Colour Changes in Chemistry

Colour changes are the observations that earn marks in 0620 Papers 4 and 6. This reference collects every colour change you need, organised by context.

Flame test colours

Dip a cleaned nichrome wire into concentrated HCl, then into the solid, and hold in a Bunsen flame. See flame tests.

Ion	Flame colour
Li+	Red (crimson)
Na+	Yellow (persistent)
K+	Lilac (use cobalt blue glass to see through Na yellow)
Ca2+	Orange-red
Cu2+	Blue-green

Precipitate colours (NaOH test)

Add sodium hydroxide solution to the test solution containing the cation.

Cation	Precipitate colour	Excess NaOH
Cu2+	Blue	Insoluble
Fe2+	Green	Insoluble
Fe3+	Orange-brown (rust)	Insoluble
Al3+	White	Dissolves (colourless solution)
Zn2+	White	Dissolves (colourless solution)
Mg2+	White	Insoluble
Ca2+	White (slight cloudiness)	Insoluble

Distinguishing white precipitates: Al3+ and Zn2+ dissolve in excess NaOH; Mg2+ does not.

Precipitate colours (silver nitrate test for halides)

Add dilute HNO3 then AgNO3(aq). See silver nitrate.

Halide	Precipitate	Colour
Cl-	AgCl	White
Br-	AgBr	Cream
I-	AgI	Yellow

Precipitate colours (barium nitrate test for sulfate)

Add dilute nitric acid then Ba(NO3)2(aq):

SO4^2- gives white precipitate (BaSO4).

Indicator colour changes

Indicator	Acid	Neutral	Alkali
Litmus	Red	Purple	Blue
Methyl orange	Red	Orange	Yellow
Phenolphthalein	Colourless	Colourless	Pink
Universal indicator	Red/orange	Green	Blue/purple

Gas test observations

Gas	Test	Positive result
Hydrogen (H2)	Burning splint	Squeaky pop
Oxygen (O2)	Glowing splint	Relights
Carbon dioxide (CO2)	Limewater	Turns milky/cloudy
Chlorine (Cl2)	Damp litmus paper	Turns red then white (bleached)
Ammonia (NH3)	Damp red litmus paper	Turns blue (alkaline gas)

Compound colour changes

Copper(II) sulfate

Form	Colour
Hydrated CuSO4.5H2O	Blue crystals
Anhydrous CuSO4	White powder

White to blue = water is present (test for water).

Copper(II) carbonate -> Copper(II) oxide

CuCO3 -> CuO + CO2: green to black

Copper(II) oxide + acid

CuO + 2HCl -> CuCl2 + H2O: black to blue solution

Copper(II) oxide + hydrogen

CuO + H2 -> Cu + H2O: black to pink-brown (copper)

Zinc oxide

ZnO is white cold, yellow when hot, white again on cooling.

Lead(II) iodide precipitation

Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s): bright yellow precipitate

Displacement reaction colours

Reaction	Colour change
Fe in CuSO4(aq)	Blue to pale green; brown copper deposited
Mg in CuSO4(aq)	Blue to colourless; brown copper deposited
Cl2 in KBr(aq)	Colourless to orange-brown (Br2 released)
Cl2 in KI(aq)	Colourless to brown (I2 released)

Oxidation of alcohols

Acidified potassium dichromate(VI) + ethanol: orange to green (Cr2O7^2- reduced to Cr3+).

Thermal decomposition colours

Compound	Before heating	After heating
CuCO3	Green	Black (CuO)
ZnCO3	White	Yellow hot, white cold (ZnO)
Cu(NO3)2	Blue	Black (CuO) + brown fumes (NO2)
Pb(NO3)2	White	Yellow (PbO) + brown fumes (NO2)

A solution is tested with sodium hydroxide. A green precipitate forms that does not dissolve in excess. Identify the cation and write the ionic equation. [3 marks]

• The cation is Fe2+ (iron(II)) [1]
• Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s) [1]
• The green precipitate is iron(II) hydroxide [1]

A student heats copper(II) nitrate crystals. Describe two observations. [2 marks]

• The blue crystals turn black (copper(II) oxide formed) [1]
• Brown fumes are produced (nitrogen dioxide, NO2) [1]