Detecting Acidity and Alkalinity

Acids are substances that dissociate in water to produce hydrogen ions. This idea was first put forward by the Swedish chemist Arrhenius in the nineteenth century. For example, although hydrogen chloride gas has a covalent structure, it dissolves in water to form  

As knowledge about atomic structure developed, chemists realised that the hydrogen ion was simply a proton, and it was highly unlikely to be stable on its own. There is now considerable evidence, from electrolysis reactions, to suggest that hydrogen ions combine with polar water molecules to form stable oxonium ions  

When hydrogen chloride dissolves in water, there is thought to be a chemical reaction  

The same holds true for the other common acids they all dissociate, to a greater or lesser extent, in water to form oxonium ions. This idea helps to explain why acids only show their acidic nature when dissolved in water. Traditionally, acids have been defined as substances that  

This information conveniently summarises the properties of most acids, but does not tell us anything about their structures and differences in reactivity. The evidence for the ionisation of acids (such as conductivity experiments) gives us an important extra property to include in a full definition acids provide hydrogen ions, (aq), in solution. Adds are 

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