Edexcel A level Chemistry - Chemical Tests (In Progress)

Testing for Carbon Dioxide

- Method-

• Bubble the gas through limewater (saturated aqueous solution of calcium hydroxide)
• Observe the change occurring 

- Positive Result-

• White precipitate 
• The carbon dioxide reacts to form calcium carbonate which is insoluble in water and is the white precipitate

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Testing for Sulfate Ions 

-Method- 

• Add dilute nitric acid to the aqueous solution containing the sulfate ions 
• Add barium chloride or barium nitrate 
• Observe the change 

-Positive Result-

• A white precipitate will form
• The sulfate ions in the solution will react with the added barium ions to form an insoluble white precipitate of barium sulfate

Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

Flame Tests 

-Method-

• Wear safety glasses and a lab coat
• Add a few drops of concentrated HCl to the solid to dissolve the solid
• Dip a clean nichrome wire into the mixture
• Pass the wire through a lit bunsen burner in a fume cupboard
• Observe the colours produced

-Colours-

Li⁺- Red                                     Rb⁺- Red/purple                                     Ca²⁺ - Brick red

Na⁺- Yellow/orange                  Cs⁺ - Blue/Violet                                     Sr²⁺ - Crimson red 

K⁺- Lilac                                    Be²⁺ and Mg²⁺ - no colour                     Ba²⁺ - Apple green 

Testing for Ammonium Ions 

-Method-

• Add sodium hydroxide solution 
• Warm the mixture 
• Smell the gas produced

-Positive Result- 

• Damp red litmus paper will turn blue in the presence of the gas
• HCl gas reacts with ammonia to form white fumes of ammonium chloride

NH₄ (aq) + OH^- (aq) → NH₃ (g) + H₂O (l)

Testing for Halide Ions

-Method-

• Add dilute nitric acid to the halide solution
• Add a solution of silver nitrate 
• Observe the precipitate formed

-Results-

• Cl^- - White ppt, soluble in dilute and concentrated aqueous ammonia
• Br^- - Cream ppt, insoluble in dilute aqueous ammonia, soluble in concentrated aqueous ammonia
• I^- - Yellow ppt, insoluble in dilute and concentrated aqueous ammonia

Ag⁺(aq) + X^-(aq) → AgCl(s)

Testing for Alkenes 

-Method-

• Add bromine water to the organic compound 
• Shake the mixture 
• Observe the colour change 

-Positive Result-

• The mixture changes colour from orange to colourless
• The bromine has been decolourised by the C=C bond

Example:

C₂H₄ + Br₂ → C₂H₄Br₂

Testing for Aldehydes

-Method-

• Add one of the reagents below to oxidise the aldehyde to a carboxylic acid
• Observe the colour change 

-Reagents and Positive Results-

• Acidified potassium dichromate (IV) - Orange solution to green solution 
• Fehling's solution - Deep blue solution to red precipitate
• Benedict's solution - Deep blue solution to red precipitate
• Tollens' reagent - Colourless solution to silver mirror

Test for CH₃CO Group

-Method-

• Add a carbonyl compound to an alkaline solution of iodine 
• Warm the mixture and cool it 
• Observe whether a precipitate is formed 

-Positive Result-

• A yellow precipitate of insoluble CHI₃
• This indicates that the compound is ethanal or a methyl ketone

Testing for Carbonyl Compounds 

-Method-

• Add Brady's reagent (2,4-dinitrophenylhydrazine) to the compound 
• Observe the colour change 

-Positive Result-

• A brightly coloured orange solid will be produced
• This derivative can be analysed to identify the specific carbonyl compound

Colours of Complexes - Copper II

- [Cu(H₂O)₆]²⁺ - Pale blue solution

- [Cu(H₂O)₄(OH)₂] - Pale blue precipitate 

• Formed during the reaction with aqueous sodium hydroxide 
• Formed during the reaction with aqueous ammonia 

- [Cu(NH₃)₄(H₂O)₂]²⁺ - Deep blue precipitate 

• Formed during the reaction with excess ammonia 

- [CuCl₄]^2- - Yellow solution

• Formed during the reaction with concentrated HCl

Colours of Complexes - Iron II

- [Fe(H₂O)₆]²⁺ - Pale green solution

- [Fe(H₂O)₄(OH)₂] - Green precipitate

• Formed during the reaction with aqueous sodium hydroxide 
• Formed during the reaction with aqueous ammonia 

-  [Fe(H₂O)₃(OH)₃] - Brown precipitate

• Formed upon the standing of [Fe(H₂O)₄(OH)₂]
• Formed during the reaction with excess ammonia 

Colours of Complexes - Iron III

- [Fe(H₂O)6]³⁺ - Yellow/brown solution

- [Fe(H₂O)₃(OH)₃] - Brown precipitate

• Formed during the reaction with aqueous sodium hydroxide 
• Formed during the reaction with aqueous/excess ammonia 

Colours of Complexes - Cobalt II

- [Co(H₂O)6]²⁺ - Pink solution

- [Co(H₂O)₄(OH)₂] - Blue precipitate (pink precipitate on standing)

• Formed during the reaction with aqueous sodium hydroxide 
• Formed during the reaction with aqueous ammonia 

- [Co(NH₃)₆]²⁺ - Brown solution

• Formed during the addition of excess ammonia 

Colours of Complexes - Cobalt II Continued

- [Co(NH₃)₆]³⁺ - Yellow precipitate

• Formed upon the standing of [Co(NH₃)₆]²⁺

- [CoCl₄]^2- - Blue solution

• Formed during the reaction with concentrated HCl

Colours of Complexes - Chromium 

- [Cr(H₂O)6]³⁺ - Green or violet solution

- [Cr(H₂O)₃(OH)₃] - Green precipitate

• Formed during the reaction with aqueous sodium hydroxide 
• Formed during the reaction with aqueous ammonia 

- [Cr(H₂O)₂(OH)₄]^- - Green solution

• Formed during the reaction with excess aqueous sodium hydroxide 

- [Cr(OH)₆]^3- - Green solution

• Formed during the reaction with concentrated sodium hydroxide 

Colours of Complexes - Chromium Continued

- [Cr(NH₃)₆]³⁺ - Violet or purple solution

• Formed during the reaction with excess ammonia  

-  CrO₄^2- - Yellow solution, Cr₇O₄^2- - Orange solution

• Formed during the addition of H₂O₂/OH^- to [Cr(OH)₆]^3-, then the addition of H⁺

- [Cr(H₂O)6]³⁺ - Green or violet solution, [Cr(H₂O)6]²⁺ -  Blue solution

• Formed during the addition of Zn/H⁺ from CrO₄^2- and [Cr(H₂O)6]³⁺ respectively 

The Chemistry of Vanadium

V²⁺ (Vanadium (II)) - Purple 

V³⁺ (Vanadium (III)) - Green 

VO²⁺ (Oxovanadium (IV)) - Blue

VO₂⁺ (Dioxovanadium (V)) - Yellow