Certain hydrogen-containing molecules or ions react by transferring a hydrogen ion, H+, to another molecule or ion.
The reactant which is the source of the H+ is called an acid, the reactant to which the H+ becomes attached is called a base, and the reaction is called acid-base.
The acid hydrogen chloride tranfers H+ to the base ammonia to give the ionic solid ammonium chloride (composed of NH4+ and Cl¯ ions).
The gas hydrogen chloride acts as an acid as it tranfers H+ to a water molecule which is acting as a base.
A hydrogen ion, H+, is just a proton, and these acid-base reactions are called proton transfer reactions. An acid is defined as a proton donor and a base is defined as a proton acceptor. This is the Brønsted-Lowry definition. A base must have a non-bonding pair of electrons which can be used to form a bond with a proton.
Brønsted acid: a proton donor, i.e. a species which donates a proton in a chemical reaction Example: HCl in the above examples.
Brønsted base: a proton acceptor, i.e a species which accepts a proton in a chemical reaction
Example: NH3 and H2O in the above two examples.
Oxonium (hydronium ion): the cation H3O+, often written as H+(aq) or just H+ in equations in water (water, aqua, aq).
It is important to realise the proton, H+, does not exist as such in condensed matter, i.e. liquid or solid state. The term hydronium is still widely used
Hydroxide ion: the anion HO¯ (more commonly written OH¯, but it is O which has the formal charge of 1).
Conjugate acids and bases: When an acid loses a proton, its product is called the conjugate base of that acid. When a base accepts a proton its conjugate acid is formed
Example: HCl/Cl¯, NH4+/NH3, H3O+/H2O, and H2O/OH¯ are conjugate acid-base pairs.
Neutralisation: The reaction between an acid and a base
Example: reaction of aqueous HCl and NaOH to give NaCl and H2O .
Salt: the product (other than water) of a neutralisation reaction between an acid and a base. Salts are ionic compounds
Example: NaCl in the above neutralisation reaction.
Alkali: Aqueous solution of a base
Example: aqueous NaOH or aqueous ammonia .