C3.4 Further analysis and quantitative chemistry
Page 1. A range of chemical tests can be used for the detection and identification of elements and compounds.
Page 2. Titrations can be used to find the amounts of acid or alkali in a solution.
■ Li+ crimson flame
■ Na+ yellow flame
■ K+ lilac flame
■ Ca2+ red flame
■ Ba2+ green flame
Add NaOH produces a coloured precipitate:
Cu2+ blue precipitate
Fe2+ green precipitate (quickly oxidises by oxygen in air to Fe3+ brown)
Fe3+ brown precipitate
Add NaOH produces no precipitate and ammonia gas: indicates ammonium ions NH4+
Ammonia gas NH3 is colourless and has a characteristic pungent odour.Tests to confirm the presence of ammonia gas include:
1. Damp red litmus paper turns blue (this test will be positive for any alkaline gas)
2. Forms misty white fumes (due to formation of ammonium chloride NH4Cl vapour) in contact with the vapour of concentrated hydrochloric acid (this test is highly specific for ammonia gas).
NH4+(aq) + OH−(aq) ⇌ NH3(g) +H2O(l)
NH3(g) + HCl(g) ⇌ NH4Cl (g)
Add NaOH produces a white precipitate:
Al3+, Ca2+and Mg2+ ions ALL form white precipitates with NaOH.
However, only the white precipitate formed by Aluminium/NaOH dissolves in excess NaOH solution.
Furthermore, of the three ions (Al3+, Ca2+and Mg2+ ), only Ca2+ will produce a coloured (brick-red) flame test.
Add dilute hydrochloric acid HCl:
1. Carbonate ions CO32− Carbonates react with dilute acids to form carbon dioxide CO2.
Carbon dioxide produces a white precipitate with limewater. This turns limewater cloudy.
2. Sulfate ions SO42− in solution produce a white precipitate with *barium chloride BaCl2 solution in the presence of dilute hydrochloric acid.
Add dilute nitric acid HNO3, then add silver nitrate AgNO3 solution:
Halide ions in solution produce precipitates with *silver nitrate AgNO3 solution in the presence of dilute nitric acid.
silver chloride AgCl is white
silver bromide AgBr is cream
silver iodide AgI is yellow
*IMPORTANT: When using barium chloride or silver nitrate solutions as testing reagents, these tests are only valid if each of these solutions is acidified. Adding dilute acid PRIOR to adding any one of these reagents will remove any dissolved ions (like carbonates or sulfites) that would also give a ‘positive’ precipitate result.
The acid chosen to acidify the testing reagents corresponds to its negative ion, so:
dilute hydrochloric acid is used to acidify barium chloride solution
dilute nitric acid is used to acidify silver nitrate solution
Warm with NaOH, add Al powder and test for ammonia gas : a positive test for presence of nitrate ions NO3−
[Beyond most GCSE syllabi, but included for completeness] Most salts of nitrate ions NO3− are soluble, so a precipitate based test is not viable. The aluminium powder reduces the nitrate ion to the ammonium ion, and as the ammonium ion is now in contact with NaOH, ammonia gas is produced. The usual tests for ammonia gas (i.e. turns damp red litmus paper blue) may then be applied to confirm the original presence of nitrate ions.
NO3− ions → reduced by Al to NH4+ ions → react with OH− → form NH3 gas
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