B.1.12 – pH & Polarity

Defined Words: pH, oxygen, hydrogen, water, hydroxide, mole, gram, polar, nonpolar, acidic, basic

 

pH is an acronym that we use to inform us of the power of Hydrogen ions within a solution. In order to fully explain what that means we’ll first have to look into the structure of a water molecule: one oxygen atom bonded to two hydrogen atoms.

Oxygen is the name we give to the element with eight protons; it also has eight electrons in order to balance its charge. Two of its electrons fill up the first energy level, leaving six in the outer valence shell. A filled valence shell has eight electrons so the missing two electrons are supplied by nearby hydrogen atoms, the element with just one proton. Those hydrogen atoms can also fill up their valence shell with an additional electron.

Water exists as H2O but doesn’t keep that form. Oxygen has eight times as many protons than hydrogen so more of the electrons are drawn toward it. One of the hydrogen atoms will completely lose its donated electron and sever the attachment to oxygen in the process. The departing hydrogen atom now has a positive charge since it no longer has an electron. The additional negative charge was left with the rest of the molecule, giving it an overall negative charge.

H2O will go back and forth between a single molecule and two ions that are written H+ (just a proton, though it often joins with another water to form H3O+) and OH (called hydroxide.)

Atoms are small, which means that in just a single liter of water there are over 3×1025 molecules of water; that’s a 3 with 25 zeroes at the end of it! (30,000,000,000,000,000,000,000,000) In order to measure the amount of molecules, we came up with a measurement that we called a mole.

A mole is the amount of molecules required for the atomic mass, the number of hadrons within the nucleus, to be the same as the mass in grams. A gram is yet another measurement of mass and you’re probably starting to see how important conversions are in science. A mole splits up large groups of molecules into chunks of 6×1023, much easier to work with!

The measurement of moles allows us to determine concentration, denoted with brackets [H+] and determined using the moles divided by the liters. Because a mole uses such a large amount of atoms, the value of [H+] is multiplied by 10 to a negative exponent, meaning these are tiny numbers between 0 and 1.

Pure water starts out with a concentration of 1×10–7 M (otherwise written 0.0000001 M) which translates to a pH of 7 since pH is the negative exponent of the concentration. We take just the exponent and switch it back to positive just so we can have a scale that fits our usual number line. The pH of water ranges from 0 to 14. This might seem like just another conversion, and it is, but a 0-14 scale is much easier to work with than very tiny or very large numbers.

Looking back to the structure of water, the oxygen already has two sets of paired electrons which sit on one side of the molecule. The electrons cause a negative charge to build up on that side of the molecule and the protons of the hydrogen atoms cause a positive charge to build up on the other side.

A molecule with oppositely charged ends is called polar and it is attracted to charged particles and other polar things. This is the reason that water sticks together as drops or why it’s possible to skip stones on a lake. The water is sticking to itself and allowing for somewhat of a membrane on the surface.

Water can also stick to other things; this is why it clings to your mirror after a shower. Molecules with symmetric distribution of charge are called nonpolar and they are attracted to each other. In the case of polarity: like attracts like, opposites repel. The reason soap works is because it’s both polar and nonpolar. Soap molecules will latch onto nonpolar compounds and as well as polar water to wash away the nonpolar gunk that the water can’t get on its own.

The polar aspect of water means that if salts are tossed into a glass of water, the water’s ions will break the salt into its positive and negative ions and surround those charged pieces. The positive salt ions will be surrounded by negative OH ions while the negative salt ions will be surrounded by positive H+ ions.

Depending on how much of each type of ion is present in a solution, there will be a greater amount of free OHor H+ ions. This means that if there are a bunch of positive ions dissolved in the water, many water molecules will ditch one of their hydrogen atoms to surround the new positive ions. There will be a greater amount of free H+ ions and the increase will cause an increase in [H+].

The negative exponent of the concentration decreases in magnitude, lowering the pH meaning the solution is more acidic. If the reverse were to happen, there would be an increase in [OH] which would raise the pH and create a basic solution.

So much goes on within a single glass of water and it all starts on the subatomic level. You’ve been drinking water your whole life and you probably never thought about the constant exchanges of protons between water molecules!

Water is one of the most interesting substances that we have ever discovered. It is the exception to many trends of the universe, it is the most crucial molecule necessary for life, and it is the reason the world is the way it is now. I feel as though understanding exactly what’s going on with the atoms of water brings me closer to the water itself. Water is the life bringer and studying it means you can understand a part of yourself.

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