2j Preparing and analysing
Tests for cations
• Li+, Na+, K+, Ca2+ using flame tests
• NH4+ using NaOH(aq)
• Cu2+, Fe2+ and Fe3+ using NaOH(aq)
Flame tests
Mr Porter’s flame tests
Flame tests
Ion
Li+
Na+
K+
Ca2+
Colour of flame
Scarlet
Yellow
Lilac
Brick red
Why? (You don’t need to know for IGCSE)
When an atom gets
excited the electrons
can jump to a higher
orbit.
Why?
When an atom gets
excited the electrons
can jump to a higher
orbit.
Why?
When the excited
electron falls back to a
lower level it emits a
little packet of light
energy called a photon
Why?
The colour of the light
depends on the
distance the electron
falls.
Colours in Fireworks
Testing for NH4+ (“ammonium ion”)
• Add NaOH (aq)
• Warm gently
• Test gas given off with damp red litmus
Testing for NH4+ (“ammonium ion”)
•
•
•
•
Add NaOH (aq)
Warm gently
Test gas given off with damp red litmus
Ammonia gas is given off which turns the
damp red litmus BLUE.
Testing for Cu2+, Fe2+ and Fe3+ using
NaOH(aq)
Testing for Cu2+, Fe2+ and Fe3+ using
NaOH(aq)
Metal
ion
Cu2+
Fe2+
Fe3+
Result of adding NaOH (aq)
Testing for Cu2+, Fe2+ and Fe3+ using
NaOH(aq)
Metal
ion
Cu2+
Fe2+
Fe3+
Result of adding NaOH (aq)
Pale blue precipitate of Cu(OH)2
Dirty green precipitate of Fe(OH)2
Rusty brown precipitate of Fe(OH)3
Common hydroxides (EXCEPT Na, K
and NH4) are INSOLUBLE so form
precipitates