What Is an Alkaline Substance?

Alkaline is a word you just don't see often. After all, acids can fairly be described as acidic without confusing anyone, but simply calling bases basic, while true, isn't as helpful as it could be since "basic" has so many synonyms. The word "alkaline" is synonymous in chemistry with the word "basic," which means "having a pH greater than 7," but not all bases come from the class of chemicals known as alkalis.

If you have ever used litmus paper and tested a substance that turned the paper from red to blue, you have just dipped the paper into an alkaline solution. But what makes a solution alkaline, other than its pH? That is, what atomic or molecular factors underlie the result of a pH test?

Acids and Bases

Various formal definitions exist for acids and bases, but generally, an acid is a substance that can donate a proton, or hydrogen ion (H+) in aqueous solution, while a base is a substance that can accept a proton.

Equivalently, bases can donate a hydroxide ion (–OH) in solution. The reason these ions matter in acid-base chemistry is that water consists of H2O molecules, which can supply either of the ions into which it dissociates (H+ and OH) as required into the solution.

  • While the word "alkaline" means "basic," "alkali" and "base" are not synonymous. An alkali, in fact, is one of a specific group of elements on the periodic table, as you'll soon learn. So be aware of the alkali vs. alkaline distinction.

An example of a strong base is sodium hydroxide, or NaOH. It is called "strong" because it is completely ionized in water, that is, all of the molecules in the compound break up into Na+ and OH ions. Because there are no H+ ions entering the solution to balance off the OH ions, the solution has a basic (alkaline) pH.

Characteristics of Alkaline Substances

If the pKb or dissociation constant of an alkaline substance has a high enough value, the basic substance will dissociate into its component ions in aqueous solution even when the pH is already strongly basic (i.e. pH of about 12 or higher). The pH scale technically goes higher than 14.0, but this level of hydroxide ion concentration is hard to produce chemically and rarely observed.

Common Alkaline Solutions

There are quite a few examples of alkalis in everyday life. One example of an alkaline solution is more or less in the name: antacids! These are substances that can neutralize acids by donating hydroxyl groups that combine with the acid's protons to form water.

This is what occurs in the stomach when you take an antacid (a shortening of "anti-acid"). The protons liberated from the gastric hydrochloric acid are neutralized by the relevant component of the ingested compound.

These substances are taken not as tablets or capsules in all cases but as solutions, after a tablet is dissolved in water. The "fizz" comes from carbon dioxide, in turn derived from the bicarbonate (HCO3) molecules of the antacid.

Examples of Alkalis in Everyday Life

A list of alkalis found in the everyday world includes all of the alkali metals on the periodic table of elements combined with –OH to create a hydroxide: lithium, sodium, potassium, rubidium and cesium. Calcium, strontium, barium and ammonium hydroxide are also considered alkalis.

A few spots on the globe are producers of soda ash, or natural alkali. This mineral's composition usually takes the form of sodium sesquicarbonate, which has the formula Na2CO3NaHCO32H2O. The U.S. generates a large fraction of the world’s natural alkalis, getting these from very large deposits under the ground in mines in the western half of the country and in particular in California.

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