Acids and bases are fundamental chemicals in chemistry. In organic chemistry, the carboxylic acids have acidic properties, but in inorganic chemistry there is a wide range of acids with a variety of properties.

Definition of Acids and Bases

The word acid comes from the Latin word which means sour or sharp to the taste, and liquids like vinegar and lemon juice are well-known acids. Other properties of acids are that they are corrosive to some metals and turn a material called litmus, which is an indicator extracted from moss, red. Some common acids are citric acid – found in fruit juice, ethanoic acid – found in vinegar, sulfuric acid – found in car batteries, and carbonic acid, which is also known as soda water.

There are other chemicals which, when added to a mixture of acid and litmus, will turn it blue. These chemicals are called bases (alkalis when they are soluble) and their reaction with acid is called neutralisation. Common bases are calcium hydroxide (lime), sodium hydrogencarbonate (baking soda), magnesium hydroxide (milk of magnesia) and ammonia.

Acidity Depends on Hydrogen Ions

The behaviour of acids and bases is related to the presence of hydrogen ions (H+) in the solution. When an acid dissolves in water, it releases hydrogen ions, which cause the acidic properties. A strong acid, like sulfuric or hydrochloric acid, is an acid which is completely ionised in solution (Important: it has nothing to do with the concentration of the solution). A weak acid, like ethanoic or citric acid, is an acid which is not completely ionised.

The activity of a base is explained by its reaction with hydrogen ions. So, a base is able to react with hydrogen ions, reducing the acidic nature of the solution. Some bases are metal hydroxides, and the hydroxide part reacts with the hydrogen ion to make water. Other bases are carbonates or hydrogen carbonates which react with the hydrogen ions to make carbon dioxide and water. This explanation of how acids and bases work is called the Brønsted-Lowry definition of acids and bases, after the two scientists who first proposed it in 1923.

The pH Scale

A scale to demonstrate how acidic or basic a solution is has been developed, called the pH scale. It ranges from 1 to 14, in which the lowest numbers are for the most acidic and the higher numbers are the most alkaline, with pH 7 being neutral. The formula to work out the pH value is:

pH = -log [H+]

where [H+] is the concentration of hydrogen ions in the solution.

Some example of pH values:

  • pH1: stomach acid
  • pH3: vinegar
  • pH5: rainwater
  • pH7: pure water
  • pH9: baking soda
  • pH12: mineral lime
  • pH14:s odium hydroxide (Caustic Soda)