Action of Heat on Salts

Action of Heat on Salts

1. Heating a salt may cause it to decompose. The decomposition may result in
   (a) a colour change
   (b) evolution of a gas
   (c) liberation of water vapour
2. Gases such as carbon dioxide, sulphur dioxide, nitrogen dioxide, ammonia or oxygen can be evolved. By identifying the gas or gases liberated, it is possible to pinpoint the anion present in the salt.
3. Examination of the residue can provide information to identify the cation in the salt.
   

Action of Heat on Carbonate Salts

1. Most metal carbonates decompose on heating to produce metal oxides and carbon dioxide gas.
   Metal carbonate → metal oxide + carbon dioxide
   
2. When the carbon dioxide gas is bubbled through limewater, it will turn the limewater milky.

Table shows the action of heat on carbonate salts.

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Action of Heat on Carbonate Salts Experiment

Aim: To investigate the action of heat on carbonate salts.
Materials: Limewater, sodium carbonate, magnesium carbonate, calcium carbonate, zinc carbonate, lead(II) carbonate, copper(II) carbonate and potassium carbonate.
Apparatus: Test tubes, tongs, spatula, Bunsen burner and stopper with delivery tube.
Procedure:

1. About two spatulaful of copper(II) carbonate are placed in a test tube.
2. The colour of the carbonate salt is noted.
3. The test tube is stoppered with a delivery tube dipping into limewater as shown in Figure.
   
4. The carbonate salt is then heated strongly.
5. Any changes that occur to the limewater and also the colour of the residue when it is hot and when it is cold are recorded.
6. Steps 1 to 5 are repeated using each of the carbonate salts listed in Table to replace the copper(II) carbonate.

Observations:

Discussion:

1. Alkali metal carbonates such as sodium carbonate and potassium carbonate are stable to heat.
2. Most metal carbonates decompose on heating to produce metal oxides and liberate carbon dioxide gas.
3. The carbon dioxide gas forms a white precipitate with limewater, making the limewater milky.

Conclusion:
Heating a metal carbonate will decompose it into a metal oxide and liberate carbon dioxide. Group 1 metal carbonates are not decomposed by heat.

Action of Heat on Nitrate Salts

1. Nitrate salts also undergo decomposition on heating.
2. Most metal nitrates decompose to produce a metal oxide, nitrogen dioxide and oxygen.
   
3. Sodium nitrate and potassium nitrate decompose to produce nitrite salts and oxygen.
4. Nitrogen dioxide is a brown gas. It is an acidic gas that turns moist blue litmus paper red. Hence, dissolving it in water produces a colourless acidic solution.
   2NO₂(g) + H₂O(l) → HNO₂(aq) + HNO₃(aq)
5. The colourless oxygen gas rekindles a glowing wooden splint.

Table: Action of heat on nitrate salts

Action of Heat on Nitrate Salts Experiment

Aim: To investigate the action of heat on nitrate salts.
Materials: Sodium nitrate, magnesium nitrate, calcium nitrate, zinc nitrate, lead(II) nitrate, copper(II) nitrate, potassium nitrate, iron(III) nitrate, iron(II) nitrate, blue litmus paper and wooden splint.
Apparatus: Test tubes, tongs, spatula and Bunsen burner.
Procedure:

1. About two spatulaful of copper(II) nitrate are placed in a test tube.
2. The colour of the nitrate salt is noted.
3. The nitrate salt is then heated strongly as shown in Figure.
   
4. The gases liberated are tested by
   (a) lowering a glowing wooden splint into the test tube.
   (b) bringing a piece of moist blue litmus paper to the mouth of the test tube.
5. The colour of the residue when it is hot and when it is cold are recorded.
6. Steps 1 to 5 are repeated using each of the nitrate salts listed in Table to replace the copper(II) nitrate.

Observations:

Discussion:

1. When nitrate salts are heated, they decompose to liberate nitrogen dioxide and oxygen.
2. Only sodium nitrate and potassium nitrate decompose to liberate oxygen.
3. Nitrogen dioxide is a brown gas that turns moist blue litmus paper red.
4. Oxygen is a colourless gas that relights a glowing wooden splint.

Conclusion:
Most metal nitrates decompose to produce a metal oxide, nitrogen dioxide and oxygen except sodium nitrate and potassium nitrate which decompose to produce nitrite salts and oxygen.

Action of Heat on Sulphate Salts

1. The normal sulphate salts are more stable to heat compared to the carbonates and nitrates.
2. Group 1 metal sulphates such as sodium sulphate and potassium sulphate do not decompose on heating. Group 2 metal sulphates such as calcium sulphate also do not decompose when heated.
3. The sulphates of heavy metals are decomposed into metal oxides and sulphur trioxide when heated.
   
4. Sulphur trioxide is a typical acidic oxide and dissolves in water to form sulphuric acid.
   SO₃(g) + H₂O(l) → H₂SO₄(aq)
5. An exceptional case is iron(II) sulphate because it also forms sulphur dioxide gas.
   2FeSO₄(s) → Fe₂O₃(s) + SO₃(g) + SO₂(g)
   The green crystals of iron(II) sulphate turn into a brown solid of iron(III) oxide.
6. Ammonium sulphate sublimes when first heated. Further heating decomposes the salt into ammonia and hydrogen sulphate.
   (NH₄)₂SO₄(S) – 2NH₃(g) + H₂SO₄(g)

Action of Heat on Chloride Salts

1. Chloride salts are stable to heat except ammonium chloride.
2. Initial heating of ammonium chloride causes the salt to sublime.
   NH₄Cl(S) → NH₄Cl(g)
3. On further heating, decomposition takes place to produce ammonia and hydrogen chloride.
   NH₄Cl(g) → NH₃(g) + HCl(g)
4. When ammonium chloride is heated in a test tube, the lighter ammonia gas will emerge first and turn a piece of moist red litmus paper blue. Hydrogen chloride, coming up next, will change the litmus paper from blue back to red.
   

Identification of salts by action of heat

1. When a salt is heated strongly, it may decompose. One or more gases may be liberated.
2. Each gas can be identified by
   • noting its colour.
   • testing it with moist blue or red litmus paper.
   • testing it with limewater.
   • testing it with glowing wooden splint.
   • testing it with acidified potassium dichromate(VI) solution or acidified potassium manganate(VII) solution.
3. The colour of the residue when hot and cold must be noted to help in the identification of the salt.
4. The following table shows how to identify salts P, Q, R, S and T through gases liberated by the action of heat.