Effect of Heat on Salt Mr.

Cheng

Gas Test Ammonia gas, NH3

Carbon dioxide gas, CO2

Properties: Colourless gas with pungent smell

Properties: Colourless gas
Gas test 1: Place a moist red litmus paper near the
Gas test: Flow the gas through limewater using
mouth of test tube
delivery tube
Observation: Colour of red litmus paper turns to
Observation: Limewater turns cloudy
blue (NH3 gas is alkaline)
Oxygen gas, O2 (OR)
Gas Test 2: Dip a glass rod with concentrated
hydrochloric acid and place it near the mouth of
Properties: Colourless gas test tube
Gas test: Lower a glowing wooden splinter into the Observation: White fumes are produced
test tube (ammonium chloride NH4Cl is produced)
Observation: Glowing wooden splinter is rekindled Hydrogen chloride gas, HCl
Hydrogen gas, H2

Properties: Colourless gas Properties: Colourless gas with pungent smell

Gas test: Place a lighted wooden splinter near the Gas test: Dip a glass rod with concentrated

mouth of test tube ammonia solution and place it near the mouth of

Observation: ‘pop’ sound is produced test tube

Observation: White fumes are produced (NH4Cl)
Nitrogen dioxide gas, NO2
Chlorine gas, Cl2

Properties: Brown gas with pungent smell

Properties: Greenish-yellow gas with pungent smell
Gas test: Place a moist blue litmus paper near the
Gas test: Place a moist blue litmus paper near the
mouth of test tube
mouth of test tube
Observation: Colour of blue litmus paper turns to
Observation: Colour of blue litmus paper turns to
red (NO2 gas is acidic)
red, then decolourised (bleaching effect)
Effect of Heat on Salt Mr. Cheng

Effect of Heat on Salts 3. Activity: effect of heat on carbonate salts.

1. When heated, most salts decompose and release

gas.
Salt (solid) → residue (solid) + gas
Colour Colour of Residue
 Type of gas released tells us what ion is Carbonate of Salt Effect on
When When
present in the salt Salt Before Limewater
Hot Cold
Heating
 Colour change of solid also tells us what Copper(II)
carbonate,
cation is present in the salt
CuCO3
2. Memorise the colour of the following metal Zinc
carbonate,
oxides: ZnCO3
Metal Oxide When hot… When cold… Lead(II)
carbonate,
Copper(II) oxide, CuO Black Black
PbCO3
Zinc oxide, ZnO Yellow White Calcium
Lead(II) oxide, PbO Brown Yellow carbonate,
Iron(III) oxide, Fe2O3 Brown Brown CaCO3
Sodium
carbonate,
Na2CO3
A) Heating of Carbonate Salts, CO32- Magnesium
carbonate,
1. When heated, most carbonate salts decompose to MgCO3

produce metal oxide and carbon dioxide.

ZnCO3 (s) → ZnO (s) + CO2 (g)  For the heating of copper(II) carbonate, name

2. Heating of different carbonate salts: the residue formed.

Carbonate
Cation Effect of Heat
Salt  It is observed that limewater turns milky.
+
K K2CO3 What inference can be made?
Does not decompose
Na+ Na2CO3
Ca2+ CaCO3 Decompose to produce
Mg2+ MgCO3 metal oxide and CO2  Heating of which metal carbonate does not
Al3+ Al2(CO3)3 turn limewater milky. Explain why.
CuCO3  CuO + CO2
Zn2+ ZnCO3
Fe3+ Fe2(CO3)3 ZnCO3  ZnO + CO2
Pb2+ PbCO3
Cu2+ CuCO3 PbCO3  PbO + CO2
Decompose to produce Ag,
Ag+ Ag2CO3 CO2 and O2
2Ag2CO3 4Ag + 2CO2 + O2
Decompose to produce
NH4+ (NH4)2CO3 ammonia gas, CO2 and H2O
(NH4)2CO3 2NH3 +CO2 + H2O
Effect of Heat on Salt Mr. Cheng

B) Heating of Nitrate Salts, NO3- 2. Heating of different nitrate salts:

Nitrate
1. When heated, most nitrate salts decompose to Cation Effect of Heat
Salt
produce metal oxide, nitrogen dioxide and K+ KNO3 Decompose to produce
metal nitrite and oxygen
oxygen. Na+ NaNO3
2NaNO3  2NaNO2 + O2
2Pb(NO3)2 (s) → 2PbO (s) + 4NO2 (g) + O2 (g) Ca2+ Ca(NO3)2 Decompose to produce
Mg2+ Mg(NO3)2 metal oxide, nitrogen
Al3+ Al(NO3)3 dioxide and oxygen
Zn2+ Zn(NO3)2
Fe3+ Fe(NO3)3 2Zn(NO3)22ZnO+4NO2+O2
Pb2+ Pb(NO3)2
2Pb(NO3)22PbO+4NO2+O2
 Nitrogen dioxide NO2 is a brown gas with Cu2+ Cu(NO3)2
Decompose into metal,
pungent smell. It can turn damp blue litmus +
Ag AgNO3 nitrogen dioxide, oxygen
paper to red 2AgNO3  2Ag + 2NO2 + O2
 Oxygen O2 is a colourless gas. It can rekindle  Can you state the name of all the nitrate salts

glowing wooden splinter above?

 Which nitrate salt releases brown gas? Which
does not?
3. Activity: Effect of heat on nitrate salts:

Colour of Residue Test on Gas

Colour of Salt
Glowing
Nitrate Salt Before Damp Blue
When Hot When Cold Colour of Gas Wooden
Heating Litmus Paper
Splinter
Copper(II) nitrate,
Cu(NO3)2
Zinc nitrate,
Zn(NO3)2
Lead(II) nitrate,
Pb(NO3)2
Magnesium
nitrate, Mg(NO3)2
Potassium nitrate,
KNO3
Calcium nitrate,
Ca(NO3)2
Example:
1. When heated, salt X decompose to release brown coloured gas. The gas turns moist blue litmus paper to red.
The residue formed is brown when hot, but yellow when it cools.
Identify salt X.
2. When salt Y is heated, it decompose to release a colourless gas that turns limewater cloudy. The colour of
residue formed is yellow when hot and white when it is cooled.
Identify salt Y.