete definitions of acids and bases.

(Svante) Arrhenius: Acids increase the concentration of H⁺ in water.  Bases increase the concentration of OH^- in water.

Bronsted-Lowry: Acids are H⁺ donors.  Bases are H⁺ acceptors.

(Gilbert) Lewis:  Acids are molecules capable of accepting an electron pair (empty orbital).  Bases are able to donate an electron pair (unpaired electrons).

 ** I chose to use H⁺ in this post instead of the more correct H₃O⁺ for clarity reasons.

References

Zumdahl, Steven S. “Chemical Principles, 4^th Edition” (2002) Houghton Mifflin Company, Boston, MA.

Black shirt – Old Navy, circa 2006.  J