Qualitative Analysis (OCR A Level Chemistry)

Revision Note

Test yourself
Sonny

Author

Sonny

Last updated

Halide Ions

  • Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise
  • The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions
  • The halide ions will react with the silver nitrate solution as follows:

Ag+ (aq) + X- (aq) → AgX (s)

(ionic equation)

    • Where X- is the halide ion
  • The state symbols are key in this equation
  • If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)Testing for Halides, downloadable AS & A Level Chemistry revision notes
A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution

  • Silver chloride (AgCl) is a white precipitate
  • Silver bromide (AgBr) is a cream precipitate
  • Silver iodide (AgI) is a yellow precipitateTesting for halide ions results, downloadable AS & A Level Chemistry revision notes
The silver halide precipitates are dense and characteristically coloured

Adding ammonia

  • Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present
  • Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
  • If the precipitate dissolves in dilute ammonia the unknown halide is chloride
  • If the precipitate does not dissolve in dilute, but does dissolve in concentrated ammonia the unknown halide is bromide
  • If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodideAgI precipitate, downloadable AS & A Level Chemistry revision notes
Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide is insoluble in ammonia

Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions
Group 17 - Table 1_Reactions of Halide Ions, downloadable AS & A Level Chemistry revision notes

Qualitative Analysis of Ions

Test for Carbonates

  • A small amount (around 1 cm3) of dilute hydrochloric acid should be added to a test tube using a pipette
  • An equal amount of sodium carbonate solution should then be added to the test tube using a clean pipette
  • As soon as the sodium carbonate solution is added, a bung with a delivery tube should be attached to the test tube
    • The delivery tube should transfer the gas which is formed into a different test tube that contains a small amount of limewater (calcium hydroxide solution)
  • If a carbonate compound is present then effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater:

CO32- (aq) + 2H+ (aq) → CO2 (g) + H2O (l)
CO2 (g) + Ca(OH)2 (aq) → CaCO(s) + H2O (l)

Test for Carbon Dioxide, IGCSE & GCSE Chemistry revision notes
Limewater turns milky in the presence of CO2 caused by formation of insoluble calcium carbonate

Test for Sulfates

  • Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride
  • If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

  • The test can also be carried out with barium nitrate solutionSulfate Ion Test, IGCSE & GCSE Chemistry revision notes
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Testing for ammonium ions

  • We can test for the presence of ammonium ions, NH4+, by reacting with warm aqueous sodium hydroxide forming ammonia gas

NH4(aq) + OH(aq) → NH(g) + H2O (l)

  • Ammonia gas is produced. You are unlikely to see gas bubbles as ammonia is very soluble in water
  • Ammonia gas can be identified by its pungent smell or by turning red litmus paper blue

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.