AS Edexcel Observations

Some observations for different tests and reactions - useful to remember for theory as well as practical exams.

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  • Created by: Alice
  • Created on: 18-01-12 19:40

Recording 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

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Flame tests

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

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Identification of common gases

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.

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Tests for reducing agents

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

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Tests for oxidising agents

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

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Sodium hydroxide solution

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

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Silver nitrate solution

Anion                       Precipitate                       Addition of NH3(aq)

Chloride                  white, AgCl                              soluble

Bromide                 cream, AgBr               soluble in excess/concentrated

Iodide                     yellow, AgI                              insoluble

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Barium chloride solution

Anion                   Precipitate                 Addition of dilute HCl

Sulphate            white, BaSO4             insoluble precipitate

Sulphite             white, BaSO3              soluble precipitate

Carbonate         white, BaCO3     precipitate dissolves with effervescence

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