Last updated 4-29-08


Humans have been familiar with acids and bases for thousands of years. They didn’t know the exact chemistry but they knew a lot about them.
The word acid comes from Latin word acidus meaning sour, sharp, or tart taste. Vinegar as you might remember means sour wine. Vinegar is acetic acid in water. Acetum is a historical name for vinegar and comes from the word acere meaning to be sour.

Citrus fruits also have a sour or tart taste due to a different acid called citric acid.
Sweet-Tarts get their sour or tart taste from citric acid.

Remember, I mentioned in an earlier tutorial that you can taste the protons from acids. Like this girl's reaction, protons taste tart or sour.

When hydrogen's proton comes off or reacts with something, its electron is left behind. This makes the remaining molecule negatively charged. Acetic acid disassociates into the hydrogen ion (H+) and the acetate ion (C2H3O2-). If water is present, it will grab the H+ ion.

The first definition of an acid is a substance that releases a proton (H+). It's normally released in water, since water is the most common solvent. As an example, hydrochloric acid is shown disassociating into H+ and Cl-. Sometimes the equation is shown with water receiving the H+ making H3O+. In reality, water will absorb the H+, but for simplicity's sake, it's often not shown (top reaction).


This definition is usually called the Bronsted-Lowry definition of an acid.

In general hydrogen that gets released from "X" leaves its electron behind, so it comes off as H+ and the "X" part left behind becomes negatively charged X-.

Power of a Protons' (+) charge.

1) A proton can attract electrons
2) A proton can push other protons

In other words it has the power to affect other compounds. Sometimes the effect is on their color.

For example, the color of wine or grapes is affected by the acid (protons) present.

A group of compounds called anthocyanins are responsible for the colors of these plants. It is theorized that this pigment evolved as protection to the high levels of ultraviolet light in Earth’s early history. Besides ultraviolet light, these pigments absorb different frequencies of visible light. This provides plants with different colors.

The color of this pigment is affected by the concentration of protons (acid level) in the plant. The red of this rose is affected by the acidity level in the petals. Change the acid levels, and the color of the rose will change.

Red cabbage has a particular wide range of color change depending on the pH. In a lab for my CHM107 class, I have students extract the pigment in red cabbage. The pigment belongs to the anthocyanidin family of compounds.

In this picture you can see that the red cabbage pigment goes from purple (neutral) to red (strong acid) as the concentration of protons (acid) increases.

As the pigment is exposed to lower levels of acid and higher alkaline levels of hydroxide (OH-) the color goes from purple to blue to green to yellow to green. Again, the molecule of

The old favorite poem starter "Roses are Red, Violets are Blue..." has a new explanation. The pH in violets is probably different than the pH in roses.

There's been an effort to genetically modify roses to make them blue, but they haven't yet controlled the pH, so no success yet.

The bright colors of autumn are due to anthocyanidins and the acid levels in the leaves.

The next time you go by a produce section in the grocery store and see the different colors in the fruits, realize that protons (pH) are partly responsible. In other words, dip them in a strong acid, and they will change color. Dip them in a strong alkaline solution and they will change to a different color.

Compounds are classified as inorganic or organic. Organic compounds contain carbon and are often produced in living things. Inorganic compounds normally have no carbon and are created by chemical reactions without the need for a living organism.

Acids are also classified as organic or inorganic for the same reasons.

Inorganic acids are usually stronger than organic acids. You probably have heard of sulfuric acid (battery acid), hydrochloric acid (pool acid), nitric acid, and phosphoric acid.
Organic acids are usually created by living organisms. Acetic acid is made by bacteria that converts alcohol to acetic acid.
Citric acid is produced in citrus (hence the name) and other plants.
The simplest organic acid is formic acid, which is made by ants and makes up their venom.
The way to recognize an organic acid is the presence of a carbon attached to two oxygens with one oxygen attached to a hydrogen. It's called the "carboxylic" group (COOH). The hydrogen has a tendency to be released, which is why it's acidic. Organic acids are not as strong as inorganic acids. For example, only about 1% of the acetic acid molecules lose their hydrogen to water. For the inorganic acids, it’s usually 100%.

The calcium salt of oxalic acid (calcium oxalate) is found in a house plant called Dumb Cane. The plant was used as a way to punish prisoners and slaves. They were forced to chew one of the leaves. Intense pain in the mouth and throat would follow and the person could not speak (That's why it's called Dumb Cane). Dumb in the sense of deaf and dumb.

In the leaves of Dumb Cane are cells with needles of calcium oxalate. When chewed, these cells explode and shoot these needles of calcium oxalate into the mouth. So this contributes to the pain of this salt of oxalic acid.

Calcium oxalate is the main ingredient in kidney stones.

Why are acids important?
Acids often cause a chemical change.
Remember the synthesis equations I showed on making flavorings? What I didn't mention was that a strong acid, like sulfuric acid, is needed as a catalyst to speed up this reaction. The protons of the acids are attracted to the oxygen atoms, which aids in the release of water and the connecting of the two reactants (ethanol & butyric acid).

Even though acids are useful in synthesis reactions, acids are also useful for decomposition reactions.

If you have ever mowed lawns, you might have wished that there was something useful for grass clippings. Acids can turn grass clippings into something valuable: sugar.

Grass is mostly cellulose, which are chains of glucose molecules. Even though this graphic shows glucose molecules coming to together to make cellulose (with water being released), acids can cause the reverse reaction. It can help water break up the long chains of cellulose and turn it back into the glucose. The sugar (glucose) can be fed to yeast, which will make us ethanol for drinking or for mixing with our gasoline. Now there's a good use of grass clippings.

