Qualitative Analysis

Quantitative Analysis: Example is titration.
• Qualitative Analysis: Relies on observations.
• Test for Anions: Carbonate, sulphate and halide.
• Carbonate and Acids: Carbonates react with acids to form carbon dioxide.
Na2CO3 (aq) + 2HNO3 (aq) -> 2NaNO3 (aq) + CO2 (g) + H2O (l). Sodium carbonate + nitric acid.
• Carbonate Test: Test for carbonate ion, CO3
2-
.
– Add dilute nitric acid.
– Bubbles/ effervescence mean carbonate.
• Carbon Dioxide Test: To prove gas is carbon dioxide, bubble gas through lime water (aqueous
solution of calcium hydroxide). Carbon dioxide reacts to form white precipitate of calcium
carbonate, which turns lime water cloudy.
CO2 (g) + Ca(OH)2 (aq) -> CaCO3 (s) + H2O (l).

• Sulphate Test: Most sulphates soluble, but barium sulphate insoluble.
– Aqueous barium ions added (ionic compound with barium added).
– Forms white precipitate of barium sulphate if sulphate ions present.
– Ba2+ (aq) + SO4
2-
(aq) -> BaSO4 (s).
• Halide Test: Most halides soluble, but silver halides insoluble. This is a precipitation reaction.
– Add aqueous silver nitrate, AgNO3, to an aqueous solution of halide.
– White precipitate is silver chloride, cream is silver bromide and yellow is silver iodide.
– Ag+
(aq) + X-
(aq) -> AgX (s). Aqueous silver ions react with aqueous halide ions.
– Add aqueous ammonia as colours can be difficult to tell apart. Clis soluble in dilute NH3. Bris soluble in concentrated NH3. Iodide is insoluble in concentrated NH3. Soluble = precipitate
completely dissolves.
• If asks for chemical test to show contains chloride ions and confirm no other halide present, do
this above and not the tests to show reactivity.
• Sequence of Tests: If asked to analyse unknown inorganic compound, need to carry out in this
order- carbonate CO3
2-
, SO4
2- and halides Cl-
, Br-
, I-
.
– Carbonate first as neither sulphate nor halide produce bubbles with dilute acids.
– Sulphate second as if you carry out sulphate test on carbonate will get white precipitate too
as barium carbonate insoluble.
– Halide test last, as silver carbonate Ag2CO3 and silver sulphate Ag2SO4 both insoluble and form
precipitates.
• Solutions of Mixture of Ions:
– If using same solution continue adding an excess acid until bubbling stops in carbonate and
sulphate test so all of no carbonate/ sulphate ions left to react in next tests.
– If intend to test sulphate or halide ions, use dilute nitric acid HNO3. Sulfuric acid contains
sulphate ions and hydrochloric acid contains chloride ions, which show up in tests.
– If intend to test for halide ions, don’t use BaCl2 as chloride ion will show up in halide test.
• Test for Ammonium Ion: A cation. Aqueous sodium hydroxide NaOH added to solution of
ammonium ion. When heated, aqueous ammonium ions and aqueous hydroxide ions react to
form ammonia gas. NH4
+
(aq) + OH-
(aq) -> NH3 (g) + H2O (l).
• No gas bubbles and ammonia is soluble but may be able to smell ammonia. Can test ammonia
with moist pH indicator paper as ammonia is alkaline and turns paper blue.