6.10Effect of Heat on Salts
Most salts undergo thermal decomposition when heated, breaking down into simpler substances such as metal oxides and gases. By observing the colour of the residue left behind when hot and cold, as well as identifying the specific gases released through chemical tests, both the cation and anion present in an unknown salt can be deduced.
Learning Objectives
- Describe briefly chemical tests used to identify common gases: oxygen (\(O_2\)), hydrogen (\(H_2\)), carbon dioxide (\(CO_2\)), ammonia (\(NH_3\)), chlorine (\(Cl_2\)), hydrogen chloride (\(HCl\)), sulphur dioxide (\(SO_2\)), and nitrogen dioxide (\(NO_2\)).
- Investigate the effects of heat on salts through experiments, particularly carbonate salts and nitrate salts.
- Identify the cation present in a salt based on the colour of the residue when hot and when cold.
- Identify the anion present in a salt based on the gases liberated during thermal decomposition.
Flowchart for Identifying Cations and Anions via Thermal Decomposition
A quick, intuitive summary linking residue colour changes to specific metal cations and liberated gases to their corresponding anions.
Residue Colour Box
Yellow → White = Zinc oxide, \(ZnO\) → \(Zn^{2+}\)
Brown → Yellow = Lead(II) oxide, \(PbO\) → \(Pb^{2+}\)
Black → Black = Copper(II) oxide, \(CuO\) → \(Cu^{2+}\)
Gas Detection Box
Limewater turns cloudy → \(CO_2\) → carbonate salt, \(CO_3^{2-}\)
Brown fumes + glowing splinter relit → \(NO_2 + O_2\) → nitrate salt, \(NO_3^-\)
Moist red litmus turns blue → \(NH_3\) → ammonium salt, \(NH_4^+\)
Brief Explanation
Understand the key ideas behind thermal decomposition and gas identification.
Action of Heat on Carbonate Salts
Most carbonate salts decompose when heated to produce metal oxides and carbon dioxide gas:
\[\text{Metal carbonate} \xrightarrow{\Delta} \text{Metal oxide} + \text{Carbon dioxide}\]
Example: \(ZnCO_3\text{(s)} \xrightarrow{\Delta} ZnO\text{(s)} + CO_2\text{(g)}\)
Exceptions: Potassium carbonate (\(K_2CO_3\)) and sodium carbonate (\(Na_2CO_3\)) are very stable and do not decompose when heated with a Bunsen flame.
Action of Heat on Nitrate Salts
Most nitrate salts decompose upon heating to produce metal oxides, brown nitrogen dioxide gas, and oxygen gas:
\[\text{Metal nitrate} \xrightarrow{\Delta} \text{Metal oxide} + \text{Nitrogen dioxide} + \text{Oxygen}\]
Example: \(2Pb(NO_3)_2\text{(s)} \xrightarrow{\Delta} 2PbO\text{(s)} + 4NO_2\text{(g)} + O_2\text{(g)}\)
Exceptions: Sodium nitrate (\(NaNO_3\)) and potassium nitrate (\(KNO_3\)) decompose differently, producing metal nitrites and oxygen gas without \(NO_2\) gas or metal oxide.
