Lesson 17: Acids and Bases

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Acids and bases are everywhere. They are in our food, household products, even in our own bodies! They are relevant and relatable.

Textbook Reading:  Chapter 14, Acids and Bases, pp. 487-521.

People have known for centuries that acids:

  • Taste sour (like lemons)
  • Dissolve/corrode metals
  • Turn blue litmus paper red

On the other hand, bases:

  • Taste bitter (like caffeine in coffee)
  • Feel slippery
  • Turn red litmus paper blue

In addition to these criteria, chemists have been refining and honing their definitions of acids and bases.

In 1884, Svante Arrhenius from Sweden realized an acid is a material that can release a proton or hydrogen ion (H +) when in aqueous solution and a base releases hydroxide ions.

Because this definition did not apply to non aqueous solutions, other scientists continued to refine the definition until two scientists in 1923 came up with a definition that would work for any situation. The Brønsted-Lowry definition says that an acid is a proton donor and a base is a proton acceptor.  Thus ammonia, which has no hydroxide group, still acts as a base by accepting a proton.

Some molecules, such as water, can act either as an acid or as a base according to this definition. Molecules that can act as an acid or a base are called “amphoteric.”

Some acids are defined as “strong” and some as “weak,” generally based on how much they ionize. Strong bases dissociate completely, whereas weak bases only dissociate slightly.

This short video from TED explains these terms.

pH Scale

Chemists measure how acidic or basic a substance is using the pH scale. Although no one knows for sure how the name came to be, it is acceptable to think of pH as the “power” of hydronium ions, thus how many hydronium ions are present. (A hydronium ion is a water molecule with an extra proton – H3O+. Although often used interchangeably with hydrogen ion, hydronium is more technically correct.) Thus, pH = -log [H3O+], where the brackets mean “concentration of.”

When the concentration of hydronium ions is high (pH less than 7), the substance is said to be acidic. If the pH is =7, then the substance is neutral and if the pH greater than 7, then the substance is basic.

Although the scale is often labelled from 1-14 or 0-14, there are really no limits to the ends. Substances have been found with a negative pH, but it turns out that it is very difficult to measure the hydronium ion concentration in the negative range.


Bozeman Science has a particularly clear overview of pH in this video:

Be sure to let me know if you have any questions about these materials.

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