Effect of Heat on Salts

6.10

Effect of Heat on Salts

Definition: chemical method to determine the substances present in a compound through chemical tests.
Cations and anions can be identified through physical properties such as;
- Colour of salt
- Solubility of salt
- Gas test
- Effect of heat on the salt
- Confirmatory test for cations

Ion or salt	Colour in solid state	Colour in solution
Salts of \(Ca^{2+}\), \( Mg^{2+}\), \(Al^{3+}\), \(Pb^{2+}\), \(NH_4\,^{+}\)	White	Colourless
\(CuSO_4\), \(Cu(NO_3)_2\)	Blue
\(CuCl_2\)	Green	Blue
Salts of iron(II) ion, \(Fe^{2+}\)	Green
Salts of iron(III) ion, \(Fe^{3+}\)	Brown	Yellow/ brownish-yellow (depending on the concentration)

Gas test

The process of heating salt may dissociate the salt to release a certain type of gas.
The gas can be identified through several tests.

Gas	Confirmatory test
Oxygen gas	Lights up a glowing wooden splinter
Hydrogen gas	Produces a ‘pop’ sound with a lighted wooden splinter
Carbon dioxide gas	Turns limewater cloudy
Ammonia gas	Turns a moist red litmus paper blue Produces thick white fumes with hydrogen chloride gas
Chlorine gas	Bleaches a moist red litmus paper Turns a moist blue litmus paper red, then bleaches it
Hydrogen chloride gas	Produces thick white fumes with ammonia gas
Nitrogen dioxide gas	Turns a moist blue litmus paper red
Sulphur dioxide gas	Decolourises an acidified potassium manganate(VII) solution
Water vapour	Turns the white anhydrous copper(II) sulphate blue Turns the blue cobalt(II) chloride paper pink

Colour before heating	Colour after heating	Effect of heat
\(K_2CO_3\)		Does not decompose by heat
White	-
\(Na_2CO_3\)
White	-
\(CaCO_3\)		Formed metal oxide and carbon dioxide gas
White	White
\(MgCO_3\)
White	White
\(Al_2(CO_3)_3\)
White	White
\(ZnCO_3\)
White	Yellow when hot
	White when cool
\(FeCO_3\)
Green	Green
\(Fe_2(CO_3)_3\)
Brown	Brown
\(SnCO_3\)
White	White
\(PbCO_3\)
White	Brown when hot
	Yellow when cool
\(CuCO_3\)
Green
\(HgCO_3\)		Formed a metal, carbon dioxide gas and oxygen gas
White	Grey
\(Ag_2CO_3\)
White	Shiny grey
\(Au_2(CO_3)_3\)
White	Golden yellow

Colour before heating	Colour after heating	Effect of heat
\(KNO_3\)		Formed nitrite salt and oxygen gas
White	White
\(NaNO_3\)
White	White
\(Ca(NO_3)_2\)		Formed metal oxide, nitrogen dioxide gas and oxygen gas
White	White
\(Mg(NO_3)_2\)
White	White
\(Al(NO_3)_3\)
White	White
\(Zn(NO_3)_2\)
White	Yellow when hot
	White when cool
\(Fe(NO_3)_2\)
Green	Green
\(Fe(NO_3)_3\)
Brown	Brown
\(Sn(NO_3)_2\)
White	White
\(Pb(NO_3)_2\)
White	Brown when hot
	Yellow when cool
\(Cu(NO_3)_2\)
Blue
\(Hg(NO_3)_2\)		Formed metal, nitrogen dioxide gas and oxygen gas
White	Grey
\(AgNO_3\)
White	Shiny grey
\(Au(NO_3)_3\)
White	Golden yellow