The acidity of a substance has a strict scientific definition. People tend to have images of metals dissolving and holes burning through things when they think of acids and non-acidic substances, or bases. The truth is, how destructive a substance can be is not the factor that chemists consider when determining the acidity (or lack thereof) of something.
Definition and Detection of an Acid
There are three different definitions that chemists use when defining an acid and base.
The Arrhenius definition: Acids are substances that, when dissolved in water, increase the concentration of H+ ions (that is, positive hydrogen ions, or protons). Bases are substances that, when dissolved in water, increase the concentration of OH- ions (also known as hydroxide ions).
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The Bronstead-Lowry defintion: An acid is a substance that can transfer a proton (H) to another substance. A base is a substance that can accept a proton (H).
The Lewis definition: An acid is defined as an electron-pair acceptor, and a base as an electron-pair donor.
In practice most chemists (unless your an organic chemist) think of acids and bases in terms of the first two definitions.
While these definitions may seem highly technical, one surefire way to understand acids in the kitchen, for example, is to preform a simple reaction with baking soda. If you have a liquid and you want to know if it is acidic, an easy way to tell is to mix in a little baking soda. The baking soda reacts with acids to produce bubbles.
You may be familiar with building a homemade kitchen volcano. You mix vinegar (an acid) with baking soda. It foams up as the baking soda reacts with the acid. This is in essence what you can do to test if a solution is acidic or not. If there is no acid present, the solution will not bubble when you add the baking soda.
Relative Strengths of Acids
Some acids are stronger than others. We are very familiar with this concept when we take a drink of soda and leave it on our tongues. The burning sensation is from the acid in the soda. We do not get this sensation when we hold pure water in our mouths. The difference is the strength of the acid. Of course, caution should be used before you put something into your mouth.
Scientifically speaking, a strong acid would be one that completely transfers its protons (H+ atoms) to water leaving no undissociated molecules in solution. A weak acid would be one that only partly dissociates in aqueous solution and exists in the solution as a mix of acid molecules and component ions. A substance with negligible acidity is one that contains hydrogen but does not demonstrate any acidic behavior in water (that is, the hydrogen does not dissociate or detach from the molecule).
The pH Scale
The use of the pH scale is one practical way of quantitatively determining how acidic something is. If the pH of a solution is less than 7, it is acidic. If the pH is 7, the solution is neutral and if the pH is greater than 7,the solution is basic. This scale indicates the amount of actual H+ ions (acidity) floating around in the solution, which is directly related to the definition of an acid.
Detecting the pH of a Solution
A few different ways of measuring the pH of a solution exist. The most commonly known method is the use of litmus paper. The litmus paper is coated with a chemical that reacts with acids to change the color of the paper. You can then compare the paper with a standard color chart to find the pH value. It is also common to use solution indicators to find out the concentration of acid in a solution. This works similarly to the litmus paper but is instead added to the solution and the color of the entire solution changes to a color indicative of the pH value. In the chemistry lab scientists carry out titration experiments to determine the pH value. A particular amount of technical skill is needed to use this method. The most common and more accurate method is via the use of a pH meter. The electronic meter contains a probe that is immersed in the liquid and a electric current is measured that can be directly related to the pH value. The value is then dictated to the user on the display of the meter. These pH meters have increased in reliability and user friendliness over the years and are the standard way to go. Most of this equipment is not in the household kitchen. One can order pH testing strips (litmus papers) from a cooking store if needed.
Examples of the pH Value of Various Substances
These values are approximate, but can give you a sense of where substances fall on the pH scale. Household bleach: 12.5 Milk of magnesia: 10 Baking soda: 8 Pure water: 7 Black coffee: 5 Wine: 3.5 Cola, vinegar: 2.9 Gastric juice: 1.2
Numbers higher than 7 are basic and numbers less than 7 are acidic.