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Definition: ion from The Penguin Dictionary of Physics

An electrically charged atom, molecule, or group of atoms or molecules. A negative ion (or ANION) contains more electrons than are necessary for the atom or group to be neutral, a positive ion (or CATION) contains less. See also GASEOUS IONS.

Summary Article: ion
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Atom, or group of atoms, that is either positively charged (cation) or negatively charged (anion), as a result of the loss or gain of electrons during chemical reactions or exposure to certain forms of radiation. In solution or in the molten state, ionic compounds such as salts, acids, alkalis, and metal oxides give rise to mobile ions and therefore conduct electricity. These compounds are known as electrolytes.

Ions are produced during electrolysis, for example the salt zinc chloride (ZnCl2) dissociates into the positively-charged Zn2+ and negatively-charged Cl when electrolysed.

Tests for negative ions The presence of negative ions can be determined by performing a number of different tests.

Bromide (Br): addition of silver nitrate solution to bromide solution immediately yields a whitish precipitate of silver bromide, which is partially soluble in concentrated ammonia solution, for example:

KBr + AgNO3 → AgBr + KNO3

Carbonate (CO32−): a solid carbonate treated with dilute hydrochloric acid gives off carbon dioxide gas, which turns limewater milky:

CaCO3 + 2HCl → CaCl2 + H2O + CO2

Chloride (Cl): treatment of a chloride with concentrated sulphuric acid produces colourless hydrogen chloride gas, which forms thick white fumes of ammonium chloride on mixing with gaseous ammonia:

NH3 + HCl ⇌ NH4Cl(s)

Hydrogen carbonate (HCO3): heating a solution of a hydrogen carbonate produces carbon dioxide, which turns limewater milky:

Ca(HCO3)2 → CaCO3 + H2O + CO2

Hydrogen carbonates react with dilute hydrochloric acid giving off carbon dioxide, in a similar way to carbonates.

Iodide (I): on addition of silver nitrate solution to an acidified solution of an iodide, a yellow precipitate of silver iodide is formed immediately, which is insoluble in ammonia solution:

KI + AgNO3 → AgI + KNO3

Nitrate (NO3): there are two tests for the nitrate ion in solution.

Sodium hydroxide solution and aluminium powder (or Devarda's alloy, which contains aluminium) are added to a solution of the nitrate. The mixture is warmed and the ammonia gas produced turns red litmus paper blue:

3NO3 + 5OH + 2H2O + 8Al → 3NH3 + 8AlO2

The brown ring test: an equal volume of iron(II) sulphate solution (acidified with dilute sulphuric acid) is added to the nitrate solution in a test tube. Concentrated sulphuric acid is carefully poured down the side of the test tube, so that it forms a separate layer at the bottom of the tube. A brown ring is formed at the junction of the two layers. This is FeSO4.NO, which is produced by the reduction of nitrate ions to nitrogen monoxide by the iron(II) ions:

NO3 + 4H+ + 3Fe2+ → NO(g) + 3Fe3+ + 2H2O

Care should be taken with this test, as nitrites and bromides can give similar results.

Nitrite (NO2): addition of dilute sulphuric acid to a nitrite produces brown nitrogen dioxide gas, which turns blue litmus paper red without bleaching it. The solution turns pale blue. No heating is required.

Sulphate (SO42−): addition of dilute hydrochloric acid and barium chloride solution to a solution of a sulphate results in the immediate precipitation of barium sulphate:

Na2SO4 + BaCl2 → BaSO4 + 2NaCl

Sulphide (S2−): addition of dilute hydrochloric acid to a sulphide results in the production of colourless hydrogen sulphide gas, which smells of rotten eggs and turns lead nitrate (soaked into filter paper) black.

Na2S + 2HCl → 2NaCl + H2S

Sulphite (SO32−): addition of dilute hydrochloric acid to a sulphite, with heating, produces colourless sulphur dioxide gas. This turns potassium dichromate from orange to green, but does not change the colour of lead nitrate solution.

K2SO3 + 2HCl → 2KCl + SO2 + H2O


Molecular and Ionic Compounds

Periodic table: reactivity in metals and non-metal groups

pH scale

Titration method


electrolysis of aluminium oxide

© RM, 2018. All rights reserved.

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