| Gas | Test Method | Observation |
|---|---|---|
| Oxygen, \(O_2\) | Insert a glowing wooden splinter into the gas. | Glowing splinter rekindles. |
| Hydrogen, \(H_2\) | Place a lighted wooden splinter near the mouth of the test tube. | A 'pop' sound is produced and the splinter is extinguished. |
| Carbon dioxide, \(CO_2\) | Bubble the gas through limewater. | Limewater turns cloudy / chalky. |
| Ammonia, \(NH_3\) | Place moist red litmus paper at the mouth of the test tube. | Moist red litmus paper turns blue. |
| Chlorine, \(Cl_2\) | Place moist blue litmus paper at the mouth of the test tube. | Moist blue litmus paper turns red and is then bleached. |
| Hydrogen chloride, \(HCl\) | Hold a glass rod dipped in concentrated ammonia solution at the mouth. | Dense white fumes of ammonium chloride are formed. |
| Sulphur dioxide, \(SO_2\) | Flow the gas into acidified potassium manganate(VII) solution. | Purple solution turns colourless. |
| Nitrogen dioxide, \(NO_2\) | Place moist blue litmus paper at the mouth of the test tube. | Brown gas is released; moist blue litmus paper turns red. |
| Colour when Hot | Colour when Cold | Metal Oxide | Cation Present in Salt |
|---|---|---|---|
| Yellow | White | Zinc oxide, \(ZnO\) | Zinc ion, \(Zn^{2+}\) |
| Brown | Yellow | Lead(II) oxide, \(PbO\) | Lead(II) ion, \(Pb^{2+}\) |
| Black | Black | Copper(II) oxide, \(CuO\) | Copper(II) ion, \(Cu^{2+}\) |
Try These Questions First
Answer mentally, then check the explanation.
What is observed when solid lead(II) nitrate, \(Pb(NO_3)_2\), is heated strongly in a dry boiling tube?
Why does sodium carbonate, \(Na_2CO_3\), not produce any gas when heated with a Bunsen burner?
Why must litmus paper be moistened before being placed at the mouth of a test tube to test for gas?
Common Mistakes
Avoid these errors when testing gases and interpreting thermal decomposition.
Common Misconceptions
Compare each misconception with the correct fact.
Chlorine gas turns moist blue litmus paper red only.
Chlorine gas first turns moist blue litmus paper red because it is acidic, but subsequently bleaches it white due to its strong bleaching (oxidising) property.
All salts decompose into gases upon heating.
Many chloride and sulphate salts, as well as group 1 carbonates (\(Na_2CO_3\), \(K_2CO_3\)), are thermally stable and do not decompose under standard laboratory Bunsen heating.
White fumes formed in the test for \(HCl\) gas are pure \(HCl\) gas.
The dense white fumes are solid ammonium chloride (\(NH_4Cl\)) particles formed from the reaction between \(HCl\) gas and ammonia gas (\(NH_3\)).
Summary
Key facts to remember.
- \(O_2\): Rekindles glowing splinter. \(H_2\): 'Pop' sound with lighted splinter. \(CO_2\): Limewater turns cloudy.
- \(NH_3\): Moist red litmus turns blue. \(Cl_2\): Moist blue litmus turns red, then bleached. \(HCl\): Forms dense white fumes with \(NH_3\).
- \(SO_2\): Decolourises purple acidified \(KMnO_4\). \(NO_2\): Brown gas, turns moist blue litmus red.
- \(\text{Carbonate salts} \xrightarrow{\Delta} \text{metal oxide} + CO_2\) (except \(Na_2CO_3\), \(K_2CO_3\)).
- \(\text{Nitrate salts} \xrightarrow{\Delta} \text{metal oxide} + NO_2 + O_2\) (except \(NaNO_3\), \(KNO_3\)).
- Residue colour diagnostics: Yellow hot, white cold → \(Zn^{2+}\) (\(ZnO\)); Brown hot, yellow cold → \(Pb^{2+}\) (\(PbO\)); Black hot, black cold → \(Cu^{2+}\) (\(CuO\)).
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
A green solid salt is heated strongly in a boiling tube. It produces a colourless gas that turns limewater cloudy, and leaves behind a black residue both when hot and cold. What is the identity of the salt?
Which gas decolourises purple acidified potassium manganate(VII) solution?
B. Fill in the Blanks
When heated, zinc carbonate decomposes to produce zinc oxide which is ________ when hot and white when cold.
Hydrogen chloride gas reacts with ammonia gas to form dense white fumes of ________.
C. Match the Gas with the Confirmatory Observation
Drag each gas on desktop. On mobile, tap a gas and then tap the matching box.
Gas
Oxygen gas (\(O_2\)) Ammonia gas (\(NH_3\)) Chlorine gas (\(Cl_2\)) Nitrogen dioxide gas (\(NO_2\))Confirmatory Observation
Keywords
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