Testing for Ions

flame test

• use nichrome wire and dip in HCl to clean in

• then dip in solid salt

• hold the wire to a bunsen burner non-luminous flame

nichrome wire

• alloy of nickel and chromium

• has a high melting point and doesn’t change the colour of the flame

lithium flame colour

crimson

sodium flame colour

yellow

potassium flame colour

lilac

calcium flame colour

brick-red

copper flame colour

blue-green

flame test experiment improvements

• contamination of wires: use a different wire

• easy to confuse similar colours: use flame photometry

flame emission spectroscopy

• more accurate

• faster

• more sensitive

• more expensive

test for cations

• add a few drops of NaOH to sample

• shake the tube to mix its contents

• observe colour of precipitate formed

aluminium precipitate colour

white (ppt dissolves when excess NaOH added)

calcium precipitate colour

white (ppt doesn’t dissolve when excess NaOH added)

copper (II) precipitate colour

blue

iron (II) precipitate colour

green

iron (III) precipitate colour

orange

test for ammonium ion

• add a few drops of NaOH

• heat and ammonia gas is produced

• damp red litmus paper turns blue as its an alkaline gas

test for halide ions

• add a few drops of dilute HNO₃ and shake

• add a few drops of AgNO₃

• observe colour of precipitate formed

chloride precipitate colour (AgCl)

white

bromide precipitate colour (AgBr)

cream

iodide precipitate colour (AgI)

yellow

test for sulphate ions

• add a few drops of dilute HCl and shake

• add a few drops of BaCl

• white precipitate should form

test for carbonate ions

• add a few drops of HNO₃/HCl

• effervescence should occur

why dilute acids are added to test solution first

to remove and react with any unwanted ions to stop them from giving false positive results