A3.3 - Qualitative analysis

relies on simple observations and can often be carried out quickly on a test-tube scale

• gas bubbles • precipitates • colour changes • identification of gases

• add sodium hydroxide to the sample • warm the solution • place damp red litmus paper near the mouth of the test tube • if ammonium ions in the test tube, goes blue

ammonia –> very volatile, so ammonia gas is given off –> alkaline and smelly

Carbonate –> sulphate –> halide

• add dilute nitric acid to sample • if you see bubble the compounds could be a carbonate • bubble gas produced through limewater • if carbonate ions present, limewater turns cloudy

almost all carbonates are insoluble –> if you add Ba2+/Ag+ ions to substances with carbonate ions, you will get a white precipitate and make the wrong identification

• add dilute nitric acid to the sample to remove carbonate ions • add barium chloride OR barium nitrate • if sulphate ions present, will see white precipitate

as Ag2SO4 is insoluble so if there are sulphate ions present and you add silver nitrate –> make wrong identification

• add dilute nitric acid to the sample to remove carbonate ions • add silver nitrate • if halide ions are present, then you will see a precipitate

white = chloride ion cream = bromine ion yellow = iodine ion (milk, cream, cheese)

must come last, as you will get false results if you have not identified carbonate/sulphate ions first

soluble in concentrated NH3

only soluble in concentrated NH3

not soluble in dilute or concentrated NH3