Tests for ions and hydrogen

Explain the test for halide ions

Halide ions in solutions are detected using silver nitrate solutions. The test solution is acidified using a few drops of dilute nitric acid, and then a few drops of silver nitrate solution are added. Different coloured silver halide precipitates form, depending on the halide ions present:

Explain the test for metal ions

It is possible to use a flame test to detect the presence of an alkali metal ion. You just place the substance in a flame and see what colour the flame goes.

Explain the test for hydrogen

If you hold a lit splint at the open end of a tube containing hydrogen you'll hear a "squeaky pop" which is created by the hydrogen burning quickly with the oxygen in the air.

What do the halide ions give off

Chloride ions = white precipitate of silver chloride
Bromide ions = cream precipitate of silver bromide
Iodide ions = yellow precipitate of silver iodide

What colour flame does metal ions give off?

Lithium = red flame
Sodium = yellow-orange flame
Potassium = lilac flame
Calcium = brick red flame
Barium = green flame

Instructions to carry out the flame test

1. Collect a damp splint into the solid sample being tested

2. pick up a few crystals of one of the compounds and put it into a blue flame of the bunsen burner

3. Record colour of the flame ignited using fresh splints

Instructions to carry out the silver nitrate test

1. Place 2cm depth of potassium chloride solution into a test tube

2. Add 2 - 3 drops of silver nitrate

3. record the colour of precipitate produced

4. carry out the experiment with potassium bromide and potassium iodin

Half equations for the silver nitrate test

Chloride = Ag+ (aq) + Cl- (aq) → AgCl(s)

Bromide = Ag+ (aq) + Br- (aq) → AgBr(s)

Iodide = Ag+ (aq) + I- (aq) → AgI(s)