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by Ron Kurtus (revised 15 September 2015)

Acids are chemical compounds that can react with metals and other substances to "eat them away" or damage them.

You can usually identify an acid by its pH value, such characteristics as taste or feel, and its chemical formula. One common place acids are found is in soft drinks.

Questions you may have include:

This lesson will answer those questions.


Acids are chemical compounds that usually have a caustic action on plant and animal tissue, as well as metals.

pH scale

The pH scale is a measurement of the strength of an acid or base. An acid is a solution with a pH less than 7.0.

Litmus paper is often used to give a rough estimate of the pH. When the paper turns red, the material is acidic, and when the paper turns blue, it contains a base.

Gardeners use the pH scale to determine how acidic or alkaline their soil is. The pH scale is also used to help determine water quality.

Physical characteristics

Mild acids that are dissolved in water have a sour taste. A common example of such an acid is carbonic acid (HCO3) used in carbonated drinks. Lemon juice is also acidic and certainly tastes sour. An acid is considered "mild" if it does not readily attack living tissue. Usually, such an acid has a pH slightly less than 7.0.

You should never try to taste stronger acids, because they can harm you. Even trying to smell a strong acid is not a good idea, because the fumes can burn your nostrils. A strong acid usually has a pH reading much lower than 7.0.

Even after diluting a strong acid such as sulfuric acid (H2SO4) in water, you must use caution.

Combining with bases

Acids will react with bases—sometimes violently—to create salts. Usually, both the base and the acid are diluted with water to buffer the reaction.

For example, a water solution of hydrochloric acid (HCl) combined with a water solution of sodium hydroxide base (NaOH) combine to form common salt (NaCl) and water:

HCl + NaOH → NaCl + H2O

(Note: The extra water used to create the solutions is not included in the above chemical equation since it isn't part of the reaction and for the sake of simplicity.)


In general, acids can be chemically identified by the hydrogen term in the front of its chemical formula. For example, the formula for hydrochloric acid is HCl. Sulfuric acid also has an H in the front of its formula H2SO4.


There are some exceptions to this rule. For example, some organic acids--such as acetic acid (CH3COOH)--have their formulae designated according to their structure.

(Note: The plural of formula is formulae, from Latin)

These exceptions occur mostly in organic chemistry and follow a more general description of acids and bases. In this definition Lewis Acids are those which can form a new covalent bond by accepting a pair of electrons and Lewis Bases are those that can form a new covalent bond by donating a pair of electrons.


Water (H2O) is an interesting compound in that it can be an acid (H in front), but it also can be a base (HOH), where OH indicates a base. It can also be a neutral substance. Whether it is an acid, base or neutral depends on the chemical reaction.

A substance that is both an acid and base is called amphoteric. (You probably will never see that word again, unless you go to advanced chemistry.)

Uses for acids

There are numerous uses for acids.

Car batteries

Car batteries use sulfuric acid to help create and store electricity. Sulfuric acid is a very strong acid that will eat a hole in a piece of iron, as well as eat through your clothes and skin. You should always use extreme caution when handling a car battery.

Stomach acids

Your stomach has acids that help break down and digest food you have eaten. Concentrated stomach acid can irritate your stomach lining and even eat a hole in it.


If the body has secreted excess acid, because it is having trouble digesting the food you have eaten, you can get a burning pain in your stomach area. If you take an antacid, it will buffer the stomach acid.

Some antacids contain a base-type chemical compound. Adding a base to an acid neutralizes the acid and produces water and a salt.


Aspirin is an acid and can irritate you if taken on an empty stomach. Buffered aspirin has a small amount of antacid to neutralize the acidic effect on you.

Other uses

Acids are used in industry both to dissolve materials and to create new compounds. We drink very mild acids in our carbonated and fruit drinks.

Working with acids

Acids usually must be mixed with water to dilute them and make them more usable. Although they dissolve in water, they also can react and bubble when mixed with water. Care must be taken when mixing strong acids and water, because if the mixture explodes, the acid can be sprayed all over.


An acid can eat through materials and has formula that usually begins with an H (hydrogen). Acids are found in soft drinks and in the stomach. Acids are also used in industry.

Chemistry rocks!

Resources and references

Ron Kurtus' Credentials


Weak Acids and Bases - From University of Waterloo, Canada

Experiments with Acids and Bases

Red Cabbage Indicator - Substitute for Litmus paper

Chemistry Resources


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