Chemical Changes: Strong and Weak Acids

An acid is a substance that donates hydrogen ions (H+) when it dissolves in water.
The strength of an acid is determined by the proportion of acid molecules that ionise (split up) in water.
The ionisation is a reversible process, hence exists as an equilibrium.
In a strong acid, almost all the acid molecules ionise to release hydrogen ions. Examples of strong acids include sulfuric, nitric and hydrochloric.
A weak acid only partially ionises in water. This means that in a weak acid, only a small proportion of the acid molecules release hydrogen ions. Examples of weak acids include ethanoic, citric and carbonic.
pH is a measure of the concentration of H+ ions in a solution. The scale goes from 0 to 14. The lower the pH, the higher the concentration of H+ ions, and the stronger the acid. A pH of 7 is neutral.
The pH of a solution can be measured using Universal Indicator or a pH probe connected to a pH metre.
Each one-unit change in pH represents a tenfold change in the acidity or alkalinity (H+ ion concentration) of the solution.
In neutralisation reactions between an acid and an alkali, hydrogen ions (H+) react with hydroxide ions (OH-) to form water.
Reacting an acid with a metal generally results in a salt and hydrogen gas. The salt is made from the metal and the non-hydrogen part of the acid.
Metal carbonates and metal oxides react with acids to form a salt and water. Additionally, metal carbonates produce carbon dioxide.
The name of the salt formed during these reactions is linked to the metal and the acid used: e.g., magnesium + hydrochloric acid produces magnesium chloride + hydrogen.