November 2003

Hydrochloric Acid. Your stomach makes it naturally to help digest your lunch. It's used industrially to process steel, the material of choice for suspension bridges and cars and trucks. Hydrochloric acid is also used in the production of batteries, photoflash bulbs and fireworks. It's even used to process sugar and make gelatin. Hydrochloric acid, like last month's chlorine compound, sodium chloride, is another "workhorse" chemical because it is incredibly useful in a wide variety of ways.

Unlike sodium chloride, hydrochloric acid is not easy to handle and safety precautions are a MUST! This acid has a sharp, irritating odor and is highly corrosive, meaning, it damages most things it touches. You may be wondering how such a reactive liquid can be stored without ruining its container. Metal containers are out for this acid, but plastic containers, such as those made of PVC (polyvinyl chloride) stand up very well.

This is a photo of the Greer Steel Company steel pickling operation in Dover,
Ohio. In steel-pickling, a hydrochloric acid solution is used to remove rust and
scale to prepare the steel surface for a protective coating. Notice the rusty
appearance of the steel rolls in the foreground and the shiny steel rolls, already
"pickled," on the conveyor belt. About ¼ of the hydrochloric acid produced in the
U.S. is used for pickling steel. (Photo courtesy of Greer Steel Company)

HCl is the compound hydrogen chloride. Each molecule of HCl is composed of a one-to-one ratio of hydrogen and chlorine. (See the diagram at the top left of the page). At room temperature, HCl is a colorless, poisonous gas. Dissolve it in water, and, voilà, you have hydrochloric acid. For the record, acids are substances that release hydrogen ions in water. The more hydrogen ions an acid releases in water, the stronger the acid (see "What is an ion?" below). If you concluded from the above discussion that HCl releases many hydrogen ions in water, you are right!

What is an ion?

An ion is an atom that has gained or lost electrons. Electrons are negatively charged sub-atomic particles that balance the positively charged protons in uncharged (neutral) atoms.

When HCl is dissolved in water, ions of H and Cl are formed. Hydrogen loses an electron (becoming an ion of +1 charge) and chlorine gains an electron (becoming an ion of -1 charge).

Strong acids, like HCl, release many more hydrogen ions in water than do weak acids, like vinegar or lemon juice.

Follow-Up Questions:

  1. Find out the names and chemical formulas of at least two naturally occurring acids. What do the chemical formulas of acids have in common?

  2. What is meant by the pH of an acid? How does it relate to an acid's strength?

  3. Bases are compounds that react with acids to form water and salt:
    Acid + Base ? Salt + Water.

In other words, acids and bases neutralize one another. Write the reaction of HCl with the base sodium hydroxide (NaOH) and describe in your own words what happens. (Hint: October's chlorine compound of the month plays a role in this reaction!)

Science Project Ideas:

  1. How is it that hydrochloric acid does not destroy the inner lining of the human stomach? Research the reason and also some of the diseases associated with "acid stomach."

  2. Research the methods used to clean up accidental spills of acids. How are the principles of chemistry used in these operations?

For a list of previous "Chlorine Compound of the Month" features, click here.

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