Print Version: Wolf's Wisdom: What's an Acid

What's an Acid

(A pretty serious chemistry lesson)

In the Pumproom Press we looked at some of the unusual properties of plain old water, now it's time to go farther. The fact that the water molecule forms an electric dipole, and that all those dipoles bond with other dipoles in their vicinity (think of it as flirting with everybody in the neighborhood), explains the high melting and boiling points as well as the unusually high heat of vaporization. But these water molecules are not completely stable. Before we explore the instability, some definitions are in order.

To write very large or very small numbers, scientists use a special notation called scientific notation. Lets look at some examples. The number 2.3 million can be represented as 2.3 times one million. Now one million is expressed commonly as 1,000,000. If you count the number of zeros in 1,000,000, you'll see that there are six of them. The scientific notation for 1,000,000 is 106, where the upper number (called the exponent) represents the six zeros in that number. 2.3 million, or 2,300,000 is then simply written as 2.3 x 106. Another example: the mass of the earth is 5.9742 septillion kilograms, that is 5,974,200,000,000,000,000,000,000 kg. Put a decimal point after the 5, and count the number of digits after it. You will find 24. So the mass of the earth is much shorter written as 5.9742 x 1024 kg or, rounded as we do in the pool world, 6 x 1024 kg. We'll have some more examples later.

That's OK for very large numbers, but how about very small ones? The whole procedure is very similar; but you must remember to write the leading zero before the decimal point; it's 0.007, and not .007 (no relations to James Bond). You count all the zeros before the first digit that are not zero, add a minus sign and, voilà, you are done. In the example of 0.007, the scientific notation sees three zeros before the digit 7, so it's 7 x 10-3, or, spoken, seven times ten to the minus three. One more example: The Bohr radius for the hydrogen atom (essentially half the diameter of the hydrogen atom) is: 0.000,000,000,0529 meters. Count all the zeros; there are eleven. So the Bohr radius is 5.29 x 10-11 m. Enough of that.

We need a couple more definitions. In the periodic table of elements each element has a so-called atomic mass number. The mass number for hydrogen, the lightest element, is about 1, for carbon it's 12, oxygen's is 16, chlorine's is 35.5, calcium's number is 40, and so on. If you combine atoms to form a molecule, you simply add up all the atomic mass numbers to get the molecular mass number, commonly referred to as the molecular weight. Example: water has the formula H2O, meaning that the water molecule consists of two hydrogen and one oxygen atom. With the numbers given above, the molecular weight is 1 + 1 + 16 or 18. One final example: calcium carbonate, the scale formed in pool water with a high pH and high hardness, has the formula CaCO3. This means one calcium plus one carbon plus three oxygen atoms. With the numbers from above this gives a molecular weight of 100: 40 + 12 + 3 x 16 = 100.

If you take the molecular weight of a substance in grams, you have a mole. (No, not the critters that dig holes in your lawn…) With the examples above, a mole of water weighs 18 grams (g), a mole of CaCO3 weighs 100 g.

So much for the basic definitions; let's get back to water: Some water molecules occasionally react with nearby ones in an exchange:

(1) H2O + H2O <=> H3O+ + OH-

The double arrow indicates that the reaction is reversible. This very fundamental reaction is called the "hydrolysis" of water. Chemists, being just as lazy as anybody else, abbreviate this reaction to:

(2) H2O <=> H+ + OH-

This is really only an abbreviation; in real life there are no H+ ions or bare protons floating around in the water. So in the future, when you see an H+ in a chemical formula, it's really H3O+. We call the former lazy chemistry.

Now we're getting into the real chemistry, but we'll stay lazy by calling the H3O+ ions "H+". These H+ ions are the things in water responsible for acidity. (An "ion" is any molecule that has either some additional or some missing electrical charges.) The more of these H+ bodies you have floating around in water, the more it reacts as an "acid". If you have one mole H+ in one liter of water, (i.e., 19 grams of H3O+ ions per liter) then you have a really strong acid. Battery acid, which is sulfuric acid, falls into that category. With 1/10 of a mole per liter, or 0.1 mol/L, you still have a strong acid; muriatic acid is in that range. Remember that we can write 1/10 as 0.1, also as 1 x 10-1? The carbonic acid in your soda pop has about 0.000'05 mol/L (5x10-5mol/L) of these H+ ions, roughly the same as acid rain!

So, you may ask, what in the world has all of this chemical …stuff to do with pH? If you're still with me, the end is close. If you have water with lots and lots of H+ ions, you have an acid. And if you have an acid you have a low pH. More precisely: if you have 100 or 1 mole per liter (mol/L) H+ ions, then the pH is zero (0). With 10-1 mol/L H+ ions, the pH is equal to one (1). And so on. If the concentration has fallen to 1/10'000'000 = 10-7 mol/L, then the pH has reached 7. All clear? In other words, you take the exponent of the molar concentration of H+ ions, change the sign from negative to positive, and you've got yourself the pH of that aqueous solution. If the concentration is 10-7, you look at -7, change the sign: the pH is 7. (And now you have an explanation for the official definition of the pH, found in every elementary chemistry book: pH is the negative log [that's the same as taking the exponent and changing the sign] of the molar concentration [i.e. mole per liter] of hydrogen ions. Whoooow.

So we have finally reached the answer to the question in the title. An acid is an aqueous solution (meaning: it must be a solution of chemicals in WATER) that contains a high concentration of H+ ions. The higher the concentration, the lower the pH, which is the scale used to measure the strength of acids.

One of the problems with pH is that it is clearly not linear. You buy a pound of apples for $0.89; ten pounds will cost you $8.90 That's linear. A gallon of gas is $1.899; ten gallon will set you back $18.99 – that is linear, and a rip-off, too. But if your pH drops from 8.0 to 7.0, you have ten times as many H+ ions in the water, and if you add more acid to reach a pH of 6.0, 100 times the H+ concentration. This is clearly NOT linear; depending on how you look at it, it's either called a logarithmic scale for the pH or an exponential scale for the ion concentration.

There are many more things to discover about pH. Why do we call a pH of 7 neutral? Is this something the chemists cooked up to make life easier for them? Given that we splashed around enough acid today, and that your brain is probably in a state of acid overload, we'll come back to this and explore some of the practical applications of pH. You can count on utilizing strong acids (like HCl) and weak acids (like HOCl), and bases (like soda ash and bicarbonate of soda). And in the next PrP you'll find a foundation for explaining alkalinity. Until then, keep the pH of your pool around, say, 7.3; and enjoy your pH 4.5 soda pops.

~Wolfram Hartwig