So far we focused on acids, but what is the opposite of an acid? The opposite would be something that can neutralize or cancel the acid. The name for the opposite of acid is alkaline or basic. Let's see where the word "alkaline" came from.

Al Kali is the Arabic name for the plant we call the saltwort plant. What was discovered a couple thousand years ago was that the ashes from the saltwort plant had the ability to neutralize the power of acids.

In the ashes were sodium hydroxide and potassium hydroxide. The hydroxide ion (OH-) would react with the hydrogen ion (H+ to form water. So this neutralized the acid. Anything that could neutralize acids like the Al Kali plant did was called alkali or alkaline.

The common alkaline compounds come from the Alkali Metals or the Alkali Earth Metals (no big surprise). These metal hydroxides are alkaline because they release "OH-" that can neutralize "H+" by turning it into water.

Above the excess H+ ions are taken out of circulation by turning them into HOH (water) by combining with OH-. Another compound that can take H+ ions out of circulation is nitrogen compounds known as amines. That's because nitrogen has an extra pair of electrons that a positive hydrogen will be attracted to. So H+ will latch onto the pair of electrons leaving fewer H+ ions in solution.

Another class of compounds are called "Alkaloids." (see below)        From the name, we guess they must be alkaline in nature meaning they can trap H+ ions. That's true. They contain nitrogen and, as you just learned, nitrogen has an extra pair of electrons that will attract H+ ions. (Roll cursor over image to see H+ attachment.)

Bronsted & Lowry defined acids as substances that released H+. They also defined bases (alkaline) as substances that released OH-.

Lewis Base: Gilbert Lewis broadened the definition of acid and base. A Lewis base possesses a pair of electrons that H+ can bind to, like the amines and alkaloids shown above. Here we see ammonia (NH3), methoxide (CH3O-), fluoride ion (F-), and hydroxide (OH-). They all have a pair of unshared electrons available for H+ to bind to. These unshared pair of electrons are often referred to as a lone pair.

Lewis Acid:
On the top we see the acid (H+) being attracted to a Lewis base (a substance with an unpaired electrons (a lone pair) on nitrogen in ammonia. On the bottom, we see the compound, boron trifluoride, that is also attracted to the same lone pair of electrons. So it behaves like the H+ acid and is called a Lewis Acid. Boron has 3 outer electrons that it shares with 3 fluorines. If boron shares the lone pair electrons of nitrogen, then boron will have a stable octet (eight) outer electrons. That's why boron trifluoride binds with ammonia (bottom right). All atoms have their outer shell full of electrons (hydrogen with two; nitrogen, fluorine, and boron with eight).

A discussion of acids and bases is not complete without an explanation of "pH". Everyone has heard of pH but few actually know what that means. First of all "pH" gets its name from "potential for Hydrogen" more specifically, the hydrogen ion (H+). So it's a scale that reflects the concentration of H+.

The numbers in the scale is counting the number of zeros in the denominator of a fraction. So a neutral pH of 7 is 1 over 10,000,000 (which has 7 zeros). pH 3 means 1 over 1,000 and so forth. Being a fraction, the more zeros the smaller the number. 1/1,000 is a bigger amount than 1/10,000,000, right? So the scale is reverse. Higher pH means smaller amounts of H+.

So we have the fraction, but a fraction of what? Well, it's the preferred way of counting in chemistry, which is the mole. Since this is concentration, it's moles per liter (molar=M). So pH is counting the moles of the H+ ion per liter but as a fraction. So pH3 is 1/1000 mole of H+ ions per liter. This is also written as 0.001 moles/liter (M) or 10-3 moles/liter (M).

This product says its liquid has a pH of 5, which is slightly acidic and will neutralize the alkaline residue from mortar. What is the concentration of H+ with a pH of 5?
The quick way is to put 1 over 1 followed by five zeros ( 1/100,000). So the concentration is 1/100,000 moles per liter. As a decimal fraction, it's 0.00001 M. The other way is to put the negative of the pH number as the exponent of 10. So pH5 is 10-5M (moles per liter H+).

This healing cream is listed as having pH 5.5. What is the concentration of H+?

Our shortcut doesn't work because we can't write out 5.5 zeros, but we can use the other method of changing it to -5.5 and putting it as the exponent of 10. 10-5.5. Putting 10-5.5 into the calculator will come back as 3.16x10-6, so the concentration is 3.16x10-6 M (moles per liter) or 0.00000316M. So pH 5.5 has less H+ ions than pH 5.
0.00001000 = pH 5
= pH 5.5
Remember the higher the pH the less H+ concentration. Also, note that a change from pH 4 to pH 3 is 10 times more H+. For example pH 2 is 1,000 times more H+ than pH 5. (5-2=3 which is 10x10x10)

Let's end on a bit of trivia. What is the strongest acid known?

It's fluoroantimonic acid (HSbF6). The yellow spheres are fluorine and the purple one is antimony (Sb). It's prepared by adding hydrofluoric acid to antimony pentafluoride.
HF + SbF5 -> HSbF6
This acid is 20,000,000,000,000,000,000 times stronger than 100% sulfuric acid. It has a syrupy consistency and dissolves pretty much everything. It can be stored in Teflon bottles. In contact with water, it explodes. If added to hydrocarbons like propane, gasoline, oil, or wax, it causes them to gain a positive charge making them quite reactive. Imagine a wax candle that would be dangerous to touch. So even if the acid is gone, its affect on the wax would make the wax extremely dangerous. In essence the wax itself becomes a very strong acid, more specifically a Lewis acid because it will be hunting out a pair of unshared electrons and those could be in the proteins of your skin.


Since April. 30, 2008