How to Identify the Ions in Aluminium Chloride Salt

Aluminium chloride is white crystal solid. It is very soluble in water and it hydrolyses in water to form acidic solution. The ease with which the anhydrous salt hydrolyses explains why hydrogen chloride fumes are evolved from it in damp air. Aluminium chloride is a covalent compound which reacts with water. The two ions in the compound are aluminium ion and Chloride ion.

How to identify the Aluminium ion (cation)

1.    Test with sodium hydroxide: add a few drops sodium hydroxide solution to the solution the unknown salt. The formation of a white gelatinous precipitate which dissolves in excess sodium hydroxide solution shows that aluminium ion is present.

Note: lead ions and zinc ions give similar reactions so this may not be the final test.

2.    Test with aqueous ammonia: add a few drops of aqueous ammonia to the unknown salt solution; the formation of a white gelatinous precipitate which is insoluble in excess aqueous ammonia confirms the presence aluminium ion. Zinc ion and lead ion give similar reactions. But lead ions give white precipitate with hydrochloric acid while aluminium does not. Zinc ions give white precipitate with aqueous ammonia but the precipitate is soluble in excess aqueous ammonia.

How to identify chloride ion (anion)

Add few drops of silver trioxonitrate V solution into the solution of the unknown salt, and then add dilute trioxonitrate V acid. White precipitate is formed which insoluble in dilute trioxonitrate V acid and which is darken on exposure to sunlight confirms the presence of chloride ion.

Example question

C is a sample of inorganic compound (AlCl₃). Carry out the test as stated below and complete the table.

Test	Observation	Inference
a(I). C + distilled water and divide into  three portions	Soluble and gives a colourless solution
(ii) to the first portion from a(i), add dilute NaOH solution in drops and then excess		Pb2+,Zn2+, Al3+ likely present
(iii) to the second portion, add dilute HCl	No visible reaction
(iv)to the mixture from a(iii), add aqueous ammonia in drops and then excess.		Al3+ present
(b) to the third portion from a(i), add AgNO3 solution and dilute HNO3 acid.	White precipitate insoluble in dilute HNO3 acid

 

  

Answer

 

Test	Observation	Inference
a(I). C + distilled water and divide into  three portions	Soluble and gives a colourless solution	Coloured transition metal ions likely absent and soluble ions like K+, Na+ , NH4+ maybe present
(ii) to the first portion from a(i), add dilute NaOH solution in drops and then excess	White precipitate form, soluble in excess NaOH	Pb2+,Zn2+, Al3+ likely present
(iii) to the second portion, add dilute HCl	No visible reaction	Zn2+, Al3+ likely present
(iv)to the mixture from a(iii), add aqueous ammonia in drops and then excess.	White precipitate, insoluble in excess aqueous ammonia	Al3+ present
(b) to the third portion from a(i), add AgNO3 solution and dilute HNO3 acid.	White precipitate insoluble in dilute HNO3 acid	Cl-  confirm

 

Explaining the inferences using the observations

      In test a(i), the unknown salt  dissolved in water to give a colourless solution. This means that coloured transition metal ions are not likely to be present because most transition ions are known to give out colour due to the d level reaction with water or ammonia solution but soluble ions like K⁺, Na⁺, NH₄⁺ likely present because these ions are known to give colourless solution with water in all cases.

  In test a(ii), white precipitate is formed, when sodium hydroxide solution was added and the precipitate dissolved in excess sodium hydroxide solution. Note that Zinc, lead and aluminium ions are the only ions which can show this reaction with sodium hydroxide. This automatically shows that sodium, potassium, and ammonium ions are absent.

 In test a(iii), when dilute hydrochloric acid was added to the unknown solution, there was no visible reaction. This means that lead ions are absent because lead ions form white precipitate with hydrochloric acid but zinc and aluminium ions are soluble in hydrochloric acid.

 In test a(iv), the unknown solution formed white precipitate with aqueous ammonia and the precipitate is insoluble in excess aqueous ammonia. There are two ions which can give insoluble white precipitate with aqueous ammonia which are lead and aluminium ions but the previous test (test a(iii)) has shown that lead is absent in the solution, so this test confirm that aluminium ion is present.

 Test b, in addition of silver trioxonitrate v, white precipitate is formed. This is as a result of silver ions reacting with chloride ions to give white insoluble silver chloride which remains insoluble in dilute trioxonitrate v acid. This confirm Chloride ion because silver can form white precipitate with only trioxocarbonate IV and chloride however, trioxocarbonates of silver are soluble in acid but chlorides of silver do not dissolve in acid