The Bronsted-Lowry theory: An acid is a H+ donor and a base is an H+ acceptor.
H3O (oxonium ion) is present in every acidic solution. The acid donates a H+ to the water molecule.
The oxonium ion can itslef act as an acid and donate an H+.
An alkali is a substance which forms OH- ions in solution.
Every acid has a conjugate base and every base has a conjugate acid. They are called conjugate acid-base pairs.
Many substances can behvave as both acid and base - they are described as amphoteric.
Not all acids have the same strength - some are powerful H+ donors and are described as strong acids.
Others are weak acids - they are moderate or weak H+ donors.
A strong acid will have a weak conjugate base and a weak acid will have a strong conjugate base.
Acid-base indicators, such as litmus, are coloured organic substances which are themselves weak acids. The conjugate acid and base will form different colours.
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