An acid can be regarded for practical purposes as a proton donor, while an alkali, sometimes called a base, can be thought of as a proton acceptor.
In 1884, Svante Arrhenius and Wilhelm Ostwald independently defined acids as substances which release hydrogen ions when they are dissolved in water.
In 1923, Johannes Bronsted defined acids as substances acting as proton sources, and bases as substances acting as proton acceptors, regardless of the solvent.
Neutralization is the reaction of an acid with an alkali, and in essence, it involves hydrogen ions combining with hydroxyl ions to form water.
Acids and alkalis are of different strengths as measured on the pH scale, which is a logarithmic scale based on the concentration of hydrogen ions.
The pH of a solution may be assessed with indicators, which are organic dyes that can add or lose hydrogen ions, and then change colour as a result.
Robert Boyle described in his 'experimental History of Colours' how some vegetable dyes change colour in acids and alkalis and introduced litmus.
As a general rule, acids react with metals, releasing hydrogen. To be more precise, the stronger acids react with the more active of the metals.
A buffer solution is one that retains a fairly constant pH, even when acid or base is added to the solution, because it is able to absorb or donate protons.
Some parts of the world are troubled by acid rain, an effect which is caused when acidic gases produced by burning fuels react with water vapour.
Carbon dioxide in water can dissolve limestone. Caves may be formed when limestone is dissolved to make hydrogen carbonate ions which can be taken elsewhere.