AS Edexcel Observations

Possible change                                                  Example of observation

Colour change in solution            -                    yellow solution turns orange
Formation of precipitate               -                  white precipitate/ppte if formed
Precipitate dissolves in                -               precipitate dissolves in excess to form
   excess reagent                                                         a green solution
A gas is evolved                          -                 bubbles of gas or effervescence
Tests on a gas                             -              the gas turned red litmus paper blue
Flame tests                                  -                               yellow flame
A solid dissolves                          -                   dissolves to give a blue solution
A reaction is exothermic              -                          mixture becomes hot
A reaction is endothermic            -                            mixture feels cold
A coloured solution loses colour  -             the yellow solution turns colourless

Flame colour                                              Inference

Yellow                                                     Sodium ion

Lilac                                                      Potassium ion

Yellow-red*                                             Calcium ion

Red*                                                Lithium or strontium ion

Pale green                                              Barium ion

* Further tests need to be carried out in order to distinguish these ions

Gas                                   Observations

Oxygen                             colourless gas, relights a glowing splint
Carbon dioxide                 colourless gas, white ppte with lime water
Ammonia                          colourless gas, moist red litmus paper -> blue,
                                         white smoke with HCl gas
Nitrogen dioxide               brown gas*
Hydrogen                         colourless gas, ignites with a 'pop'
Hydrogen chloride            steamy fumes on exposure to moist air, acidic,
                                         white smoke with ammonia
Chlorine                            pale green gas, bleaches moist litmus paper
Bromine                            brown gas*
Iodine                               purple vapour
Water vapour                    turns blue cobalt chloride paper pink

* Bromine dissolves in organic solvents to form brown solution, nitrogen dioxide is insoluble.

Reducing agents usually:

decolourise aqueous acidified potassium manganate(VII)

may turn aqueous, acidified potassium dichromate (VI) from orange to green.

Reducing agents include:

Iron(II) ions

Iodide ions

Hydrogen peroxide

Oxidising agents usually:

liberate iodine as a brown solution or black solid from aqueous potassium iodide

Oxidising agents include:

Acidified manganate(VII) ions

Acidified dichromate(VI) ions

Hydrogen peroxide (can be both)

Copper(II) ions

Aqueous chlorine/bromine

Observation                                            Likely ions
on adding excess
dilute NaOH

Insoluble, white ppte             Magnesium, barium, strontium, calcium

No precipitate                                     Sodium, potassium

No precipitate but ammonia                     Ammonium
evolved on warming

Anion                       Precipitate                       Addition of NH3(aq)

Chloride                  white, AgCl                              soluble

Bromide                 cream, AgBr               soluble in excess/concentrated

Iodide                     yellow, AgI                              insoluble

Anion                   Precipitate                 Addition of dilute HCl

Sulphate            white, BaSO4             insoluble precipitate

Sulphite             white, BaSO3              soluble precipitate

Carbonate         white, BaCO3     precipitate dissolves with effervescence