Introduction to Acids, Bases & Salts

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The Language of Chemistry - 04

Introduction to Acids, Bases & Salts

The hydrogen cation (H+) and the hydroxide (hydroxyl) anion (OH) are two of the most important ions, since they are the basis for two very important classes of compounds in chemistry – acids and bases. While a hydrogen cation is simply a proton (since the hydrogen atom consists of a single proton and a single electron), a hydroxide anion is a polyatomic anion, in which one oxygen atom covalently bonds with one hydrogen atom.


Acids are compounds which contain hydrogen which is replaceable by a metal in a chemical reaction.

An acid is a compound that provides H+ ions in water.

Some common acids are hydrochloric acid (HCl), nitric acid (HNO3), sulphuric acid (H2SO4), and phosphoric acid (H3PO4).

Examples of dissociation of acids into ions in solution are:

(a) hydrochloric acid:
HCl ( g ) dissolves in  H 2 O H + ( aq ) + Cl ( aq )
(b) nitric acid:
HNO 3 ( l ) dissolves in  H 2 O H + ( aq ) + NO 3 ( aq )
(c) sulphuric acid:
H 2 SO 4 ( l ) dissolves in  H 2  O H + ( aq ) + HSO 4 ( aq ) HSO 4 ( aq ) dissolves in   H 2  O H + ( aq ) + SO 4 2 ( aq )

Hydracids (Binary Acids)

A binary acid, or hydracid, contains hydrogen and a non-metal, but not oxygen. An example is hydrochloric acid ( HCl ( a q ) ), which is hydrogen chloride dissolved in water.

Oxoacids (Oxyacids)

An oxoacid, or oxyacid, in addition to hydrogen, contains an oxoanion, which is a polyatomic anion containing oxygen. Most acids are oxoacids. Some examples are nitric acid (HNO3) and sulphuric acid (H2SO4).


Bases or alkalis are compounds which provide hydroxide radicals and react with acids to form salts and water only.

A base is a compound which provides OH ions in water.

Some common bases are sodium hydroxide (NaOH) or caustic soda, potassium hydroxide (KOH) or caustic potash, and barium hydroxide (Ba(OH)2).

Examples of dissociation of alkalis into ions in solution are:

(a) sodium hydroxide:
NaOH ( s ) dissolves in  H 2 O Na + ( aq ) + O H ( aq )
(b) barium hydroxide:
Ba ( OH ) 2 ( s ) dissolves in  H 2 O Ba 2 + ( aq ) + 2 O H ( aq )


A salt is a compound formed by the replacement of the hydrogen atom(s) of an acid by metal atom(s).

(a) Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), an alkali, to form the salt sodium chloride along with water. HCl + NaOH NaCl + H 2 O (b) Sulphuric acid (H2SO4) reacts with potassium hydroxide (KOH), an alkali, to form the salt potassium sulphate along with water. H 2 SO 4 + 2 KOH K 2 SO 4 + 2 H 2 O

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McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.