Tests for ions

It is important to be able to test for certain chemical groups in a compound. One of those important groups are ions.

Ions are charged particles Cations are positive (in the words of JD paws-ative) anions are negative (A N-egative- Ion).

Chemical tests are simply reactions whose products give visual clues as to the reactants. for example if universal indicator is turned red it means there is an H+ ion in the substance being tested or if a gas turns limewater milky then a calcium carbonate precipitate has been produced we use this reaction to show that the gas was carbon dioxide.

the tests that you should know are…

Tests for cations

Hydrogen ion H+*

add universal indicator
indicator turns red if hydrogen is present

or

add calcium carbonate
carbon dioxide is given off if hydrogen is present

or

any other typical reaction of an acid
H+ is effectively an acid and so you can expect it to produce Hydrogen in a reaction with a metal, a salt with an alkali, carbon dioxide with a carbonate

For Hydrogen gas see the common tests page

Copper (commonly Cu2+)*

Add drop of ammonium hydroxide
light blue ppt forms

Excess ammonium hydroxide
deep blue solution forms (this takes a large quantity of water to dilute down)

Iron (II)*

Add aqueous sodium hydroxide
Green ppt forms

Iron (III)*

Add drop aqueous sodium hydroxide
Brown ppt forms

Zinc (II)*

Add aqueous sodium hydroxide
White ppt forms

Excess sodium hydroxide
White ppt re-dissolve

or

Add drop of ammonium hydroxide
White ppt forms

Excess ammonium hydroxide
White ppt re-dissolve

Ammonium (NH4+)*

Add 5cm3 of sodium hydroxide and warm
Ammonia detected

Aluminium (commonly 3+)

Add aqueous sodium hydroxide
White ppt forms

Excess sodium hydroxide
White ppt re-dissolve

Calcium (Ca2+)

Add aqueous sodium hydroxide
Fine white ppt forms

Excess sodium hydroxide
White ppt does not re-dissolve

Tests for anions

Hydroxide OH–*

Add universal indicator
Indicator turns blue

or

Add ammonium compound and warm
Ammonia detected

Carbonate CO32-*

Add a little dilute acid
Carbon dioxide given off (see common molecules page)

Sulfate SO42-*

Add dilute hydrochloric acid then a few drops of aqueous barium chloride
White precipitate formed

Sulfite SO32-

Add a little dilute hydrochloric acid, warm
Sulfur dioxide detected (see the common molecules tests page)

Nitrate*

Add sodium hydroxide and add aluminium foil
Warm Carefully
Ammonia detected

Chloride Cl–*

Add dilute nitric acid then a few drops of silver nitrate
White precipitate forms
Will dissolve in dilute ammonia

Bromide Br–

Add dilute nitric acid then a few drops of silver nitrate
Cream precipitate forms
Will only dissolve in conc ammonia

Iodide I–

Add dilute nitric acid then a few drops of silver nitrate
Yellow precipitate forms
Will not dissolve in ammonia

* denotes that these are CIE GCSE tests for ions