4.8.3 identification of ions by chemical means

what does a lithium compound result in?

a crimson flame

what does a sodium compound result in?

a yellow flame

what does a potassium compound result in?

a lilac flame

what does a calcium compound result in?

an orange-red flame

what does a copper compound result in?

a green flame

what happens when metal hydroxides are formed in a reaction with sodium hydroxide?

precipitates are formed, which can identify metal ions

what colour precipitate does aluminium hydroxide form?

a white precipitate that dissolves in excess sodium hydroxide

what colour precipitate does calcium hydroxide form?

a white precipitate that doesn’t dissolve in excess sodium hydroxide

what colour precipitate does magnesium hydroxide form?

a white precipitate that doesn’t dissolve in excess sodium hydroxide

what colour precipitate does iron (II) hydroxide form?

a green precipitate

what happens to iron (II) hydroxide when left standing in air?

it turns brown as it oxidizes to form iron hydroxide

what colour precipitate does iron (III) hydroxide form?

a brown precipitate

what colour precipitate does copper (II) hydroxide form?

a blue precipitate

how can you distinguish between aluminium and calcium ions using sodium hydroxide?

aluminium hydroxide dissolves in excess sodium hydroxide but calcium hydroxide doesn’t

how do you test for the presence of carbonate ions?

add dilute acid (e.g. hydrochloric acid) to a substance

what will happen if carbonate ions are present?

carbon dioxide gas will be produced

how can you confirm that carbon dioxide in produced in the carbonate water?

bubble the gas through limewater and if it turns cloudy, carbon dioxide is present

how do you test for halide ions (Cl-, Br-, I-)?

add dilute nitric acid to remove impurities, followed by silver nitrate solution

what is the observation if chloride ions are present in the halide test?

a white precipitate of silver chloride forms

what is the observation if bromide ions are present in the halide test?

a cream precipitate of silver bromide forms

what is the observation if iodide ions are present in the halide test?

a yellow precipitate of silver iodide forms

why is nitric acid added before silver nitrate in the halide test?

to remove any carbonate or sulfite ions that might interfere with the test

how do you test for sulfate ions?

add dilute hydrochloric acid to remove impurities, followed by barium chloride solution

what is the observation if sulfate ions are present?

a white precipitate of barium sulfate forms

why is hydrochloric acid added before barium chloride in the sulfate test?

to remove any carbonate or sulfite ions that could form a white precipitate and give a false positive result

what are instrumental methods of analysis?

techniques that use instruments to analyze the composition of substances

what are two advantages of instrumental methods over chemical tests?

they’re more accurate, sensitive and rapid

give an example of an instrumental method of analysis

flame emission spectroscopy

what is flame emission spectroscopy used for?

to identify metal ions and determine their concentrations in a solution

how does flame emission spectroscopy work?

the sample is heated in a flame and the light emitted by the metal ions is analyzed using a spectroscope. each metal produces a unique line spectrum

why is flame emission spectroscopy better than a flame test?

it can identify multiple ions in a mixture and provide quantitative results